TypeScript/src/compiler/checker.ts

/// <reference path="binder.ts"/>

/* @internal */
namespace ts {
    let nextSymbolId = 1;
    let nextNodeId = 1;
    let nextMergeId = 1;
    let nextFlowId = 1;

    export function getNodeId(node: Node): number {
        if (!node.id) {
            node.id = nextNodeId;
            nextNodeId++;
        }
        return node.id;
    }

    export function getSymbolId(symbol: Symbol): number {
        if (!symbol.id) {
            symbol.id = nextSymbolId;
            nextSymbolId++;
        }

        return symbol.id;
    }

    export function createTypeChecker(host: TypeCheckerHost, produceDiagnostics: boolean): TypeChecker {
        // Cancellation that controls whether or not we can cancel in the middle of type checking.
        // In general cancelling is *not* safe for the type checker.  We might be in the middle of
        // computing something, and we will leave our internals in an inconsistent state.  Callers
        // who set the cancellation token should catch if a cancellation exception occurs, and
        // should throw away and create a new TypeChecker.
        //
        // Currently we only support setting the cancellation token when getting diagnostics.  This
        // is because diagnostics can be quite expensive, and we want to allow hosts to bail out if
        // they no longer need the information (for example, if the user started editing again).
        let cancellationToken: CancellationToken;

        const Symbol = objectAllocator.getSymbolConstructor();
        const Type = objectAllocator.getTypeConstructor();
        const Signature = objectAllocator.getSignatureConstructor();

        let typeCount = 0;
        let symbolCount = 0;

        const emptyArray: any[] = [];
        const emptySymbols = createMap<Symbol>();

        const compilerOptions = host.getCompilerOptions();
        const languageVersion = compilerOptions.target || ScriptTarget.ES3;
        const modulekind = getEmitModuleKind(compilerOptions);
        const noUnusedIdentifiers = !!compilerOptions.noUnusedLocals || !!compilerOptions.noUnusedParameters;
        const allowSyntheticDefaultImports = typeof compilerOptions.allowSyntheticDefaultImports !== "undefined" ? compilerOptions.allowSyntheticDefaultImports : modulekind === ModuleKind.System;
        const strictNullChecks = compilerOptions.strictNullChecks;

        const emitResolver = createResolver();

        const undefinedSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "undefined");
        undefinedSymbol.declarations = [];
        const argumentsSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "arguments");

        const checker: TypeChecker = {
            getNodeCount: () => sum(host.getSourceFiles(), "nodeCount"),
            getIdentifierCount: () => sum(host.getSourceFiles(), "identifierCount"),
            getSymbolCount: () => sum(host.getSourceFiles(), "symbolCount") + symbolCount,
            getTypeCount: () => typeCount,
            isUndefinedSymbol: symbol => symbol === undefinedSymbol,
            isArgumentsSymbol: symbol => symbol === argumentsSymbol,
            isUnknownSymbol: symbol => symbol === unknownSymbol,
            getDiagnostics,
            getGlobalDiagnostics,
            getTypeOfSymbolAtLocation,
            getSymbolsOfParameterPropertyDeclaration,
            getDeclaredTypeOfSymbol,
            getPropertiesOfType,
            getPropertyOfType,
            getSignaturesOfType,
            getIndexTypeOfType,
            getBaseTypes,
            getReturnTypeOfSignature,
            getNonNullableType,
            getSymbolsInScope,
            getSymbolAtLocation,
            getShorthandAssignmentValueSymbol,
            getExportSpecifierLocalTargetSymbol,
            getTypeAtLocation: getTypeOfNode,
            getPropertySymbolOfDestructuringAssignment,
            typeToString,
            getSymbolDisplayBuilder,
            symbolToString,
            getAugmentedPropertiesOfType,
            getRootSymbols,
            getContextualType,
            getFullyQualifiedName,
            getResolvedSignature,
            getConstantValue,
            isValidPropertyAccess,
            getSignatureFromDeclaration,
            isImplementationOfOverload,
            getAliasedSymbol: resolveAlias,
            getEmitResolver,
            getExportsOfModule: getExportsOfModuleAsArray,

            getJsxElementAttributesType,
            getJsxIntrinsicTagNames,
            isOptionalParameter
        };

        const tupleTypes = createMap<TupleType>();
        const unionTypes = createMap<UnionType>();
        const intersectionTypes = createMap<IntersectionType>();
        const stringLiteralTypes = createMap<LiteralType>();
        const numericLiteralTypes = createMap<LiteralType>();

        const unknownSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "unknown");
        const resolvingSymbol = createSymbol(SymbolFlags.Transient, "__resolving__");

        const anyType = createIntrinsicType(TypeFlags.Any, "any");
        const unknownType = createIntrinsicType(TypeFlags.Any, "unknown");
        const undefinedType = createIntrinsicType(TypeFlags.Undefined, "undefined");
        const undefinedWideningType = strictNullChecks ? undefinedType : createIntrinsicType(TypeFlags.Undefined | TypeFlags.ContainsWideningType, "undefined");
        const nullType = createIntrinsicType(TypeFlags.Null, "null");
        const nullWideningType = strictNullChecks ? nullType : createIntrinsicType(TypeFlags.Null | TypeFlags.ContainsWideningType, "null");
        const stringType = createIntrinsicType(TypeFlags.String, "string");
        const numberType = createIntrinsicType(TypeFlags.Number, "number");
        const trueType = createIntrinsicType(TypeFlags.BooleanLiteral, "true");
        const falseType = createIntrinsicType(TypeFlags.BooleanLiteral, "false");
        const booleanType = createBooleanType([trueType, falseType]);
        const esSymbolType = createIntrinsicType(TypeFlags.ESSymbol, "symbol");
        const voidType = createIntrinsicType(TypeFlags.Void, "void");
        const neverType = createIntrinsicType(TypeFlags.Never, "never");

        const emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        const emptyGenericType = <GenericType><ObjectType>createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        emptyGenericType.instantiations = createMap<TypeReference>();

        const anyFunctionType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
        // The anyFunctionType contains the anyFunctionType by definition. The flag is further propagated
        // in getPropagatingFlagsOfTypes, and it is checked in inferFromTypes.
        anyFunctionType.flags |= TypeFlags.ContainsAnyFunctionType;

        const noConstraintType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);

        const anySignature = createSignature(undefined, undefined, undefined, emptyArray, anyType, /*typePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);
        const unknownSignature = createSignature(undefined, undefined, undefined, emptyArray, unknownType, /*typePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false);

        const enumNumberIndexInfo = createIndexInfo(stringType, /*isReadonly*/ true);

        const globals = createMap<Symbol>();
        /**
         * List of every ambient module with a "*" wildcard.
         * Unlike other ambient modules, these can't be stored in `globals` because symbol tables only deal with exact matches.
         * This is only used if there is no exact match.
        */
        let patternAmbientModules: PatternAmbientModule[];

        let getGlobalESSymbolConstructorSymbol: () => Symbol;

        let getGlobalPromiseConstructorSymbol: () => Symbol;

        let globalObjectType: ObjectType;
        let globalFunctionType: ObjectType;
        let globalArrayType: GenericType;
        let globalReadonlyArrayType: GenericType;
        let globalStringType: ObjectType;
        let globalNumberType: ObjectType;
        let globalBooleanType: ObjectType;
        let globalRegExpType: ObjectType;
        let anyArrayType: Type;
        let anyReadonlyArrayType: Type;

        // The library files are only loaded when the feature is used.
        // This allows users to just specify library files they want to used through --lib
        // and they will not get an error from not having unrelated library files
        let getGlobalTemplateStringsArrayType: () => ObjectType;

        let getGlobalESSymbolType: () => ObjectType;
        let getGlobalIterableType: () => GenericType;
        let getGlobalIteratorType: () => GenericType;
        let getGlobalIterableIteratorType: () => GenericType;

        let getGlobalClassDecoratorType: () => ObjectType;
        let getGlobalParameterDecoratorType: () => ObjectType;
        let getGlobalPropertyDecoratorType: () => ObjectType;
        let getGlobalMethodDecoratorType: () => ObjectType;
        let getGlobalTypedPropertyDescriptorType: () => ObjectType;
        let getGlobalPromiseType: () => ObjectType;
        let tryGetGlobalPromiseType: () => ObjectType;
        let getGlobalPromiseLikeType: () => ObjectType;
        let getInstantiatedGlobalPromiseLikeType: () => ObjectType;
        let getGlobalPromiseConstructorLikeType: () => ObjectType;
        let getGlobalThenableType: () => ObjectType;

        let jsxElementClassType: Type;

        let deferredNodes: Node[];
        let deferredUnusedIdentifierNodes: Node[];

        let flowLoopStart = 0;
        let flowLoopCount = 0;
        let visitedFlowCount = 0;

        const emptyStringType = getLiteralTypeForText(TypeFlags.StringLiteral, "");
        const zeroType = getLiteralTypeForText(TypeFlags.NumberLiteral, "0");

        const resolutionTargets: TypeSystemEntity[] = [];
        const resolutionResults: boolean[] = [];
        const resolutionPropertyNames: TypeSystemPropertyName[] = [];

        const mergedSymbols: Symbol[] = [];
        const symbolLinks: SymbolLinks[] = [];
        const nodeLinks: NodeLinks[] = [];
        const flowLoopCaches: Map<Type>[] = [];
        const flowLoopNodes: FlowNode[] = [];
        const flowLoopKeys: string[] = [];
        const flowLoopTypes: Type[][] = [];
        const visitedFlowNodes: FlowNode[] = [];
        const visitedFlowTypes: FlowType[] = [];
        const potentialThisCollisions: Node[] = [];
        const awaitedTypeStack: number[] = [];

        const diagnostics = createDiagnosticCollection();

        const enum TypeFacts {
            None = 0,
            TypeofEQString = 1 << 0,      // typeof x === "string"
            TypeofEQNumber = 1 << 1,      // typeof x === "number"
            TypeofEQBoolean = 1 << 2,     // typeof x === "boolean"
            TypeofEQSymbol = 1 << 3,      // typeof x === "symbol"
            TypeofEQObject = 1 << 4,      // typeof x === "object"
            TypeofEQFunction = 1 << 5,    // typeof x === "function"
            TypeofEQHostObject = 1 << 6,  // typeof x === "xxx"
            TypeofNEString = 1 << 7,      // typeof x !== "string"
            TypeofNENumber = 1 << 8,      // typeof x !== "number"
            TypeofNEBoolean = 1 << 9,     // typeof x !== "boolean"
            TypeofNESymbol = 1 << 10,     // typeof x !== "symbol"
            TypeofNEObject = 1 << 11,     // typeof x !== "object"
            TypeofNEFunction = 1 << 12,   // typeof x !== "function"
            TypeofNEHostObject = 1 << 13, // typeof x !== "xxx"
            EQUndefined = 1 << 14,        // x === undefined
            EQNull = 1 << 15,             // x === null
            EQUndefinedOrNull = 1 << 16,  // x == undefined / x == null
            NEUndefined = 1 << 17,        // x !== undefined
            NENull = 1 << 18,             // x !== null
            NEUndefinedOrNull = 1 << 19,  // x != undefined / x != null
            Truthy = 1 << 20,             // x
            Falsy = 1 << 21,              // !x
            Discriminatable = 1 << 22,    // May have discriminant property
            All = (1 << 23) - 1,
            // The following members encode facts about particular kinds of types for use in the getTypeFacts function.
            // The presence of a particular fact means that the given test is true for some (and possibly all) values
            // of that kind of type.
            BaseStringStrictFacts = TypeofEQString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseStringFacts = BaseStringStrictFacts  | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            StringStrictFacts = BaseStringStrictFacts | Truthy | Falsy,
            StringFacts = BaseStringFacts | Truthy,
            EmptyStringStrictFacts = BaseStringStrictFacts | Falsy,
            EmptyStringFacts = BaseStringFacts,
            NonEmptyStringStrictFacts = BaseStringStrictFacts | Truthy,
            NonEmptyStringFacts = BaseStringFacts | Truthy,
            BaseNumberStrictFacts = TypeofEQNumber | TypeofNEString | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseNumberFacts = BaseNumberStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            NumberStrictFacts = BaseNumberStrictFacts | Truthy | Falsy,
            NumberFacts = BaseNumberFacts | Truthy,
            ZeroStrictFacts = BaseNumberStrictFacts | Falsy,
            ZeroFacts = BaseNumberFacts,
            NonZeroStrictFacts = BaseNumberStrictFacts | Truthy,
            NonZeroFacts = BaseNumberFacts | Truthy,
            BaseBooleanStrictFacts = TypeofEQBoolean | TypeofNEString | TypeofNENumber | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull,
            BaseBooleanFacts = BaseBooleanStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            BooleanStrictFacts = BaseBooleanStrictFacts | Truthy | Falsy,
            BooleanFacts = BaseBooleanFacts | Truthy,
            FalseStrictFacts = BaseBooleanStrictFacts | Falsy,
            FalseFacts = BaseBooleanFacts,
            TrueStrictFacts = BaseBooleanStrictFacts | Truthy,
            TrueFacts = BaseBooleanFacts | Truthy,
            SymbolStrictFacts = TypeofEQSymbol | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
            SymbolFacts = SymbolStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
            ObjectFacts = ObjectStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
            FunctionFacts = FunctionStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
            UndefinedFacts = TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | EQUndefined | EQUndefinedOrNull | NENull | Falsy,
            NullFacts = TypeofEQObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | TypeofNEHostObject | EQNull | EQUndefinedOrNull | NEUndefined | Falsy,
        }

        const typeofEQFacts = createMap({
            "string": TypeFacts.TypeofEQString,
            "number": TypeFacts.TypeofEQNumber,
            "boolean": TypeFacts.TypeofEQBoolean,
            "symbol": TypeFacts.TypeofEQSymbol,
            "undefined": TypeFacts.EQUndefined,
            "object": TypeFacts.TypeofEQObject,
            "function": TypeFacts.TypeofEQFunction
        });

        const typeofNEFacts = createMap({
            "string": TypeFacts.TypeofNEString,
            "number": TypeFacts.TypeofNENumber,
            "boolean": TypeFacts.TypeofNEBoolean,
            "symbol": TypeFacts.TypeofNESymbol,
            "undefined": TypeFacts.NEUndefined,
            "object": TypeFacts.TypeofNEObject,
            "function": TypeFacts.TypeofNEFunction
        });

        const typeofTypesByName = createMap<Type>({
            "string": stringType,
            "number": numberType,
            "boolean": booleanType,
            "symbol": esSymbolType,
            "undefined": undefinedType
        });

        let jsxElementType: ObjectType;
        /** Things we lazy load from the JSX namespace */
        const jsxTypes = createMap<ObjectType>();
        const JsxNames = {
            JSX: "JSX",
            IntrinsicElements: "IntrinsicElements",
            ElementClass: "ElementClass",
            ElementAttributesPropertyNameContainer: "ElementAttributesProperty",
            Element: "Element",
            IntrinsicAttributes: "IntrinsicAttributes",
            IntrinsicClassAttributes: "IntrinsicClassAttributes"
        };

        const subtypeRelation = createMap<RelationComparisonResult>();
        const assignableRelation = createMap<RelationComparisonResult>();
        const comparableRelation = createMap<RelationComparisonResult>();
        const identityRelation = createMap<RelationComparisonResult>();

        // This is for caching the result of getSymbolDisplayBuilder. Do not access directly.
        let _displayBuilder: SymbolDisplayBuilder;

        type TypeSystemEntity = Symbol | Type | Signature;

        const enum TypeSystemPropertyName {
            Type,
            ResolvedBaseConstructorType,
            DeclaredType,
            ResolvedReturnType
        }

        const builtinGlobals = createMap<Symbol>();
        builtinGlobals[undefinedSymbol.name] = undefinedSymbol;

        initializeTypeChecker();

        return checker;

        function getEmitResolver(sourceFile: SourceFile, cancellationToken: CancellationToken) {
            // Ensure we have all the type information in place for this file so that all the
            // emitter questions of this resolver will return the right information.
            getDiagnostics(sourceFile, cancellationToken);
            return emitResolver;
        }

        function error(location: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): void {
            const diagnostic = location
                ? createDiagnosticForNode(location, message, arg0, arg1, arg2)
                : createCompilerDiagnostic(message, arg0, arg1, arg2);
            diagnostics.add(diagnostic);
        }

        function createSymbol(flags: SymbolFlags, name: string): Symbol {
            symbolCount++;
            return new Symbol(flags, name);
        }

        function getExcludedSymbolFlags(flags: SymbolFlags): SymbolFlags {
            let result: SymbolFlags = 0;
            if (flags & SymbolFlags.BlockScopedVariable) result |= SymbolFlags.BlockScopedVariableExcludes;
            if (flags & SymbolFlags.FunctionScopedVariable) result |= SymbolFlags.FunctionScopedVariableExcludes;
            if (flags & SymbolFlags.Property) result |= SymbolFlags.PropertyExcludes;
            if (flags & SymbolFlags.EnumMember) result |= SymbolFlags.EnumMemberExcludes;
            if (flags & SymbolFlags.Function) result |= SymbolFlags.FunctionExcludes;
            if (flags & SymbolFlags.Class) result |= SymbolFlags.ClassExcludes;
            if (flags & SymbolFlags.Interface) result |= SymbolFlags.InterfaceExcludes;
            if (flags & SymbolFlags.RegularEnum) result |= SymbolFlags.RegularEnumExcludes;
            if (flags & SymbolFlags.ConstEnum) result |= SymbolFlags.ConstEnumExcludes;
            if (flags & SymbolFlags.ValueModule) result |= SymbolFlags.ValueModuleExcludes;
            if (flags & SymbolFlags.Method) result |= SymbolFlags.MethodExcludes;
            if (flags & SymbolFlags.GetAccessor) result |= SymbolFlags.GetAccessorExcludes;
            if (flags & SymbolFlags.SetAccessor) result |= SymbolFlags.SetAccessorExcludes;
            if (flags & SymbolFlags.TypeParameter) result |= SymbolFlags.TypeParameterExcludes;
            if (flags & SymbolFlags.TypeAlias) result |= SymbolFlags.TypeAliasExcludes;
            if (flags & SymbolFlags.Alias) result |= SymbolFlags.AliasExcludes;
            return result;
        }

        function recordMergedSymbol(target: Symbol, source: Symbol) {
            if (!source.mergeId) {
                source.mergeId = nextMergeId;
                nextMergeId++;
            }
            mergedSymbols[source.mergeId] = target;
        }

        function cloneSymbol(symbol: Symbol): Symbol {
            const result = createSymbol(symbol.flags | SymbolFlags.Merged, symbol.name);
            result.declarations = symbol.declarations.slice(0);
            result.parent = symbol.parent;
            if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
            if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;
            if (symbol.members) result.members = cloneMap(symbol.members);
            if (symbol.exports) result.exports = cloneMap(symbol.exports);
            recordMergedSymbol(result, symbol);
            return result;
        }

        function mergeSymbol(target: Symbol, source: Symbol) {
            if (!(target.flags & getExcludedSymbolFlags(source.flags))) {
                if (source.flags & SymbolFlags.ValueModule && target.flags & SymbolFlags.ValueModule && target.constEnumOnlyModule && !source.constEnumOnlyModule) {
                    // reset flag when merging instantiated module into value module that has only const enums
                    target.constEnumOnlyModule = false;
                }
                target.flags |= source.flags;
                if (source.valueDeclaration &&
                    (!target.valueDeclaration ||
                        (target.valueDeclaration.kind === SyntaxKind.ModuleDeclaration && source.valueDeclaration.kind !== SyntaxKind.ModuleDeclaration))) {
                    // other kinds of value declarations take precedence over modules
                    target.valueDeclaration = source.valueDeclaration;
                }
                forEach(source.declarations, node => {
                    target.declarations.push(node);
                });
                if (source.members) {
                    if (!target.members) target.members = createMap<Symbol>();
                    mergeSymbolTable(target.members, source.members);
                }
                if (source.exports) {
                    if (!target.exports) target.exports = createMap<Symbol>();
                    mergeSymbolTable(target.exports, source.exports);
                }
                recordMergedSymbol(target, source);
            }
            else {
                const message = target.flags & SymbolFlags.BlockScopedVariable || source.flags & SymbolFlags.BlockScopedVariable
                    ? Diagnostics.Cannot_redeclare_block_scoped_variable_0 : Diagnostics.Duplicate_identifier_0;
                forEach(source.declarations, node => {
                    error(node.name ? node.name : node, message, symbolToString(source));
                });
                forEach(target.declarations, node => {
                    error(node.name ? node.name : node, message, symbolToString(source));
                });
            }
        }

        function mergeSymbolTable(target: SymbolTable, source: SymbolTable) {
            for (const id in source) {
                let targetSymbol = target[id];
                if (!targetSymbol) {
                    target[id] = source[id];
                }
                else {
                    if (!(targetSymbol.flags & SymbolFlags.Merged)) {
                        target[id] = targetSymbol = cloneSymbol(targetSymbol);
                    }
                    mergeSymbol(targetSymbol, source[id]);
                }
            }
        }

        function mergeModuleAugmentation(moduleName: LiteralExpression): void {
            const moduleAugmentation = <ModuleDeclaration>moduleName.parent;
            if (moduleAugmentation.symbol.declarations[0] !== moduleAugmentation) {
                // this is a combined symbol for multiple augmentations within the same file.
                // its symbol already has accumulated information for all declarations
                // so we need to add it just once - do the work only for first declaration
                Debug.assert(moduleAugmentation.symbol.declarations.length > 1);
                return;
            }

            if (isGlobalScopeAugmentation(moduleAugmentation)) {
                mergeSymbolTable(globals, moduleAugmentation.symbol.exports);
            }
            else {
                // find a module that about to be augmented
                // do not validate names of augmentations that are defined in ambient context
                const moduleNotFoundError = !isInAmbientContext(moduleName.parent.parent)
                    ? Diagnostics.Invalid_module_name_in_augmentation_module_0_cannot_be_found
                    : undefined;
                let mainModule = resolveExternalModuleNameWorker(moduleName, moduleName, moduleNotFoundError);
                if (!mainModule) {
                    return;
                }
                // obtain item referenced by 'export='
                mainModule = resolveExternalModuleSymbol(mainModule);
                if (mainModule.flags & SymbolFlags.Namespace) {
                    // if module symbol has already been merged - it is safe to use it.
                    // otherwise clone it
                    mainModule = mainModule.flags & SymbolFlags.Merged ? mainModule : cloneSymbol(mainModule);
                    mergeSymbol(mainModule, moduleAugmentation.symbol);
                }
                else {
                    error(moduleName, Diagnostics.Cannot_augment_module_0_because_it_resolves_to_a_non_module_entity, moduleName.text);
                }
            }
        }

        function addToSymbolTable(target: SymbolTable, source: SymbolTable, message: DiagnosticMessage) {
            for (const id in source) {
                if (target[id]) {
                    // Error on redeclarations
                    forEach(target[id].declarations, addDeclarationDiagnostic(id, message));
                }
                else {
                    target[id] = source[id];
                }
            }

            function addDeclarationDiagnostic(id: string, message: DiagnosticMessage) {
                return (declaration: Declaration) => diagnostics.add(createDiagnosticForNode(declaration, message, id));
            }
        }

        function getSymbolLinks(symbol: Symbol): SymbolLinks {
            if (symbol.flags & SymbolFlags.Transient) return <TransientSymbol>symbol;
            const id = getSymbolId(symbol);
            return symbolLinks[id] || (symbolLinks[id] = {});
        }

        function getNodeLinks(node: Node): NodeLinks {
            const nodeId = getNodeId(node);
            return nodeLinks[nodeId] || (nodeLinks[nodeId] = {});
        }

        function isGlobalSourceFile(node: Node) {
            return node.kind === SyntaxKind.SourceFile && !isExternalOrCommonJsModule(<SourceFile>node);
        }

        function getSymbol(symbols: SymbolTable, name: string, meaning: SymbolFlags): Symbol {
            if (meaning) {
                const symbol = symbols[name];
                if (symbol) {
                    Debug.assert((symbol.flags & SymbolFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
                    if (symbol.flags & meaning) {
                        return symbol;
                    }
                    if (symbol.flags & SymbolFlags.Alias) {
                        const target = resolveAlias(symbol);
                        // Unknown symbol means an error occurred in alias resolution, treat it as positive answer to avoid cascading errors
                        if (target === unknownSymbol || target.flags & meaning) {
                            return symbol;
                        }
                    }
                }
            }
            // return undefined if we can't find a symbol.
        }

        /**
         * Get symbols that represent parameter-property-declaration as parameter and as property declaration
         * @param parameter a parameterDeclaration node
         * @param parameterName a name of the parameter to get the symbols for.
         * @return a tuple of two symbols
         */
        function getSymbolsOfParameterPropertyDeclaration(parameter: ParameterDeclaration, parameterName: string): [Symbol, Symbol] {
            const constructorDeclaration = parameter.parent;
            const classDeclaration = parameter.parent.parent;

            const parameterSymbol = getSymbol(constructorDeclaration.locals, parameterName, SymbolFlags.Value);
            const propertySymbol = getSymbol(classDeclaration.symbol.members, parameterName, SymbolFlags.Value);

            if (parameterSymbol && propertySymbol) {
                return [parameterSymbol, propertySymbol];
            }

            Debug.fail("There should exist two symbols, one as property declaration and one as parameter declaration");
        }

        function isBlockScopedNameDeclaredBeforeUse(declaration: Declaration, usage: Node): boolean {
            const declarationFile = getSourceFileOfNode(declaration);
            const useFile = getSourceFileOfNode(usage);
            if (declarationFile !== useFile) {
                if ((modulekind && (declarationFile.externalModuleIndicator || useFile.externalModuleIndicator)) ||
                    (!compilerOptions.outFile && !compilerOptions.out)) {
                    // nodes are in different files and order cannot be determines
                    return true;
                }

                const sourceFiles = host.getSourceFiles();
                return indexOf(sourceFiles, declarationFile) <= indexOf(sourceFiles, useFile);
            }

            if (declaration.pos <= usage.pos) {
                // declaration is before usage
                // still might be illegal if usage is in the initializer of the variable declaration
                return declaration.kind !== SyntaxKind.VariableDeclaration ||
                    !isImmediatelyUsedInInitializerOfBlockScopedVariable(<VariableDeclaration>declaration, usage);
            }

            // declaration is after usage
            // can be legal if usage is deferred (i.e. inside function or in initializer of instance property)
            return isUsedInFunctionOrNonStaticProperty(declaration, usage);

            function isImmediatelyUsedInInitializerOfBlockScopedVariable(declaration: VariableDeclaration, usage: Node): boolean {
                const container = getEnclosingBlockScopeContainer(declaration);

                switch (declaration.parent.parent.kind) {
                    case SyntaxKind.VariableStatement:
                    case SyntaxKind.ForStatement:
                    case SyntaxKind.ForOfStatement:
                        // variable statement/for/for-of statement case,
                        // use site should not be inside variable declaration (initializer of declaration or binding element)
                        if (isSameScopeDescendentOf(usage, declaration, container)) {
                            return true;
                        }
                        break;
                }

                switch (declaration.parent.parent.kind) {
                    case SyntaxKind.ForInStatement:
                    case SyntaxKind.ForOfStatement:
                        // ForIn/ForOf case - use site should not be used in expression part
                        if (isSameScopeDescendentOf(usage, (<ForInStatement | ForOfStatement>declaration.parent.parent).expression, container)) {
                            return true;
                        }
                }

                return false;
            }

            function isUsedInFunctionOrNonStaticProperty(declaration: Declaration, usage: Node): boolean {
                const container = getEnclosingBlockScopeContainer(declaration);
                let current = usage;
                while (current) {
                    if (current === container) {
                        return false;
                    }

                    if (isFunctionLike(current)) {
                        return true;
                    }

                    const initializerOfNonStaticProperty = current.parent &&
                        current.parent.kind === SyntaxKind.PropertyDeclaration &&
                        (current.parent.flags & NodeFlags.Static) === 0 &&
                        (<PropertyDeclaration>current.parent).initializer === current;

                    if (initializerOfNonStaticProperty) {
                        return true;
                    }

                    current = current.parent;
                }
                return false;
            }
        }

        // Resolve a given name for a given meaning at a given location. An error is reported if the name was not found and
        // the nameNotFoundMessage argument is not undefined. Returns the resolved symbol, or undefined if no symbol with
        // the given name can be found.
        function resolveName(location: Node | undefined, name: string, meaning: SymbolFlags, nameNotFoundMessage: DiagnosticMessage, nameArg: string | Identifier): Symbol {
            let result: Symbol;
            let lastLocation: Node;
            let propertyWithInvalidInitializer: Node;
            const errorLocation = location;
            let grandparent: Node;
            let isInExternalModule = false;

            loop: while (location) {
                // Locals of a source file are not in scope (because they get merged into the global symbol table)
                if (location.locals && !isGlobalSourceFile(location)) {
                    if (result = getSymbol(location.locals, name, meaning)) {
                        let useResult = true;
                        if (isFunctionLike(location) && lastLocation && lastLocation !== (<FunctionLikeDeclaration>location).body) {
                            // symbol lookup restrictions for function-like declarations
                            // - Type parameters of a function are in scope in the entire function declaration, including the parameter
                            //   list and return type. However, local types are only in scope in the function body.
                            // - parameters are only in the scope of function body
                            // This restriction does not apply to JSDoc comment types because they are parented
                            // at a higher level than type parameters would normally be
                            if (meaning & result.flags & SymbolFlags.Type && lastLocation.kind !== SyntaxKind.JSDocComment) {
                                useResult = result.flags & SymbolFlags.TypeParameter
                                    // type parameters are visible in parameter list, return type and type parameter list
                                    ? lastLocation === (<FunctionLikeDeclaration>location).type ||
                                    lastLocation.kind === SyntaxKind.Parameter ||
                                    lastLocation.kind === SyntaxKind.TypeParameter
                                    // local types not visible outside the function body
                                    : false;
                            }
                            if (meaning & SymbolFlags.Value && result.flags & SymbolFlags.FunctionScopedVariable) {
                                // parameters are visible only inside function body, parameter list and return type
                                // technically for parameter list case here we might mix parameters and variables declared in function,
                                // however it is detected separately when checking initializers of parameters
                                // to make sure that they reference no variables declared after them.
                                useResult =
                                    lastLocation.kind === SyntaxKind.Parameter ||
                                    (
                                        lastLocation === (<FunctionLikeDeclaration>location).type &&
                                        result.valueDeclaration.kind === SyntaxKind.Parameter
                                    );
                            }
                        }

                        if (useResult) {
                            break loop;
                        }
                        else {
                            result = undefined;
                        }
                    }
                }
                switch (location.kind) {
                    case SyntaxKind.SourceFile:
                        if (!isExternalOrCommonJsModule(<SourceFile>location)) break;
                        isInExternalModule = true;
                    case SyntaxKind.ModuleDeclaration:
                        const moduleExports = getSymbolOfNode(location).exports;
                        if (location.kind === SyntaxKind.SourceFile || isAmbientModule(location)) {

                            // It's an external module. First see if the module has an export default and if the local
                            // name of that export default matches.
                            if (result = moduleExports["default"]) {
                                const localSymbol = getLocalSymbolForExportDefault(result);
                                if (localSymbol && (result.flags & meaning) && localSymbol.name === name) {
                                    break loop;
                                }
                                result = undefined;
                            }

                            // Because of module/namespace merging, a module's exports are in scope,
                            // yet we never want to treat an export specifier as putting a member in scope.
                            // Therefore, if the name we find is purely an export specifier, it is not actually considered in scope.
                            // Two things to note about this:
                            //     1. We have to check this without calling getSymbol. The problem with calling getSymbol
                            //        on an export specifier is that it might find the export specifier itself, and try to
                            //        resolve it as an alias. This will cause the checker to consider the export specifier
                            //        a circular alias reference when it might not be.
                            //     2. We check === SymbolFlags.Alias in order to check that the symbol is *purely*
                            //        an alias. If we used &, we'd be throwing out symbols that have non alias aspects,
                            //        which is not the desired behavior.
                            if (moduleExports[name] &&
                                moduleExports[name].flags === SymbolFlags.Alias &&
                                getDeclarationOfKind(moduleExports[name], SyntaxKind.ExportSpecifier)) {
                                break;
                            }
                        }

                        if (result = getSymbol(moduleExports, name, meaning & SymbolFlags.ModuleMember)) {
                            break loop;
                        }
                        break;
                    case SyntaxKind.EnumDeclaration:
                        if (result = getSymbol(getSymbolOfNode(location).exports, name, meaning & SymbolFlags.EnumMember)) {
                            break loop;
                        }
                        break;
                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.PropertySignature:
                        // TypeScript 1.0 spec (April 2014): 8.4.1
                        // Initializer expressions for instance member variables are evaluated in the scope
                        // of the class constructor body but are not permitted to reference parameters or
                        // local variables of the constructor. This effectively means that entities from outer scopes
                        // by the same name as a constructor parameter or local variable are inaccessible
                        // in initializer expressions for instance member variables.
                        if (isClassLike(location.parent) && !(location.flags & NodeFlags.Static)) {
                            const ctor = findConstructorDeclaration(<ClassLikeDeclaration>location.parent);
                            if (ctor && ctor.locals) {
                                if (getSymbol(ctor.locals, name, meaning & SymbolFlags.Value)) {
                                    // Remember the property node, it will be used later to report appropriate error
                                    propertyWithInvalidInitializer = location;
                                }
                            }
                        }
                        break;
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                    case SyntaxKind.InterfaceDeclaration:
                        if (result = getSymbol(getSymbolOfNode(location).members, name, meaning & SymbolFlags.Type)) {
                            if (lastLocation && lastLocation.flags & NodeFlags.Static) {
                                // TypeScript 1.0 spec (April 2014): 3.4.1
                                // The scope of a type parameter extends over the entire declaration with which the type
                                // parameter list is associated, with the exception of static member declarations in classes.
                                error(errorLocation, Diagnostics.Static_members_cannot_reference_class_type_parameters);
                                return undefined;
                            }
                            break loop;
                        }
                        if (location.kind === SyntaxKind.ClassExpression && meaning & SymbolFlags.Class) {
                            const className = (<ClassExpression>location).name;
                            if (className && name === className.text) {
                                result = location.symbol;
                                break loop;
                            }
                        }
                        break;

                    // It is not legal to reference a class's own type parameters from a computed property name that
                    // belongs to the class. For example:
                    //
                    //   function foo<T>() { return '' }
                    //   class C<T> { // <-- Class's own type parameter T
                    //       [foo<T>()]() { } // <-- Reference to T from class's own computed property
                    //   }
                    //
                    case SyntaxKind.ComputedPropertyName:
                        grandparent = location.parent.parent;
                        if (isClassLike(grandparent) || grandparent.kind === SyntaxKind.InterfaceDeclaration) {
                            // A reference to this grandparent's type parameters would be an error
                            if (result = getSymbol(getSymbolOfNode(grandparent).members, name, meaning & SymbolFlags.Type)) {
                                error(errorLocation, Diagnostics.A_computed_property_name_cannot_reference_a_type_parameter_from_its_containing_type);
                                return undefined;
                            }
                        }
                        break;
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.Constructor:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.ArrowFunction:
                        if (meaning & SymbolFlags.Variable && name === "arguments") {
                            result = argumentsSymbol;
                            break loop;
                        }
                        break;
                    case SyntaxKind.FunctionExpression:
                        if (meaning & SymbolFlags.Variable && name === "arguments") {
                            result = argumentsSymbol;
                            break loop;
                        }

                        if (meaning & SymbolFlags.Function) {
                            const functionName = (<FunctionExpression>location).name;
                            if (functionName && name === functionName.text) {
                                result = location.symbol;
                                break loop;
                            }
                        }
                        break;
                    case SyntaxKind.Decorator:
                        // Decorators are resolved at the class declaration. Resolving at the parameter
                        // or member would result in looking up locals in the method.
                        //
                        //   function y() {}
                        //   class C {
                        //       method(@y x, y) {} // <-- decorator y should be resolved at the class declaration, not the parameter.
                        //   }
                        //
                        if (location.parent && location.parent.kind === SyntaxKind.Parameter) {
                            location = location.parent;
                        }
                        //
                        //   function y() {}
                        //   class C {
                        //       @y method(x, y) {} // <-- decorator y should be resolved at the class declaration, not the method.
                        //   }
                        //
                        if (location.parent && isClassElement(location.parent)) {
                            location = location.parent;
                        }
                        break;
                }
                lastLocation = location;
                location = location.parent;
            }

            if (result && nameNotFoundMessage && noUnusedIdentifiers) {
                result.isReferenced = true;
            }

            if (!result) {
                result = getSymbol(globals, name, meaning);
            }

            if (!result) {
                if (nameNotFoundMessage) {
                    if (!errorLocation ||
                        !checkAndReportErrorForMissingPrefix(errorLocation, name, nameArg) &&
                        !checkAndReportErrorForExtendingInterface(errorLocation)) {
                        error(errorLocation, nameNotFoundMessage, typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg));
                    }
                }
                return undefined;
            }

            // Perform extra checks only if error reporting was requested
            if (nameNotFoundMessage) {
                if (propertyWithInvalidInitializer) {
                    // We have a match, but the reference occurred within a property initializer and the identifier also binds
                    // to a local variable in the constructor where the code will be emitted.
                    const propertyName = (<PropertyDeclaration>propertyWithInvalidInitializer).name;
                    error(errorLocation, Diagnostics.Initializer_of_instance_member_variable_0_cannot_reference_identifier_1_declared_in_the_constructor,
                        declarationNameToString(propertyName), typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg));
                    return undefined;
                }

                // Only check for block-scoped variable if we are looking for the
                // name with variable meaning
                //      For example,
                //          declare module foo {
                //              interface bar {}
                //          }
                //      const foo/*1*/: foo/*2*/.bar;
                // The foo at /*1*/ and /*2*/ will share same symbol with two meaning
                // block - scope variable and namespace module. However, only when we
                // try to resolve name in /*1*/ which is used in variable position,
                // we want to check for block- scoped
                if (meaning & SymbolFlags.BlockScopedVariable) {
                    const exportOrLocalSymbol = getExportSymbolOfValueSymbolIfExported(result);
                    if (exportOrLocalSymbol.flags & SymbolFlags.BlockScopedVariable) {
                        checkResolvedBlockScopedVariable(exportOrLocalSymbol, errorLocation);
                    }
                }

                // If we're in an external module, we can't reference symbols created from UMD export declarations
                if (result && isInExternalModule) {
                    const decls = result.declarations;
                    if (decls && decls.length === 1 && decls[0].kind === SyntaxKind.NamespaceExportDeclaration) {
                        error(errorLocation, Diagnostics.Identifier_0_must_be_imported_from_a_module, name);
                    }
                }
            }
            return result;
        }

        function checkAndReportErrorForMissingPrefix(errorLocation: Node, name: string, nameArg: string | Identifier): boolean {
            if ((errorLocation.kind === SyntaxKind.Identifier && (isTypeReferenceIdentifier(<Identifier>errorLocation)) || isInTypeQuery(errorLocation))) {
                return false;
            }

            const container = getThisContainer(errorLocation, /* includeArrowFunctions */ true);
            let location = container;
            while (location) {
                if (isClassLike(location.parent)) {
                    const classSymbol = getSymbolOfNode(location.parent);
                    if (!classSymbol) {
                        break;
                    }

                    // Check to see if a static member exists.
                    const constructorType = getTypeOfSymbol(classSymbol);
                    if (getPropertyOfType(constructorType, name)) {
                        error(errorLocation, Diagnostics.Cannot_find_name_0_Did_you_mean_the_static_member_1_0, typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg), symbolToString(classSymbol));
                        return true;
                    }

                    // No static member is present.
                    // Check if we're in an instance method and look for a relevant instance member.
                    if (location === container && !(location.flags & NodeFlags.Static)) {
                        const instanceType = (<InterfaceType>getDeclaredTypeOfSymbol(classSymbol)).thisType;
                        if (getPropertyOfType(instanceType, name)) {
                            error(errorLocation, Diagnostics.Cannot_find_name_0_Did_you_mean_the_instance_member_this_0, typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg));
                            return true;
                        }
                    }
                }

                location = location.parent;
            }
            return false;
        }


        function checkAndReportErrorForExtendingInterface(errorLocation: Node): boolean {
            const expression = getEntityNameForExtendingInterface(errorLocation);
            const isError = !!(expression && resolveEntityName(expression, SymbolFlags.Interface, /*ignoreErrors*/ true));
            if (isError) {
                error(errorLocation, Diagnostics.Cannot_extend_an_interface_0_Did_you_mean_implements, getTextOfNode(expression));
            }
            return isError;
        }
        /**
         * Climbs up parents to an ExpressionWithTypeArguments, and returns its expression,
         * but returns undefined if that expression is not an EntityNameExpression.
         */
        function getEntityNameForExtendingInterface(node: Node): EntityNameExpression | undefined {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.PropertyAccessExpression:
                    return node.parent ? getEntityNameForExtendingInterface(node.parent) : undefined;
                case SyntaxKind.ExpressionWithTypeArguments:
                    Debug.assert(isEntityNameExpression((<ExpressionWithTypeArguments>node).expression));
                    return <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression;
                default:
                    return undefined;
            }
        }


        function checkResolvedBlockScopedVariable(result: Symbol, errorLocation: Node): void {
            Debug.assert((result.flags & SymbolFlags.BlockScopedVariable) !== 0);
            // Block-scoped variables cannot be used before their definition
            const declaration = forEach(result.declarations, d => isBlockOrCatchScoped(d) ? d : undefined);

            Debug.assert(declaration !== undefined, "Block-scoped variable declaration is undefined");

            if (!isInAmbientContext(declaration) && !isBlockScopedNameDeclaredBeforeUse(<Declaration>getAncestor(declaration, SyntaxKind.VariableDeclaration), errorLocation)) {
                error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, declarationNameToString(declaration.name));
            }
        }

        /* Starting from 'initial' node walk up the parent chain until 'stopAt' node is reached.
         * If at any point current node is equal to 'parent' node - return true.
         * Return false if 'stopAt' node is reached or isFunctionLike(current) === true.
         */
        function isSameScopeDescendentOf(initial: Node, parent: Node, stopAt: Node): boolean {
            if (!parent) {
                return false;
            }
            for (let current = initial; current && current !== stopAt && !isFunctionLike(current); current = current.parent) {
                if (current === parent) {
                    return true;
                }
            }
            return false;
        }

        function getAnyImportSyntax(node: Node): AnyImportSyntax {
            if (isAliasSymbolDeclaration(node)) {
                if (node.kind === SyntaxKind.ImportEqualsDeclaration) {
                    return <ImportEqualsDeclaration>node;
                }

                while (node && node.kind !== SyntaxKind.ImportDeclaration) {
                    node = node.parent;
                }
                return <ImportDeclaration>node;
            }
        }

        function getDeclarationOfAliasSymbol(symbol: Symbol): Declaration {
            return find(symbol.declarations, d => isAliasSymbolDeclaration(d) ? d : undefined);
        }

        function getTargetOfImportEqualsDeclaration(node: ImportEqualsDeclaration): Symbol {
            if (node.moduleReference.kind === SyntaxKind.ExternalModuleReference) {
                return resolveExternalModuleSymbol(resolveExternalModuleName(node, getExternalModuleImportEqualsDeclarationExpression(node)));
            }
            return getSymbolOfPartOfRightHandSideOfImportEquals(<EntityName>node.moduleReference, node);
        }

        function getTargetOfImportClause(node: ImportClause): Symbol {
            const moduleSymbol = resolveExternalModuleName(node, (<ImportDeclaration>node.parent).moduleSpecifier);

            if (moduleSymbol) {
                const exportDefaultSymbol = isShorthandAmbientModule(moduleSymbol.valueDeclaration) ?
                    moduleSymbol :
                    moduleSymbol.exports["export="] ?
                        getPropertyOfType(getTypeOfSymbol(moduleSymbol.exports["export="]), "default") :
                        resolveSymbol(moduleSymbol.exports["default"]);

                if (!exportDefaultSymbol && !allowSyntheticDefaultImports) {
                    error(node.name, Diagnostics.Module_0_has_no_default_export, symbolToString(moduleSymbol));
                }
                else if (!exportDefaultSymbol && allowSyntheticDefaultImports) {
                    return resolveExternalModuleSymbol(moduleSymbol) || resolveSymbol(moduleSymbol);
                }
                return exportDefaultSymbol;
            }
        }

        function getTargetOfNamespaceImport(node: NamespaceImport): Symbol {
            const moduleSpecifier = (<ImportDeclaration>node.parent.parent).moduleSpecifier;
            return resolveESModuleSymbol(resolveExternalModuleName(node, moduleSpecifier), moduleSpecifier);
        }

        // This function creates a synthetic symbol that combines the value side of one symbol with the
        // type/namespace side of another symbol. Consider this example:
        //
        //   declare module graphics {
        //       interface Point {
        //           x: number;
        //           y: number;
        //       }
        //   }
        //   declare var graphics: {
        //       Point: new (x: number, y: number) => graphics.Point;
        //   }
        //   declare module "graphics" {
        //       export = graphics;
        //   }
        //
        // An 'import { Point } from "graphics"' needs to create a symbol that combines the value side 'Point'
        // property with the type/namespace side interface 'Point'.
        function combineValueAndTypeSymbols(valueSymbol: Symbol, typeSymbol: Symbol): Symbol {
            if (valueSymbol.flags & (SymbolFlags.Type | SymbolFlags.Namespace)) {
                return valueSymbol;
            }
            const result = createSymbol(valueSymbol.flags | typeSymbol.flags, valueSymbol.name);
            result.declarations = concatenate(valueSymbol.declarations, typeSymbol.declarations);
            result.parent = valueSymbol.parent || typeSymbol.parent;
            if (valueSymbol.valueDeclaration) result.valueDeclaration = valueSymbol.valueDeclaration;
            if (typeSymbol.members) result.members = typeSymbol.members;
            if (valueSymbol.exports) result.exports = valueSymbol.exports;
            return result;
        }

        function getExportOfModule(symbol: Symbol, name: string): Symbol {
            if (symbol.flags & SymbolFlags.Module) {
                const exportedSymbol = getExportsOfSymbol(symbol)[name];
                if (exportedSymbol) {
                    return resolveSymbol(exportedSymbol);
                }
            }
        }

        function getPropertyOfVariable(symbol: Symbol, name: string): Symbol {
            if (symbol.flags & SymbolFlags.Variable) {
                const typeAnnotation = (<VariableDeclaration>symbol.valueDeclaration).type;
                if (typeAnnotation) {
                    return resolveSymbol(getPropertyOfType(getTypeFromTypeNode(typeAnnotation), name));
                }
            }
        }

        function getExternalModuleMember(node: ImportDeclaration | ExportDeclaration, specifier: ImportOrExportSpecifier): Symbol {
            const moduleSymbol = resolveExternalModuleName(node, node.moduleSpecifier);
            const targetSymbol = resolveESModuleSymbol(moduleSymbol, node.moduleSpecifier);
            if (targetSymbol) {
                const name = specifier.propertyName || specifier.name;
                if (name.text) {
                    if (isShorthandAmbientModule(moduleSymbol.valueDeclaration)) {
                        return moduleSymbol;
                    }

                    let symbolFromVariable: Symbol;
                    // First check if module was specified with "export=". If so, get the member from the resolved type
                    if (moduleSymbol && moduleSymbol.exports && moduleSymbol.exports["export="]) {
                        symbolFromVariable = getPropertyOfType(getTypeOfSymbol(targetSymbol), name.text);
                    }
                    else {
                        symbolFromVariable = getPropertyOfVariable(targetSymbol, name.text);
                    }
                    // If the export member we're looking for is default, and there is no real default but allowSyntheticDefaultImports is on, return the entire module as the default
                    if (!symbolFromVariable && allowSyntheticDefaultImports && name.text === "default") {
                        symbolFromVariable = resolveExternalModuleSymbol(moduleSymbol) || resolveSymbol(moduleSymbol);
                    }
                    // if symbolFromVariable is export - get its final target
                    symbolFromVariable = resolveSymbol(symbolFromVariable);
                    const symbolFromModule = getExportOfModule(targetSymbol, name.text);
                    const symbol = symbolFromModule && symbolFromVariable ?
                        combineValueAndTypeSymbols(symbolFromVariable, symbolFromModule) :
                        symbolFromModule || symbolFromVariable;
                    if (!symbol) {
                        error(name, Diagnostics.Module_0_has_no_exported_member_1, getFullyQualifiedName(moduleSymbol), declarationNameToString(name));
                    }
                    return symbol;
                }
            }
        }

        function getTargetOfImportSpecifier(node: ImportSpecifier): Symbol {
            return getExternalModuleMember(<ImportDeclaration>node.parent.parent.parent, node);
        }

        function getTargetOfNamespaceExportDeclaration(node: NamespaceExportDeclaration): Symbol {
            return resolveExternalModuleSymbol(node.parent.symbol);
        }

        function getTargetOfExportSpecifier(node: ExportSpecifier): Symbol {
            return (<ExportDeclaration>node.parent.parent).moduleSpecifier ?
                getExternalModuleMember(<ExportDeclaration>node.parent.parent, node) :
                resolveEntityName(node.propertyName || node.name, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace);
        }

        function getTargetOfExportAssignment(node: ExportAssignment): Symbol {
            return resolveEntityName(<EntityNameExpression>node.expression, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace);
        }

        function getTargetOfAliasDeclaration(node: Declaration): Symbol {
            switch (node.kind) {
                case SyntaxKind.ImportEqualsDeclaration:
                    return getTargetOfImportEqualsDeclaration(<ImportEqualsDeclaration>node);
                case SyntaxKind.ImportClause:
                    return getTargetOfImportClause(<ImportClause>node);
                case SyntaxKind.NamespaceImport:
                    return getTargetOfNamespaceImport(<NamespaceImport>node);
                case SyntaxKind.ImportSpecifier:
                    return getTargetOfImportSpecifier(<ImportSpecifier>node);
                case SyntaxKind.ExportSpecifier:
                    return getTargetOfExportSpecifier(<ExportSpecifier>node);
                case SyntaxKind.ExportAssignment:
                    return getTargetOfExportAssignment(<ExportAssignment>node);
                case SyntaxKind.NamespaceExportDeclaration:
                    return getTargetOfNamespaceExportDeclaration(<NamespaceExportDeclaration>node);
            }
        }

        function resolveSymbol(symbol: Symbol): Symbol {
            return symbol && symbol.flags & SymbolFlags.Alias && !(symbol.flags & (SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace)) ? resolveAlias(symbol) : symbol;
        }

        function resolveAlias(symbol: Symbol): Symbol {
            Debug.assert((symbol.flags & SymbolFlags.Alias) !== 0, "Should only get Alias here.");
            const links = getSymbolLinks(symbol);
            if (!links.target) {
                links.target = resolvingSymbol;
                const node = getDeclarationOfAliasSymbol(symbol);
                const target = getTargetOfAliasDeclaration(node);
                if (links.target === resolvingSymbol) {
                    links.target = target || unknownSymbol;
                }
                else {
                    error(node, Diagnostics.Circular_definition_of_import_alias_0, symbolToString(symbol));
                }
            }
            else if (links.target === resolvingSymbol) {
                links.target = unknownSymbol;
            }
            return links.target;
        }

        function markExportAsReferenced(node: ImportEqualsDeclaration | ExportAssignment | ExportSpecifier) {
            const symbol = getSymbolOfNode(node);
            const target = resolveAlias(symbol);
            if (target) {
                const markAlias = target === unknownSymbol ||
                    ((target.flags & SymbolFlags.Value) && !isConstEnumOrConstEnumOnlyModule(target));

                if (markAlias) {
                    markAliasSymbolAsReferenced(symbol);
                }
            }
        }

        // When an alias symbol is referenced, we need to mark the entity it references as referenced and in turn repeat that until
        // we reach a non-alias or an exported entity (which is always considered referenced). We do this by checking the target of
        // the alias as an expression (which recursively takes us back here if the target references another alias).
        function markAliasSymbolAsReferenced(symbol: Symbol) {
            const links = getSymbolLinks(symbol);
            if (!links.referenced) {
                links.referenced = true;
                const node = getDeclarationOfAliasSymbol(symbol);
                if (node.kind === SyntaxKind.ExportAssignment) {
                    // export default <symbol>
                    checkExpressionCached((<ExportAssignment>node).expression);
                }
                else if (node.kind === SyntaxKind.ExportSpecifier) {
                    // export { <symbol> } or export { <symbol> as foo }
                    checkExpressionCached((<ExportSpecifier>node).propertyName || (<ExportSpecifier>node).name);
                }
                else if (isInternalModuleImportEqualsDeclaration(node)) {
                    // import foo = <symbol>
                    checkExpressionCached(<Expression>(<ImportEqualsDeclaration>node).moduleReference);
                }
            }
        }

        // This function is only for imports with entity names
        function getSymbolOfPartOfRightHandSideOfImportEquals(entityName: EntityName, importDeclaration: ImportEqualsDeclaration, dontResolveAlias?: boolean): Symbol {
            // There are three things we might try to look for. In the following examples,
            // the search term is enclosed in |...|:
            //
            //     import a = |b|; // Namespace
            //     import a = |b.c|; // Value, type, namespace
            //     import a = |b.c|.d; // Namespace
            if (entityName.kind === SyntaxKind.Identifier && isRightSideOfQualifiedNameOrPropertyAccess(entityName)) {
                entityName = <QualifiedName>entityName.parent;
            }
            // Check for case 1 and 3 in the above example
            if (entityName.kind === SyntaxKind.Identifier || entityName.parent.kind === SyntaxKind.QualifiedName) {
                return resolveEntityName(entityName, SymbolFlags.Namespace, /*ignoreErrors*/ false, dontResolveAlias);
            }
            else {
                // Case 2 in above example
                // entityName.kind could be a QualifiedName or a Missing identifier
                Debug.assert(entityName.parent.kind === SyntaxKind.ImportEqualsDeclaration);
                return resolveEntityName(entityName, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace, /*ignoreErrors*/ false, dontResolveAlias);
            }
        }

        function getFullyQualifiedName(symbol: Symbol): string {
            return symbol.parent ? getFullyQualifiedName(symbol.parent) + "." + symbolToString(symbol) : symbolToString(symbol);
        }

        // Resolves a qualified name and any involved aliases
        function resolveEntityName(name: EntityNameOrEntityNameExpression, meaning: SymbolFlags, ignoreErrors?: boolean, dontResolveAlias?: boolean): Symbol | undefined {
            if (nodeIsMissing(name)) {
                return undefined;
            }

            let symbol: Symbol;
            if (name.kind === SyntaxKind.Identifier) {
                const message = meaning === SymbolFlags.Namespace ? Diagnostics.Cannot_find_namespace_0 : Diagnostics.Cannot_find_name_0;

                symbol = resolveName(name, (<Identifier>name).text, meaning, ignoreErrors ? undefined : message, <Identifier>name);
                if (!symbol) {
                    return undefined;
                }
            }
            else if (name.kind === SyntaxKind.QualifiedName || name.kind === SyntaxKind.PropertyAccessExpression) {
                const left = name.kind === SyntaxKind.QualifiedName ? (<QualifiedName>name).left : (<PropertyAccessEntityNameExpression>name).expression;
                const right = name.kind === SyntaxKind.QualifiedName ? (<QualifiedName>name).right : (<PropertyAccessExpression>name).name;

                const namespace = resolveEntityName(left, SymbolFlags.Namespace, ignoreErrors);
                if (!namespace || nodeIsMissing(right)) {
                    return undefined;
                }
                else if (namespace === unknownSymbol) {
                    return namespace;
                }

                symbol = getSymbol(getExportsOfSymbol(namespace), right.text, meaning);
                if (!symbol) {
                    if (!ignoreErrors) {
                        error(right, Diagnostics.Module_0_has_no_exported_member_1, getFullyQualifiedName(namespace), declarationNameToString(right));
                    }
                    return undefined;
                }
            }
            else {
                Debug.fail("Unknown entity name kind.");
            }
            Debug.assert((symbol.flags & SymbolFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
            return (symbol.flags & meaning) || dontResolveAlias ? symbol : resolveAlias(symbol);
        }

        function resolveExternalModuleName(location: Node, moduleReferenceExpression: Expression): Symbol {
            return resolveExternalModuleNameWorker(location, moduleReferenceExpression, Diagnostics.Cannot_find_module_0);
        }

        function resolveExternalModuleNameWorker(location: Node, moduleReferenceExpression: Expression, moduleNotFoundError: DiagnosticMessage): Symbol {
            if (moduleReferenceExpression.kind !== SyntaxKind.StringLiteral) {
                return;
            }

            const moduleReferenceLiteral = <LiteralExpression>moduleReferenceExpression;

            // Module names are escaped in our symbol table.  However, string literal values aren't.
            // Escape the name in the "require(...)" clause to ensure we find the right symbol.
            const moduleName = escapeIdentifier(moduleReferenceLiteral.text);

            if (moduleName === undefined) {
                return;
            }

            const isRelative = isExternalModuleNameRelative(moduleName);
            if (!isRelative) {
                const symbol = getSymbol(globals, '"' + moduleName + '"', SymbolFlags.ValueModule);
                if (symbol) {
                    // merged symbol is module declaration symbol combined with all augmentations
                    return getMergedSymbol(symbol);
                }
            }

            const resolvedModule = getResolvedModule(getSourceFileOfNode(location), moduleReferenceLiteral.text);
            const sourceFile = resolvedModule && host.getSourceFile(resolvedModule.resolvedFileName);
            if (sourceFile) {
                if (sourceFile.symbol) {
                    // merged symbol is module declaration symbol combined with all augmentations
                    return getMergedSymbol(sourceFile.symbol);
                }
                if (moduleNotFoundError) {
                    // report errors only if it was requested
                    error(moduleReferenceLiteral, Diagnostics.File_0_is_not_a_module, sourceFile.fileName);
                }
                return undefined;
            }

            if (patternAmbientModules) {
                const pattern = findBestPatternMatch(patternAmbientModules, _ => _.pattern, moduleName);
                if (pattern) {
                    return getMergedSymbol(pattern.symbol);
                }
            }

            if (moduleNotFoundError) {
                // report errors only if it was requested
                error(moduleReferenceLiteral, moduleNotFoundError, moduleName);
            }
            return undefined;
        }

        // An external module with an 'export =' declaration resolves to the target of the 'export =' declaration,
        // and an external module with no 'export =' declaration resolves to the module itself.
        function resolveExternalModuleSymbol(moduleSymbol: Symbol): Symbol {
            return moduleSymbol && getMergedSymbol(resolveSymbol(moduleSymbol.exports["export="])) || moduleSymbol;
        }

        // An external module with an 'export =' declaration may be referenced as an ES6 module provided the 'export ='
        // references a symbol that is at least declared as a module or a variable. The target of the 'export =' may
        // combine other declarations with the module or variable (e.g. a class/module, function/module, interface/variable).
        function resolveESModuleSymbol(moduleSymbol: Symbol, moduleReferenceExpression: Expression): Symbol {
            let symbol = resolveExternalModuleSymbol(moduleSymbol);
            if (symbol && !(symbol.flags & (SymbolFlags.Module | SymbolFlags.Variable))) {
                error(moduleReferenceExpression, Diagnostics.Module_0_resolves_to_a_non_module_entity_and_cannot_be_imported_using_this_construct, symbolToString(moduleSymbol));
                symbol = undefined;
            }
            return symbol;
        }

        function hasExportAssignmentSymbol(moduleSymbol: Symbol): boolean {
            return moduleSymbol.exports["export="] !== undefined;
        }

        function getExportsOfModuleAsArray(moduleSymbol: Symbol): Symbol[] {
            return symbolsToArray(getExportsOfModule(moduleSymbol));
        }

        function getExportsOfSymbol(symbol: Symbol): SymbolTable {
            return symbol.flags & SymbolFlags.Module ? getExportsOfModule(symbol) : symbol.exports || emptySymbols;
        }

        function getExportsOfModule(moduleSymbol: Symbol): SymbolTable {
            const links = getSymbolLinks(moduleSymbol);
            return links.resolvedExports || (links.resolvedExports = getExportsForModule(moduleSymbol));
        }

        interface ExportCollisionTracker {
            specifierText: string;
            exportsWithDuplicate: ExportDeclaration[];
        }

        /**
         * Extends one symbol table with another while collecting information on name collisions for error message generation into the `lookupTable` argument
         * Not passing `lookupTable` and `exportNode` disables this collection, and just extends the tables
         */
        function extendExportSymbols(target: SymbolTable, source: SymbolTable, lookupTable?: Map<ExportCollisionTracker>, exportNode?: ExportDeclaration) {
            for (const id in source) {
                if (id !== "default" && !target[id]) {
                    target[id] = source[id];
                    if (lookupTable && exportNode) {
                        lookupTable[id] = {
                            specifierText: getTextOfNode(exportNode.moduleSpecifier)
                        } as ExportCollisionTracker;
                    }
                }
                else if (lookupTable && exportNode && id !== "default" && target[id] && resolveSymbol(target[id]) !== resolveSymbol(source[id])) {
                    if (!lookupTable[id].exportsWithDuplicate) {
                        lookupTable[id].exportsWithDuplicate = [exportNode];
                    }
                    else {
                        lookupTable[id].exportsWithDuplicate.push(exportNode);
                    }
                }
            }
        }

        function getExportsForModule(moduleSymbol: Symbol): SymbolTable {
            const visitedSymbols: Symbol[] = [];
            return visit(moduleSymbol) || moduleSymbol.exports;

            // The ES6 spec permits export * declarations in a module to circularly reference the module itself. For example,
            // module 'a' can 'export * from "b"' and 'b' can 'export * from "a"' without error.
            function visit(symbol: Symbol): SymbolTable {
                if (!(symbol && symbol.flags & SymbolFlags.HasExports && !contains(visitedSymbols, symbol))) {
                    return;
                }
                visitedSymbols.push(symbol);
                const symbols = cloneMap(symbol.exports);
                // All export * declarations are collected in an __export symbol by the binder
                const exportStars = symbol.exports["__export"];
                if (exportStars) {
                    const nestedSymbols = createMap<Symbol>();
                    const lookupTable = createMap<ExportCollisionTracker>();
                    for (const node of exportStars.declarations) {
                        const resolvedModule = resolveExternalModuleName(node, (node as ExportDeclaration).moduleSpecifier);
                        const exportedSymbols = visit(resolvedModule);
                        extendExportSymbols(
                            nestedSymbols,
                            exportedSymbols,
                            lookupTable,
                            node as ExportDeclaration
                        );
                    }
                    for (const id in lookupTable) {
                        const { exportsWithDuplicate } = lookupTable[id];
                        // It's not an error if the file with multiple `export *`s with duplicate names exports a member with that name itself
                        if (id === "export=" || !(exportsWithDuplicate && exportsWithDuplicate.length) || symbols[id]) {
                            continue;
                        }
                        for (const node of exportsWithDuplicate) {
                            diagnostics.add(createDiagnosticForNode(
                                node,
                                Diagnostics.Module_0_has_already_exported_a_member_named_1_Consider_explicitly_re_exporting_to_resolve_the_ambiguity,
                                lookupTable[id].specifierText,
                                id
                            ));
                        }
                    }
                    extendExportSymbols(symbols, nestedSymbols);
                }
                return symbols;
            }
        }

        function getMergedSymbol(symbol: Symbol): Symbol {
            let merged: Symbol;
            return symbol && symbol.mergeId && (merged = mergedSymbols[symbol.mergeId]) ? merged : symbol;
        }

        function getSymbolOfNode(node: Node): Symbol {
            return getMergedSymbol(node.symbol);
        }

        function getParentOfSymbol(symbol: Symbol): Symbol {
            return getMergedSymbol(symbol.parent);
        }

        function getExportSymbolOfValueSymbolIfExported(symbol: Symbol): Symbol {
            return symbol && (symbol.flags & SymbolFlags.ExportValue) !== 0
                ? getMergedSymbol(symbol.exportSymbol)
                : symbol;
        }

        function symbolIsValue(symbol: Symbol): boolean {
            // If it is an instantiated symbol, then it is a value if the symbol it is an
            // instantiation of is a value.
            if (symbol.flags & SymbolFlags.Instantiated) {
                return symbolIsValue(getSymbolLinks(symbol).target);
            }

            // If the symbol has the value flag, it is trivially a value.
            if (symbol.flags & SymbolFlags.Value) {
                return true;
            }

            // If it is an alias, then it is a value if the symbol it resolves to is a value.
            if (symbol.flags & SymbolFlags.Alias) {
                return (resolveAlias(symbol).flags & SymbolFlags.Value) !== 0;
            }

            return false;
        }

        function findConstructorDeclaration(node: ClassLikeDeclaration): ConstructorDeclaration {
            const members = node.members;
            for (const member of members) {
                if (member.kind === SyntaxKind.Constructor && nodeIsPresent((<ConstructorDeclaration>member).body)) {
                    return <ConstructorDeclaration>member;
                }
            }
        }

        function createType(flags: TypeFlags): Type {
            const result = new Type(checker, flags);
            typeCount++;
            result.id = typeCount;
            return result;
        }

        function createIntrinsicType(kind: TypeFlags, intrinsicName: string): IntrinsicType {
            const type = <IntrinsicType>createType(kind);
            type.intrinsicName = intrinsicName;
            return type;
        }

        function createBooleanType(trueFalseTypes: Type[]): IntrinsicType & UnionType {
            const type = <IntrinsicType & UnionType>getUnionType(trueFalseTypes);
            type.flags |= TypeFlags.Boolean;
            type.intrinsicName = "boolean";
            return type;
        }

        function createObjectType(kind: TypeFlags, symbol?: Symbol): ObjectType {
            const type = <ObjectType>createType(kind);
            type.symbol = symbol;
            return type;
        }

        // A reserved member name starts with two underscores, but the third character cannot be an underscore
        // or the @ symbol. A third underscore indicates an escaped form of an identifer that started
        // with at least two underscores. The @ character indicates that the name is denoted by a well known ES
        // Symbol instance.
        function isReservedMemberName(name: string) {
            return name.charCodeAt(0) === CharacterCodes._ &&
                name.charCodeAt(1) === CharacterCodes._ &&
                name.charCodeAt(2) !== CharacterCodes._ &&
                name.charCodeAt(2) !== CharacterCodes.at;
        }

        function getNamedMembers(members: SymbolTable): Symbol[] {
            let result: Symbol[];
            for (const id in members) {
                if (!isReservedMemberName(id)) {
                    if (!result) result = [];
                    const symbol = members[id];
                    if (symbolIsValue(symbol)) {
                        result.push(symbol);
                    }
                }
            }
            return result || emptyArray;
        }

        function setObjectTypeMembers(type: ObjectType, members: SymbolTable, callSignatures: Signature[], constructSignatures: Signature[], stringIndexInfo: IndexInfo, numberIndexInfo: IndexInfo): ResolvedType {
            (<ResolvedType>type).members = members;
            (<ResolvedType>type).properties = getNamedMembers(members);
            (<ResolvedType>type).callSignatures = callSignatures;
            (<ResolvedType>type).constructSignatures = constructSignatures;
            if (stringIndexInfo) (<ResolvedType>type).stringIndexInfo = stringIndexInfo;
            if (numberIndexInfo) (<ResolvedType>type).numberIndexInfo = numberIndexInfo;
            return <ResolvedType>type;
        }

        function createAnonymousType(symbol: Symbol, members: SymbolTable, callSignatures: Signature[], constructSignatures: Signature[], stringIndexInfo: IndexInfo, numberIndexInfo: IndexInfo): ResolvedType {
            return setObjectTypeMembers(createObjectType(TypeFlags.Anonymous, symbol),
                members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function forEachSymbolTableInScope<T>(enclosingDeclaration: Node, callback: (symbolTable: SymbolTable) => T): T {
            let result: T;
            for (let location = enclosingDeclaration; location; location = location.parent) {
                // Locals of a source file are not in scope (because they get merged into the global symbol table)
                if (location.locals && !isGlobalSourceFile(location)) {
                    if (result = callback(location.locals)) {
                        return result;
                    }
                }
                switch (location.kind) {
                    case SyntaxKind.SourceFile:
                        if (!isExternalOrCommonJsModule(<SourceFile>location)) {
                            break;
                        }
                    case SyntaxKind.ModuleDeclaration:
                        if (result = callback(getSymbolOfNode(location).exports)) {
                            return result;
                        }
                        break;
                }
            }

            return callback(globals);
        }

        function getQualifiedLeftMeaning(rightMeaning: SymbolFlags) {
            // If we are looking in value space, the parent meaning is value, other wise it is namespace
            return rightMeaning === SymbolFlags.Value ? SymbolFlags.Value : SymbolFlags.Namespace;
        }

        function getAccessibleSymbolChain(symbol: Symbol, enclosingDeclaration: Node, meaning: SymbolFlags, useOnlyExternalAliasing: boolean): Symbol[] {
            function getAccessibleSymbolChainFromSymbolTable(symbols: SymbolTable): Symbol[] {
                function canQualifySymbol(symbolFromSymbolTable: Symbol, meaning: SymbolFlags) {
                    // If the symbol is equivalent and doesn't need further qualification, this symbol is accessible
                    if (!needsQualification(symbolFromSymbolTable, enclosingDeclaration, meaning)) {
                        return true;
                    }

                    // If symbol needs qualification, make sure that parent is accessible, if it is then this symbol is accessible too
                    const accessibleParent = getAccessibleSymbolChain(symbolFromSymbolTable.parent, enclosingDeclaration, getQualifiedLeftMeaning(meaning), useOnlyExternalAliasing);
                    return !!accessibleParent;
                }

                function isAccessible(symbolFromSymbolTable: Symbol, resolvedAliasSymbol?: Symbol) {
                    if (symbol === (resolvedAliasSymbol || symbolFromSymbolTable)) {
                        // if the symbolFromSymbolTable is not external module (it could be if it was determined as ambient external module and would be in globals table)
                        // and if symbolFromSymbolTable or alias resolution matches the symbol,
                        // check the symbol can be qualified, it is only then this symbol is accessible
                        return !forEach(symbolFromSymbolTable.declarations, hasExternalModuleSymbol) &&
                            canQualifySymbol(symbolFromSymbolTable, meaning);
                    }
                }

                // If symbol is directly available by its name in the symbol table
                if (isAccessible(symbols[symbol.name])) {
                    return [symbol];
                }

                // Check if symbol is any of the alias
                return forEachProperty(symbols, symbolFromSymbolTable => {
                    if (symbolFromSymbolTable.flags & SymbolFlags.Alias
                        && symbolFromSymbolTable.name !== "export="
                        && !getDeclarationOfKind(symbolFromSymbolTable, SyntaxKind.ExportSpecifier)) {
                        if (!useOnlyExternalAliasing || // We can use any type of alias to get the name
                            // Is this external alias, then use it to name
                            ts.forEach(symbolFromSymbolTable.declarations, isExternalModuleImportEqualsDeclaration)) {

                            const resolvedImportedSymbol = resolveAlias(symbolFromSymbolTable);
                            if (isAccessible(symbolFromSymbolTable, resolveAlias(symbolFromSymbolTable))) {
                                return [symbolFromSymbolTable];
                            }

                            // Look in the exported members, if we can find accessibleSymbolChain, symbol is accessible using this chain
                            // but only if the symbolFromSymbolTable can be qualified
                            const accessibleSymbolsFromExports = resolvedImportedSymbol.exports ? getAccessibleSymbolChainFromSymbolTable(resolvedImportedSymbol.exports) : undefined;
                            if (accessibleSymbolsFromExports && canQualifySymbol(symbolFromSymbolTable, getQualifiedLeftMeaning(meaning))) {
                                return [symbolFromSymbolTable].concat(accessibleSymbolsFromExports);
                            }
                        }
                    }
                });
            }

            if (symbol) {
                if (!(isPropertyOrMethodDeclarationSymbol(symbol))) {
                    return forEachSymbolTableInScope(enclosingDeclaration, getAccessibleSymbolChainFromSymbolTable);
                }
            }
        }

        function needsQualification(symbol: Symbol, enclosingDeclaration: Node, meaning: SymbolFlags) {
            let qualify = false;
            forEachSymbolTableInScope(enclosingDeclaration, symbolTable => {
                // If symbol of this name is not available in the symbol table we are ok
                let symbolFromSymbolTable = symbolTable[symbol.name];
                if (!symbolFromSymbolTable) {
                    // Continue to the next symbol table
                    return false;
                }
                // If the symbol with this name is present it should refer to the symbol
                if (symbolFromSymbolTable === symbol) {
                    // No need to qualify
                    return true;
                }

                // Qualify if the symbol from symbol table has same meaning as expected
                symbolFromSymbolTable = (symbolFromSymbolTable.flags & SymbolFlags.Alias && !getDeclarationOfKind(symbolFromSymbolTable, SyntaxKind.ExportSpecifier)) ? resolveAlias(symbolFromSymbolTable) : symbolFromSymbolTable;
                if (symbolFromSymbolTable.flags & meaning) {
                    qualify = true;
                    return true;
                }

                // Continue to the next symbol table
                return false;
            });

            return qualify;
        }

        function isPropertyOrMethodDeclarationSymbol(symbol: Symbol) {
            if (symbol.declarations && symbol.declarations.length) {
                for (const declaration of symbol.declarations) {
                    switch (declaration.kind) {
                        case SyntaxKind.PropertyDeclaration:
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                            continue;
                        default:
                            return false;
                    }
                }
                return true;
            }
            return false;
        }

        function isSymbolAccessible(symbol: Symbol, enclosingDeclaration: Node, meaning: SymbolFlags): SymbolAccessibilityResult {
            if (symbol && enclosingDeclaration && !(symbol.flags & SymbolFlags.TypeParameter)) {
                const initialSymbol = symbol;
                let meaningToLook = meaning;
                while (symbol) {
                    // Symbol is accessible if it by itself is accessible
                    const accessibleSymbolChain = getAccessibleSymbolChain(symbol, enclosingDeclaration, meaningToLook, /*useOnlyExternalAliasing*/ false);
                    if (accessibleSymbolChain) {
                        const hasAccessibleDeclarations = hasVisibleDeclarations(accessibleSymbolChain[0]);
                        if (!hasAccessibleDeclarations) {
                            return <SymbolAccessibilityResult>{
                                accessibility: SymbolAccessibility.NotAccessible,
                                errorSymbolName: symbolToString(initialSymbol, enclosingDeclaration, meaning),
                                errorModuleName: symbol !== initialSymbol ? symbolToString(symbol, enclosingDeclaration, SymbolFlags.Namespace) : undefined,
                            };
                        }
                        return hasAccessibleDeclarations;
                    }

                    // If we haven't got the accessible symbol, it doesn't mean the symbol is actually inaccessible.
                    // It could be a qualified symbol and hence verify the path
                    // e.g.:
                    // module m {
                    //     export class c {
                    //     }
                    // }
                    // const x: typeof m.c
                    // In the above example when we start with checking if typeof m.c symbol is accessible,
                    // we are going to see if c can be accessed in scope directly.
                    // But it can't, hence the accessible is going to be undefined, but that doesn't mean m.c is inaccessible
                    // It is accessible if the parent m is accessible because then m.c can be accessed through qualification
                    meaningToLook = getQualifiedLeftMeaning(meaning);
                    symbol = getParentOfSymbol(symbol);
                }

                // This could be a symbol that is not exported in the external module
                // or it could be a symbol from different external module that is not aliased and hence cannot be named
                const symbolExternalModule = forEach(initialSymbol.declarations, getExternalModuleContainer);
                if (symbolExternalModule) {
                    const enclosingExternalModule = getExternalModuleContainer(enclosingDeclaration);
                    if (symbolExternalModule !== enclosingExternalModule) {
                        // name from different external module that is not visible
                        return {
                            accessibility: SymbolAccessibility.CannotBeNamed,
                            errorSymbolName: symbolToString(initialSymbol, enclosingDeclaration, meaning),
                            errorModuleName: symbolToString(symbolExternalModule)
                        };
                    }
                }

                // Just a local name that is not accessible
                return {
                    accessibility: SymbolAccessibility.NotAccessible,
                    errorSymbolName: symbolToString(initialSymbol, enclosingDeclaration, meaning),
                };
            }

            return { accessibility: SymbolAccessibility.Accessible };

            function getExternalModuleContainer(declaration: Node) {
                for (; declaration; declaration = declaration.parent) {
                    if (hasExternalModuleSymbol(declaration)) {
                        return getSymbolOfNode(declaration);
                    }
                }
            }
        }

        function hasExternalModuleSymbol(declaration: Node) {
            return isAmbientModule(declaration) || (declaration.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>declaration));
        }

        function hasVisibleDeclarations(symbol: Symbol): SymbolVisibilityResult {
            let aliasesToMakeVisible: AnyImportSyntax[];
            if (forEach(symbol.declarations, declaration => !getIsDeclarationVisible(declaration))) {
                return undefined;
            }
            return { accessibility: SymbolAccessibility.Accessible, aliasesToMakeVisible };

            function getIsDeclarationVisible(declaration: Declaration) {
                if (!isDeclarationVisible(declaration)) {
                    // Mark the unexported alias as visible if its parent is visible
                    // because these kind of aliases can be used to name types in declaration file

                    const anyImportSyntax = getAnyImportSyntax(declaration);
                    if (anyImportSyntax &&
                        !(anyImportSyntax.flags & NodeFlags.Export) && // import clause without export
                        isDeclarationVisible(<Declaration>anyImportSyntax.parent)) {
                        getNodeLinks(declaration).isVisible = true;
                        if (aliasesToMakeVisible) {
                            if (!contains(aliasesToMakeVisible, anyImportSyntax)) {
                                aliasesToMakeVisible.push(anyImportSyntax);
                            }
                        }
                        else {
                            aliasesToMakeVisible = [anyImportSyntax];
                        }
                        return true;
                    }

                    // Declaration is not visible
                    return false;
                }

                return true;
            }
        }

        function isEntityNameVisible(entityName: EntityNameOrEntityNameExpression, enclosingDeclaration: Node): SymbolVisibilityResult {
            // get symbol of the first identifier of the entityName
            let meaning: SymbolFlags;
            if (entityName.parent.kind === SyntaxKind.TypeQuery || isExpressionWithTypeArgumentsInClassExtendsClause(entityName.parent)) {
                // Typeof value
                meaning = SymbolFlags.Value | SymbolFlags.ExportValue;
            }
            else if (entityName.kind === SyntaxKind.QualifiedName || entityName.kind === SyntaxKind.PropertyAccessExpression ||
                entityName.parent.kind === SyntaxKind.ImportEqualsDeclaration) {
                // Left identifier from type reference or TypeAlias
                // Entity name of the import declaration
                meaning = SymbolFlags.Namespace;
            }
            else {
                // Type Reference or TypeAlias entity = Identifier
                meaning = SymbolFlags.Type;
            }

            const firstIdentifier = getFirstIdentifier(entityName);
            const symbol = resolveName(enclosingDeclaration, (<Identifier>firstIdentifier).text, meaning, /*nodeNotFoundErrorMessage*/ undefined, /*nameArg*/ undefined);

            // Verify if the symbol is accessible
            return (symbol && hasVisibleDeclarations(symbol)) || <SymbolVisibilityResult>{
                accessibility: SymbolAccessibility.NotAccessible,
                errorSymbolName: getTextOfNode(firstIdentifier),
                errorNode: firstIdentifier
            };
        }

        function writeKeyword(writer: SymbolWriter, kind: SyntaxKind) {
            writer.writeKeyword(tokenToString(kind));
        }

        function writePunctuation(writer: SymbolWriter, kind: SyntaxKind) {
            writer.writePunctuation(tokenToString(kind));
        }

        function writeSpace(writer: SymbolWriter) {
            writer.writeSpace(" ");
        }

        function symbolToString(symbol: Symbol, enclosingDeclaration?: Node, meaning?: SymbolFlags): string {
            const writer = getSingleLineStringWriter();
            getSymbolDisplayBuilder().buildSymbolDisplay(symbol, writer, enclosingDeclaration, meaning);
            const result = writer.string();
            releaseStringWriter(writer);

            return result;
        }

        function signatureToString(signature: Signature, enclosingDeclaration?: Node, flags?: TypeFormatFlags, kind?: SignatureKind): string {
            const writer = getSingleLineStringWriter();
            getSymbolDisplayBuilder().buildSignatureDisplay(signature, writer, enclosingDeclaration, flags, kind);
            const result = writer.string();
            releaseStringWriter(writer);

            return result;
        }

        function typeToString(type: Type, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string {
            const writer = getSingleLineStringWriter();
            getSymbolDisplayBuilder().buildTypeDisplay(type, writer, enclosingDeclaration, flags);
            let result = writer.string();
            releaseStringWriter(writer);

            const maxLength = compilerOptions.noErrorTruncation || flags & TypeFormatFlags.NoTruncation ? undefined : 100;
            if (maxLength && result.length >= maxLength) {
                result = result.substr(0, maxLength - "...".length) + "...";
            }
            return result;
        }

        function typePredicateToString(typePredicate: TypePredicate, enclosingDeclaration?: Declaration, flags?: TypeFormatFlags): string {
            const writer = getSingleLineStringWriter();
            getSymbolDisplayBuilder().buildTypePredicateDisplay(typePredicate, writer, enclosingDeclaration, flags);
            const result = writer.string();
            releaseStringWriter(writer);

            return result;
        }

        function formatUnionTypes(types: Type[]): Type[] {
            const result: Type[] = [];
            let flags: TypeFlags = 0;
            for (let i = 0; i < types.length; i++) {
                const t = types[i];
                flags |= t.flags;
                if (!(t.flags & TypeFlags.Nullable)) {
                    if (t.flags & (TypeFlags.BooleanLiteral | TypeFlags.EnumLiteral)) {
                        const baseType = t.flags & TypeFlags.BooleanLiteral ? booleanType : (<EnumLiteralType>t).baseType;
                        const count = baseType.types.length;
                        if (i + count <= types.length && types[i + count - 1] === baseType.types[count - 1]) {
                            result.push(baseType);
                            i += count - 1;
                            continue;
                        }
                    }
                    result.push(t);
                }
            }
            if (flags & TypeFlags.Null) result.push(nullType);
            if (flags & TypeFlags.Undefined) result.push(undefinedType);
            return result || types;
        }

        function visibilityToString(flags: NodeFlags) {
            if (flags === NodeFlags.Private) {
                return "private";
            }
            if (flags === NodeFlags.Protected) {
                return "protected";
            }
            return "public";
        }

        function getTypeAliasForTypeLiteral(type: Type): Symbol {
            if (type.symbol && type.symbol.flags & SymbolFlags.TypeLiteral) {
                let node = type.symbol.declarations[0].parent;
                while (node.kind === SyntaxKind.ParenthesizedType) {
                    node = node.parent;
                }
                if (node.kind === SyntaxKind.TypeAliasDeclaration) {
                    return getSymbolOfNode(node);
                }
            }
            return undefined;
        }

        function isTopLevelInExternalModuleAugmentation(node: Node): boolean {
            return node && node.parent &&
                node.parent.kind === SyntaxKind.ModuleBlock &&
                isExternalModuleAugmentation(node.parent.parent);
        }

        function getSymbolDisplayBuilder(): SymbolDisplayBuilder {

            function getNameOfSymbol(symbol: Symbol): string {
                if (symbol.declarations && symbol.declarations.length) {
                    const declaration = symbol.declarations[0];
                    if (declaration.name) {
                        return declarationNameToString(declaration.name);
                    }
                    switch (declaration.kind) {
                        case SyntaxKind.ClassExpression:
                            return "(Anonymous class)";
                        case SyntaxKind.FunctionExpression:
                        case SyntaxKind.ArrowFunction:
                            return "(Anonymous function)";
                    }
                }
                return symbol.name;
            }

            /**
             * Writes only the name of the symbol out to the writer. Uses the original source text
             * for the name of the symbol if it is available to match how the user wrote the name.
             */
            function appendSymbolNameOnly(symbol: Symbol, writer: SymbolWriter): void {
                writer.writeSymbol(getNameOfSymbol(symbol), symbol);
            }

            /**
             * Writes a property access or element access with the name of the symbol out to the writer.
             * Uses the original source text for the name of the symbol if it is available to match how the user wrote the name,
             * ensuring that any names written with literals use element accesses.
             */
            function appendPropertyOrElementAccessForSymbol(symbol: Symbol, writer: SymbolWriter): void {
                const symbolName = getNameOfSymbol(symbol);
                const firstChar = symbolName.charCodeAt(0);
                const needsElementAccess = !isIdentifierStart(firstChar, languageVersion);

                if (needsElementAccess) {
                    writePunctuation(writer, SyntaxKind.OpenBracketToken);
                    if (isSingleOrDoubleQuote(firstChar)) {
                        writer.writeStringLiteral(symbolName);
                    }
                    else {
                        writer.writeSymbol(symbolName, symbol);
                    }
                    writePunctuation(writer, SyntaxKind.CloseBracketToken);
                }
                else {
                    writePunctuation(writer, SyntaxKind.DotToken);
                    writer.writeSymbol(symbolName, symbol);
                }
            }

            /**
             * Enclosing declaration is optional when we don't want to get qualified name in the enclosing declaration scope
             * Meaning needs to be specified if the enclosing declaration is given
             */
            function buildSymbolDisplay(symbol: Symbol, writer: SymbolWriter, enclosingDeclaration?: Node, meaning?: SymbolFlags, flags?: SymbolFormatFlags, typeFlags?: TypeFormatFlags): void {
                let parentSymbol: Symbol;
                function appendParentTypeArgumentsAndSymbolName(symbol: Symbol): void {
                    if (parentSymbol) {
                        // Write type arguments of instantiated class/interface here
                        if (flags & SymbolFormatFlags.WriteTypeParametersOrArguments) {
                            if (symbol.flags & SymbolFlags.Instantiated) {
                                buildDisplayForTypeArgumentsAndDelimiters(getTypeParametersOfClassOrInterface(parentSymbol),
                                    (<TransientSymbol>symbol).mapper, writer, enclosingDeclaration);
                            }
                            else {
                                buildTypeParameterDisplayFromSymbol(parentSymbol, writer, enclosingDeclaration);
                            }
                        }
                        appendPropertyOrElementAccessForSymbol(symbol, writer);
                    }
                    else {
                        appendSymbolNameOnly(symbol, writer);
                    }
                    parentSymbol = symbol;
                }

                // const the writer know we just wrote out a symbol.  The declaration emitter writer uses
                // this to determine if an import it has previously seen (and not written out) needs
                // to be written to the file once the walk of the tree is complete.
                //
                // NOTE(cyrusn): This approach feels somewhat unfortunate.  A simple pass over the tree
                // up front (for example, during checking) could determine if we need to emit the imports
                // and we could then access that data during declaration emit.
                writer.trackSymbol(symbol, enclosingDeclaration, meaning);
                function walkSymbol(symbol: Symbol, meaning: SymbolFlags): void {
                    if (symbol) {
                        const accessibleSymbolChain = getAccessibleSymbolChain(symbol, enclosingDeclaration, meaning, !!(flags & SymbolFormatFlags.UseOnlyExternalAliasing));

                        if (!accessibleSymbolChain ||
                            needsQualification(accessibleSymbolChain[0], enclosingDeclaration, accessibleSymbolChain.length === 1 ? meaning : getQualifiedLeftMeaning(meaning))) {

                            // Go up and add our parent.
                            walkSymbol(
                                getParentOfSymbol(accessibleSymbolChain ? accessibleSymbolChain[0] : symbol),
                                getQualifiedLeftMeaning(meaning));
                        }

                        if (accessibleSymbolChain) {
                            for (const accessibleSymbol of accessibleSymbolChain) {
                                appendParentTypeArgumentsAndSymbolName(accessibleSymbol);
                            }
                        }
                        else {
                            // If we didn't find accessible symbol chain for this symbol, break if this is external module
                            if (!parentSymbol && ts.forEach(symbol.declarations, hasExternalModuleSymbol)) {
                                return;
                            }

                            // if this is anonymous type break
                            if (symbol.flags & SymbolFlags.TypeLiteral || symbol.flags & SymbolFlags.ObjectLiteral) {
                                return;
                            }

                            appendParentTypeArgumentsAndSymbolName(symbol);
                        }
                    }
                }

                // Get qualified name if the symbol is not a type parameter
                // and there is an enclosing declaration or we specifically
                // asked for it
                const isTypeParameter = symbol.flags & SymbolFlags.TypeParameter;
                const typeFormatFlag = TypeFormatFlags.UseFullyQualifiedType & typeFlags;
                if (!isTypeParameter && (enclosingDeclaration || typeFormatFlag)) {
                    walkSymbol(symbol, meaning);
                    return;
                }

                return appendParentTypeArgumentsAndSymbolName(symbol);
            }

            function buildTypeDisplay(type: Type, writer: SymbolWriter, enclosingDeclaration?: Node, globalFlags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                const globalFlagsToPass = globalFlags & TypeFormatFlags.WriteOwnNameForAnyLike;
                let inObjectTypeLiteral = false;
                return writeType(type, globalFlags);

                function writeType(type: Type, flags: TypeFormatFlags) {
                    const nextFlags = flags & ~TypeFormatFlags.InTypeAlias;
                    // Write undefined/null type as any
                    if (type.flags & TypeFlags.Intrinsic) {
                        // Special handling for unknown / resolving types, they should show up as any and not unknown or __resolving
                        writer.writeKeyword(!(globalFlags & TypeFormatFlags.WriteOwnNameForAnyLike) && isTypeAny(type)
                            ? "any"
                            : (<IntrinsicType>type).intrinsicName);
                    }
                    else if (type.flags & TypeFlags.ThisType) {
                        if (inObjectTypeLiteral) {
                            writer.reportInaccessibleThisError();
                        }
                        writer.writeKeyword("this");
                    }
                    else if (type.flags & TypeFlags.Reference) {
                        writeTypeReference(<TypeReference>type, nextFlags);
                    }
                    else if (type.flags & TypeFlags.EnumLiteral) {
                        buildSymbolDisplay(getParentOfSymbol(type.symbol), writer, enclosingDeclaration, SymbolFlags.Type, SymbolFormatFlags.None, nextFlags);
                        writePunctuation(writer, SyntaxKind.DotToken);
                        appendSymbolNameOnly(type.symbol, writer);
                    }
                    else if (type.flags & (TypeFlags.Class | TypeFlags.Interface | TypeFlags.Enum | TypeFlags.TypeParameter)) {
                        // The specified symbol flags need to be reinterpreted as type flags
                        buildSymbolDisplay(type.symbol, writer, enclosingDeclaration, SymbolFlags.Type, SymbolFormatFlags.None, nextFlags);
                    }
                    else if (type.flags & TypeFlags.Tuple) {
                        writeTupleType(<TupleType>type);
                    }
                    else if (!(flags & TypeFormatFlags.InTypeAlias) && type.flags & (TypeFlags.Anonymous | TypeFlags.UnionOrIntersection) && type.aliasSymbol) {
                        const typeArguments = type.aliasTypeArguments;
                        writeSymbolTypeReference(type.aliasSymbol, typeArguments, 0, typeArguments ? typeArguments.length : 0, nextFlags);
                    }
                    else if (type.flags & TypeFlags.UnionOrIntersection) {
                        writeUnionOrIntersectionType(<UnionOrIntersectionType>type, nextFlags);
                    }
                    else if (type.flags & TypeFlags.Anonymous) {
                        writeAnonymousType(<ObjectType>type, nextFlags);
                    }
                    else if (type.flags & TypeFlags.StringLiteral) {
                        writer.writeStringLiteral(`"${escapeString((<LiteralType>type).text)}"`);
                    }
                    else if (type.flags & TypeFlags.NumberLiteral) {
                        writer.writeStringLiteral((<LiteralType>type).text);
                    }
                    else {
                        // Should never get here
                        // { ... }
                        writePunctuation(writer, SyntaxKind.OpenBraceToken);
                        writeSpace(writer);
                        writePunctuation(writer, SyntaxKind.DotDotDotToken);
                        writeSpace(writer);
                        writePunctuation(writer, SyntaxKind.CloseBraceToken);
                    }
                }

                function writeTypeList(types: Type[], delimiter: SyntaxKind) {
                    for (let i = 0; i < types.length; i++) {
                        if (i > 0) {
                            if (delimiter !== SyntaxKind.CommaToken) {
                                writeSpace(writer);
                            }
                            writePunctuation(writer, delimiter);
                            writeSpace(writer);
                        }
                        writeType(types[i], delimiter === SyntaxKind.CommaToken ? TypeFormatFlags.None : TypeFormatFlags.InElementType);
                    }
                }

                function writeSymbolTypeReference(symbol: Symbol, typeArguments: Type[], pos: number, end: number, flags: TypeFormatFlags) {
                    // Unnamed function expressions and arrow functions have reserved names that we don't want to display
                    if (symbol.flags & SymbolFlags.Class || !isReservedMemberName(symbol.name)) {
                        buildSymbolDisplay(symbol, writer, enclosingDeclaration, SymbolFlags.Type, SymbolFormatFlags.None, flags);
                    }
                    if (pos < end) {
                        writePunctuation(writer, SyntaxKind.LessThanToken);
                        writeType(typeArguments[pos], TypeFormatFlags.InFirstTypeArgument);
                        pos++;
                        while (pos < end) {
                            writePunctuation(writer, SyntaxKind.CommaToken);
                            writeSpace(writer);
                            writeType(typeArguments[pos], TypeFormatFlags.None);
                            pos++;
                        }
                        writePunctuation(writer, SyntaxKind.GreaterThanToken);
                    }
                }

                function writeTypeReference(type: TypeReference, flags: TypeFormatFlags) {
                    const typeArguments = type.typeArguments || emptyArray;
                    if (type.target === globalArrayType && !(flags & TypeFormatFlags.WriteArrayAsGenericType)) {
                        writeType(typeArguments[0], TypeFormatFlags.InElementType);
                        writePunctuation(writer, SyntaxKind.OpenBracketToken);
                        writePunctuation(writer, SyntaxKind.CloseBracketToken);
                    }
                    else {
                        // Write the type reference in the format f<A>.g<B>.C<X, Y> where A and B are type arguments
                        // for outer type parameters, and f and g are the respective declaring containers of those
                        // type parameters.
                        const outerTypeParameters = type.target.outerTypeParameters;
                        let i = 0;
                        if (outerTypeParameters) {
                            const length = outerTypeParameters.length;
                            while (i < length) {
                                // Find group of type arguments for type parameters with the same declaring container.
                                const start = i;
                                const parent = getParentSymbolOfTypeParameter(outerTypeParameters[i]);
                                do {
                                    i++;
                                } while (i < length && getParentSymbolOfTypeParameter(outerTypeParameters[i]) === parent);
                                // When type parameters are their own type arguments for the whole group (i.e. we have
                                // the default outer type arguments), we don't show the group.
                                if (!rangeEquals(outerTypeParameters, typeArguments, start, i)) {
                                    writeSymbolTypeReference(parent, typeArguments, start, i, flags);
                                    writePunctuation(writer, SyntaxKind.DotToken);
                                }
                            }
                        }
                        const typeParameterCount = (type.target.typeParameters || emptyArray).length;
                        writeSymbolTypeReference(type.symbol, typeArguments, i, typeParameterCount, flags);
                    }
                }

                function writeTupleType(type: TupleType) {
                    writePunctuation(writer, SyntaxKind.OpenBracketToken);
                    writeTypeList(type.elementTypes, SyntaxKind.CommaToken);
                    writePunctuation(writer, SyntaxKind.CloseBracketToken);
                }

                function writeUnionOrIntersectionType(type: UnionOrIntersectionType, flags: TypeFormatFlags) {
                    if (flags & TypeFormatFlags.InElementType) {
                        writePunctuation(writer, SyntaxKind.OpenParenToken);
                    }
                    if (type.flags & TypeFlags.Union) {
                        writeTypeList(formatUnionTypes(type.types), SyntaxKind.BarToken);
                    }
                    else {
                        writeTypeList(type.types, SyntaxKind.AmpersandToken);
                    }
                    if (flags & TypeFormatFlags.InElementType) {
                        writePunctuation(writer, SyntaxKind.CloseParenToken);
                    }
                }

                function writeAnonymousType(type: ObjectType, flags: TypeFormatFlags) {
                    const symbol = type.symbol;
                    if (symbol) {
                        // Always use 'typeof T' for type of class, enum, and module objects
                        if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Enum | SymbolFlags.ValueModule)) {
                            writeTypeOfSymbol(type, flags);
                        }
                        else if (shouldWriteTypeOfFunctionSymbol()) {
                            writeTypeOfSymbol(type, flags);
                        }
                        else if (contains(symbolStack, symbol)) {
                            // If type is an anonymous type literal in a type alias declaration, use type alias name
                            const typeAlias = getTypeAliasForTypeLiteral(type);
                            if (typeAlias) {
                                // The specified symbol flags need to be reinterpreted as type flags
                                buildSymbolDisplay(typeAlias, writer, enclosingDeclaration, SymbolFlags.Type, SymbolFormatFlags.None, flags);
                            }
                            else {
                                // Recursive usage, use any
                                writeKeyword(writer, SyntaxKind.AnyKeyword);
                            }
                        }
                        else {
                            // Since instantiations of the same anonymous type have the same symbol, tracking symbols instead
                            // of types allows us to catch circular references to instantiations of the same anonymous type
                            if (!symbolStack) {
                                symbolStack = [];
                            }
                            symbolStack.push(symbol);
                            writeLiteralType(type, flags);
                            symbolStack.pop();
                        }
                    }
                    else {
                        // Anonymous types with no symbol are never circular
                        writeLiteralType(type, flags);
                    }

                    function shouldWriteTypeOfFunctionSymbol() {
                        const isStaticMethodSymbol = !!(symbol.flags & SymbolFlags.Method &&  // typeof static method
                            forEach(symbol.declarations, declaration => declaration.flags & NodeFlags.Static));
                        const isNonLocalFunctionSymbol = !!(symbol.flags & SymbolFlags.Function) &&
                            (symbol.parent || // is exported function symbol
                                forEach(symbol.declarations, declaration =>
                                    declaration.parent.kind === SyntaxKind.SourceFile || declaration.parent.kind === SyntaxKind.ModuleBlock));
                        if (isStaticMethodSymbol || isNonLocalFunctionSymbol) {
                            // typeof is allowed only for static/non local functions
                            return !!(flags & TypeFormatFlags.UseTypeOfFunction) || // use typeof if format flags specify it
                                (contains(symbolStack, symbol)); // it is type of the symbol uses itself recursively
                        }
                    }
                }

                function writeTypeOfSymbol(type: ObjectType, typeFormatFlags?: TypeFormatFlags) {
                    writeKeyword(writer, SyntaxKind.TypeOfKeyword);
                    writeSpace(writer);
                    buildSymbolDisplay(type.symbol, writer, enclosingDeclaration, SymbolFlags.Value, SymbolFormatFlags.None, typeFormatFlags);
                }

                function writeIndexSignature(info: IndexInfo, keyword: SyntaxKind) {
                    if (info) {
                        if (info.isReadonly) {
                            writeKeyword(writer, SyntaxKind.ReadonlyKeyword);
                            writeSpace(writer);
                        }
                        writePunctuation(writer, SyntaxKind.OpenBracketToken);
                        writer.writeParameter(info.declaration ? declarationNameToString(info.declaration.parameters[0].name) : "x");
                        writePunctuation(writer, SyntaxKind.ColonToken);
                        writeSpace(writer);
                        writeKeyword(writer, keyword);
                        writePunctuation(writer, SyntaxKind.CloseBracketToken);
                        writePunctuation(writer, SyntaxKind.ColonToken);
                        writeSpace(writer);
                        writeType(info.type, TypeFormatFlags.None);
                        writePunctuation(writer, SyntaxKind.SemicolonToken);
                        writer.writeLine();
                    }
                }

                function writePropertyWithModifiers(prop: Symbol) {
                    if (isReadonlySymbol(prop)) {
                        writeKeyword(writer, SyntaxKind.ReadonlyKeyword);
                        writeSpace(writer);
                    }
                    buildSymbolDisplay(prop, writer);
                    if (prop.flags & SymbolFlags.Optional) {
                        writePunctuation(writer, SyntaxKind.QuestionToken);
                    }
                }

                function shouldAddParenthesisAroundFunctionType(callSignature: Signature, flags: TypeFormatFlags) {
                    if (flags & TypeFormatFlags.InElementType) {
                        return true;
                    }
                    else if (flags & TypeFormatFlags.InFirstTypeArgument) {
                        // Add parenthesis around function type for the first type argument to avoid ambiguity
                        const typeParameters = callSignature.target && (flags & TypeFormatFlags.WriteTypeArgumentsOfSignature) ?
                            callSignature.target.typeParameters : callSignature.typeParameters;
                        return typeParameters && typeParameters.length !== 0;
                    }
                    return false;
                }

                function writeLiteralType(type: ObjectType, flags: TypeFormatFlags) {
                    const resolved = resolveStructuredTypeMembers(type);
                    if (!resolved.properties.length && !resolved.stringIndexInfo && !resolved.numberIndexInfo) {
                        if (!resolved.callSignatures.length && !resolved.constructSignatures.length) {
                            writePunctuation(writer, SyntaxKind.OpenBraceToken);
                            writePunctuation(writer, SyntaxKind.CloseBraceToken);
                            return;
                        }

                        if (resolved.callSignatures.length === 1 && !resolved.constructSignatures.length) {
                            const parenthesizeSignature = shouldAddParenthesisAroundFunctionType(resolved.callSignatures[0], flags);
                            if (parenthesizeSignature) {
                                writePunctuation(writer, SyntaxKind.OpenParenToken);
                            }
                            buildSignatureDisplay(resolved.callSignatures[0], writer, enclosingDeclaration, globalFlagsToPass | TypeFormatFlags.WriteArrowStyleSignature, /*kind*/ undefined, symbolStack);
                            if (parenthesizeSignature) {
                                writePunctuation(writer, SyntaxKind.CloseParenToken);
                            }
                            return;
                        }
                        if (resolved.constructSignatures.length === 1 && !resolved.callSignatures.length) {
                            if (flags & TypeFormatFlags.InElementType) {
                                writePunctuation(writer, SyntaxKind.OpenParenToken);
                            }
                            writeKeyword(writer, SyntaxKind.NewKeyword);
                            writeSpace(writer);
                            buildSignatureDisplay(resolved.constructSignatures[0], writer, enclosingDeclaration, globalFlagsToPass | TypeFormatFlags.WriteArrowStyleSignature, /*kind*/ undefined, symbolStack);
                            if (flags & TypeFormatFlags.InElementType) {
                                writePunctuation(writer, SyntaxKind.CloseParenToken);
                            }
                            return;
                        }
                    }

                    const saveInObjectTypeLiteral = inObjectTypeLiteral;
                    inObjectTypeLiteral = true;
                    writePunctuation(writer, SyntaxKind.OpenBraceToken);
                    writer.writeLine();
                    writer.increaseIndent();
                    for (const signature of resolved.callSignatures) {
                        buildSignatureDisplay(signature, writer, enclosingDeclaration, globalFlagsToPass, /*kind*/ undefined, symbolStack);
                        writePunctuation(writer, SyntaxKind.SemicolonToken);
                        writer.writeLine();
                    }
                    for (const signature of resolved.constructSignatures) {
                        buildSignatureDisplay(signature, writer, enclosingDeclaration, globalFlagsToPass, SignatureKind.Construct, symbolStack);
                        writePunctuation(writer, SyntaxKind.SemicolonToken);
                        writer.writeLine();
                    }
                    writeIndexSignature(resolved.stringIndexInfo, SyntaxKind.StringKeyword);
                    writeIndexSignature(resolved.numberIndexInfo, SyntaxKind.NumberKeyword);
                    for (const p of resolved.properties) {
                        const t = getTypeOfSymbol(p);
                        if (p.flags & (SymbolFlags.Function | SymbolFlags.Method) && !getPropertiesOfObjectType(t).length) {
                            const signatures = getSignaturesOfType(t, SignatureKind.Call);
                            for (const signature of signatures) {
                                writePropertyWithModifiers(p);
                                buildSignatureDisplay(signature, writer, enclosingDeclaration, globalFlagsToPass, /*kind*/ undefined, symbolStack);
                                writePunctuation(writer, SyntaxKind.SemicolonToken);
                                writer.writeLine();
                            }
                        }
                        else {
                            writePropertyWithModifiers(p);
                            writePunctuation(writer, SyntaxKind.ColonToken);
                            writeSpace(writer);
                            writeType(t, TypeFormatFlags.None);
                            writePunctuation(writer, SyntaxKind.SemicolonToken);
                            writer.writeLine();
                        }
                    }
                    writer.decreaseIndent();
                    writePunctuation(writer, SyntaxKind.CloseBraceToken);
                    inObjectTypeLiteral = saveInObjectTypeLiteral;
                }
            }

            function buildTypeParameterDisplayFromSymbol(symbol: Symbol, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags) {
                const targetSymbol = getTargetSymbol(symbol);
                if (targetSymbol.flags & SymbolFlags.Class || targetSymbol.flags & SymbolFlags.Interface || targetSymbol.flags & SymbolFlags.TypeAlias) {
                    buildDisplayForTypeParametersAndDelimiters(getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol), writer, enclosingDeclaration, flags);
                }
            }

            function buildTypeParameterDisplay(tp: TypeParameter, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                appendSymbolNameOnly(tp.symbol, writer);
                const constraint = getConstraintOfTypeParameter(tp);
                if (constraint) {
                    writeSpace(writer);
                    writeKeyword(writer, SyntaxKind.ExtendsKeyword);
                    writeSpace(writer);
                    buildTypeDisplay(constraint, writer, enclosingDeclaration, flags, symbolStack);
                }
            }

            function buildParameterDisplay(p: Symbol, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                const parameterNode = <ParameterDeclaration>p.valueDeclaration;
                if (isRestParameter(parameterNode)) {
                    writePunctuation(writer, SyntaxKind.DotDotDotToken);
                }
                if (isBindingPattern(parameterNode.name)) {
                    buildBindingPatternDisplay(<BindingPattern>parameterNode.name, writer, enclosingDeclaration, flags, symbolStack);
                }
                else {
                    appendSymbolNameOnly(p, writer);
                }
                if (isOptionalParameter(parameterNode)) {
                    writePunctuation(writer, SyntaxKind.QuestionToken);
                }
                writePunctuation(writer, SyntaxKind.ColonToken);
                writeSpace(writer);

                buildTypeDisplay(getTypeOfSymbol(p), writer, enclosingDeclaration, flags, symbolStack);
            }

            function buildBindingPatternDisplay(bindingPattern: BindingPattern, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                // We have to explicitly emit square bracket and bracket because these tokens are not stored inside the node.
                if (bindingPattern.kind === SyntaxKind.ObjectBindingPattern) {
                    writePunctuation(writer, SyntaxKind.OpenBraceToken);
                    buildDisplayForCommaSeparatedList(bindingPattern.elements, writer, e => buildBindingElementDisplay(e, writer, enclosingDeclaration, flags, symbolStack));
                    writePunctuation(writer, SyntaxKind.CloseBraceToken);
                }
                else if (bindingPattern.kind === SyntaxKind.ArrayBindingPattern) {
                    writePunctuation(writer, SyntaxKind.OpenBracketToken);
                    const elements = bindingPattern.elements;
                    buildDisplayForCommaSeparatedList(elements, writer, e => buildBindingElementDisplay(e, writer, enclosingDeclaration, flags, symbolStack));
                    if (elements && elements.hasTrailingComma) {
                        writePunctuation(writer, SyntaxKind.CommaToken);
                    }
                    writePunctuation(writer, SyntaxKind.CloseBracketToken);
                }
            }

            function buildBindingElementDisplay(bindingElement: BindingElement, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                if (bindingElement.kind === SyntaxKind.OmittedExpression) {
                    return;
                }
                Debug.assert(bindingElement.kind === SyntaxKind.BindingElement);
                if (bindingElement.propertyName) {
                    writer.writeSymbol(getTextOfNode(bindingElement.propertyName), bindingElement.symbol);
                    writePunctuation(writer, SyntaxKind.ColonToken);
                    writeSpace(writer);
                }
                if (isBindingPattern(bindingElement.name)) {
                    buildBindingPatternDisplay(<BindingPattern>bindingElement.name, writer, enclosingDeclaration, flags, symbolStack);
                }
                else {
                    if (bindingElement.dotDotDotToken) {
                        writePunctuation(writer, SyntaxKind.DotDotDotToken);
                    }
                    appendSymbolNameOnly(bindingElement.symbol, writer);
                }
            }

            function buildDisplayForTypeParametersAndDelimiters(typeParameters: TypeParameter[], writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                if (typeParameters && typeParameters.length) {
                    writePunctuation(writer, SyntaxKind.LessThanToken);
                    buildDisplayForCommaSeparatedList(typeParameters, writer, p => buildTypeParameterDisplay(p, writer, enclosingDeclaration, flags, symbolStack));
                    writePunctuation(writer, SyntaxKind.GreaterThanToken);
                }
            }

            function buildDisplayForCommaSeparatedList<T>(list: T[], writer: SymbolWriter, action: (item: T) => void) {
                for (let i = 0; i < list.length; i++) {
                    if (i > 0) {
                        writePunctuation(writer, SyntaxKind.CommaToken);
                        writeSpace(writer);
                    }
                    action(list[i]);
                }
            }

            function buildDisplayForTypeArgumentsAndDelimiters(typeParameters: TypeParameter[], mapper: TypeMapper, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                if (typeParameters && typeParameters.length) {
                    writePunctuation(writer, SyntaxKind.LessThanToken);
                    let flags = TypeFormatFlags.InFirstTypeArgument;
                    for (let i = 0; i < typeParameters.length; i++) {
                        if (i > 0) {
                            writePunctuation(writer, SyntaxKind.CommaToken);
                            writeSpace(writer);
                            flags = TypeFormatFlags.None;
                        }
                        buildTypeDisplay(mapper(typeParameters[i]), writer, enclosingDeclaration, flags);
                    }
                    writePunctuation(writer, SyntaxKind.GreaterThanToken);
                }
            }

            function buildDisplayForParametersAndDelimiters(thisParameter: Symbol | undefined, parameters: Symbol[], writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                writePunctuation(writer, SyntaxKind.OpenParenToken);
                if (thisParameter) {
                    buildParameterDisplay(thisParameter, writer, enclosingDeclaration, flags, symbolStack);
                }
                for (let i = 0; i < parameters.length; i++) {
                    if (i > 0 || thisParameter) {
                        writePunctuation(writer, SyntaxKind.CommaToken);
                        writeSpace(writer);
                    }
                    buildParameterDisplay(parameters[i], writer, enclosingDeclaration, flags, symbolStack);
                }
                writePunctuation(writer, SyntaxKind.CloseParenToken);
            }

            function buildTypePredicateDisplay(predicate: TypePredicate, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]): void {
                if (isIdentifierTypePredicate(predicate)) {
                    writer.writeParameter(predicate.parameterName);
                }
                else {
                    writeKeyword(writer, SyntaxKind.ThisKeyword);
                }
                writeSpace(writer);
                writeKeyword(writer, SyntaxKind.IsKeyword);
                writeSpace(writer);
                buildTypeDisplay(predicate.type, writer, enclosingDeclaration, flags, symbolStack);
            }

            function buildReturnTypeDisplay(signature: Signature, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
                if (flags & TypeFormatFlags.WriteArrowStyleSignature) {
                    writeSpace(writer);
                    writePunctuation(writer, SyntaxKind.EqualsGreaterThanToken);
                }
                else {
                    writePunctuation(writer, SyntaxKind.ColonToken);
                }
                writeSpace(writer);

                if (signature.typePredicate) {
                    buildTypePredicateDisplay(signature.typePredicate, writer, enclosingDeclaration, flags, symbolStack);
                }
                else {
                    const returnType = getReturnTypeOfSignature(signature);
                    buildTypeDisplay(returnType, writer, enclosingDeclaration, flags, symbolStack);
                }
            }

            function buildSignatureDisplay(signature: Signature, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, kind?: SignatureKind, symbolStack?: Symbol[]) {
                if (kind === SignatureKind.Construct) {
                    writeKeyword(writer, SyntaxKind.NewKeyword);
                    writeSpace(writer);
                }

                if (signature.target && (flags & TypeFormatFlags.WriteTypeArgumentsOfSignature)) {
                    // Instantiated signature, write type arguments instead
                    // This is achieved by passing in the mapper separately
                    buildDisplayForTypeArgumentsAndDelimiters(signature.target.typeParameters, signature.mapper, writer, enclosingDeclaration);
                }
                else {
                    buildDisplayForTypeParametersAndDelimiters(signature.typeParameters, writer, enclosingDeclaration, flags, symbolStack);
                }

                buildDisplayForParametersAndDelimiters(signature.thisParameter, signature.parameters, writer, enclosingDeclaration, flags, symbolStack);

                buildReturnTypeDisplay(signature, writer, enclosingDeclaration, flags, symbolStack);
            }

            return _displayBuilder || (_displayBuilder = {
                buildSymbolDisplay,
                buildTypeDisplay,
                buildTypeParameterDisplay,
                buildTypePredicateDisplay,
                buildParameterDisplay,
                buildDisplayForParametersAndDelimiters,
                buildDisplayForTypeParametersAndDelimiters,
                buildTypeParameterDisplayFromSymbol,
                buildSignatureDisplay,
                buildReturnTypeDisplay
            });
        }

        function isDeclarationVisible(node: Declaration): boolean {
            if (node) {
                const links = getNodeLinks(node);
                if (links.isVisible === undefined) {
                    links.isVisible = !!determineIfDeclarationIsVisible();
                }
                return links.isVisible;
            }

            return false;

            function determineIfDeclarationIsVisible() {
                switch (node.kind) {
                    case SyntaxKind.BindingElement:
                        return isDeclarationVisible(<Declaration>node.parent.parent);
                    case SyntaxKind.VariableDeclaration:
                        if (isBindingPattern(node.name) &&
                            !(<BindingPattern>node.name).elements.length) {
                            // If the binding pattern is empty, this variable declaration is not visible
                            return false;
                        }
                    // Otherwise fall through
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.TypeAliasDeclaration:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.EnumDeclaration:
                    case SyntaxKind.ImportEqualsDeclaration:
                        // external module augmentation is always visible
                        if (isExternalModuleAugmentation(node)) {
                            return true;
                        }
                        const parent = getDeclarationContainer(node);
                        // If the node is not exported or it is not ambient module element (except import declaration)
                        if (!(getCombinedNodeFlags(node) & NodeFlags.Export) &&
                            !(node.kind !== SyntaxKind.ImportEqualsDeclaration && parent.kind !== SyntaxKind.SourceFile && isInAmbientContext(parent))) {
                            return isGlobalSourceFile(parent);
                        }
                        // Exported members/ambient module elements (exception import declaration) are visible if parent is visible
                        return isDeclarationVisible(<Declaration>parent);

                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.PropertySignature:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                        if (node.flags & (NodeFlags.Private | NodeFlags.Protected)) {
                            // Private/protected properties/methods are not visible
                            return false;
                        }
                    // Public properties/methods are visible if its parents are visible, so const it fall into next case statement

                    case SyntaxKind.Constructor:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.IndexSignature:
                    case SyntaxKind.Parameter:
                    case SyntaxKind.ModuleBlock:
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.TypeLiteral:
                    case SyntaxKind.TypeReference:
                    case SyntaxKind.ArrayType:
                    case SyntaxKind.TupleType:
                    case SyntaxKind.UnionType:
                    case SyntaxKind.IntersectionType:
                    case SyntaxKind.ParenthesizedType:
                        return isDeclarationVisible(<Declaration>node.parent);

                    // Default binding, import specifier and namespace import is visible
                    // only on demand so by default it is not visible
                    case SyntaxKind.ImportClause:
                    case SyntaxKind.NamespaceImport:
                    case SyntaxKind.ImportSpecifier:
                        return false;

                    // Type parameters are always visible
                    case SyntaxKind.TypeParameter:
                    // Source file is always visible
                    case SyntaxKind.SourceFile:
                        return true;

                    // Export assignments do not create name bindings outside the module
                    case SyntaxKind.ExportAssignment:
                        return false;

                    default:
                        return false;
                }
            }
        }

        function collectLinkedAliases(node: Identifier): Node[] {
            let exportSymbol: Symbol;
            if (node.parent && node.parent.kind === SyntaxKind.ExportAssignment) {
                exportSymbol = resolveName(node.parent, node.text, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias, Diagnostics.Cannot_find_name_0, node);
            }
            else if (node.parent.kind === SyntaxKind.ExportSpecifier) {
                const exportSpecifier = <ExportSpecifier>node.parent;
                exportSymbol = (<ExportDeclaration>exportSpecifier.parent.parent).moduleSpecifier ?
                    getExternalModuleMember(<ExportDeclaration>exportSpecifier.parent.parent, exportSpecifier) :
                    resolveEntityName(exportSpecifier.propertyName || exportSpecifier.name, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
            }
            const result: Node[] = [];
            if (exportSymbol) {
                buildVisibleNodeList(exportSymbol.declarations);
            }
            return result;

            function buildVisibleNodeList(declarations: Declaration[]) {
                forEach(declarations, declaration => {
                    getNodeLinks(declaration).isVisible = true;
                    const resultNode = getAnyImportSyntax(declaration) || declaration;
                    if (!contains(result, resultNode)) {
                        result.push(resultNode);
                    }

                    if (isInternalModuleImportEqualsDeclaration(declaration)) {
                        // Add the referenced top container visible
                        const internalModuleReference = <Identifier | QualifiedName>(<ImportEqualsDeclaration>declaration).moduleReference;
                        const firstIdentifier = getFirstIdentifier(internalModuleReference);
                        const importSymbol = resolveName(declaration, firstIdentifier.text, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace,
                            undefined, undefined);
                        if (importSymbol) {
                            buildVisibleNodeList(importSymbol.declarations);
                        }
                    }
                });
            }
        }

        /**
         * Push an entry on the type resolution stack. If an entry with the given target and the given property name
         * is already on the stack, and no entries in between already have a type, then a circularity has occurred.
         * In this case, the result values of the existing entry and all entries pushed after it are changed to false,
         * and the value false is returned. Otherwise, the new entry is just pushed onto the stack, and true is returned.
         * In order to see if the same query has already been done before, the target object and the propertyName both
         * must match the one passed in.
         *
         * @param target The symbol, type, or signature whose type is being queried
         * @param propertyName The property name that should be used to query the target for its type
         */
        function pushTypeResolution(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): boolean {
            const resolutionCycleStartIndex = findResolutionCycleStartIndex(target, propertyName);
            if (resolutionCycleStartIndex >= 0) {
                // A cycle was found
                const { length } = resolutionTargets;
                for (let i = resolutionCycleStartIndex; i < length; i++) {
                    resolutionResults[i] = false;
                }
                return false;
            }
            resolutionTargets.push(target);
            resolutionResults.push(/*items*/ true);
            resolutionPropertyNames.push(propertyName);
            return true;
        }

        function findResolutionCycleStartIndex(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): number {
            for (let i = resolutionTargets.length - 1; i >= 0; i--) {
                if (hasType(resolutionTargets[i], resolutionPropertyNames[i])) {
                    return -1;
                }
                if (resolutionTargets[i] === target && resolutionPropertyNames[i] === propertyName) {
                    return i;
                }
            }

            return -1;
        }

        function hasType(target: TypeSystemEntity, propertyName: TypeSystemPropertyName): Type {
            if (propertyName === TypeSystemPropertyName.Type) {
                return getSymbolLinks(<Symbol>target).type;
            }
            if (propertyName === TypeSystemPropertyName.DeclaredType) {
                return getSymbolLinks(<Symbol>target).declaredType;
            }
            if (propertyName === TypeSystemPropertyName.ResolvedBaseConstructorType) {
                Debug.assert(!!((<Type>target).flags & TypeFlags.Class));
                return (<InterfaceType>target).resolvedBaseConstructorType;
            }
            if (propertyName === TypeSystemPropertyName.ResolvedReturnType) {
                return (<Signature>target).resolvedReturnType;
            }

            Debug.fail("Unhandled TypeSystemPropertyName " + propertyName);
        }

        // Pop an entry from the type resolution stack and return its associated result value. The result value will
        // be true if no circularities were detected, or false if a circularity was found.
        function popTypeResolution(): boolean {
            resolutionTargets.pop();
            resolutionPropertyNames.pop();
            return resolutionResults.pop();
        }

        function getDeclarationContainer(node: Node): Node {
            node = getRootDeclaration(node);
            while (node) {
                switch (node.kind) {
                    case SyntaxKind.VariableDeclaration:
                    case SyntaxKind.VariableDeclarationList:
                    case SyntaxKind.ImportSpecifier:
                    case SyntaxKind.NamedImports:
                    case SyntaxKind.NamespaceImport:
                    case SyntaxKind.ImportClause:
                        node = node.parent;
                        break;

                    default:
                        return node.parent;
                }
            }
        }

        function getTypeOfPrototypeProperty(prototype: Symbol): Type {
            // TypeScript 1.0 spec (April 2014): 8.4
            // Every class automatically contains a static property member named 'prototype',
            // the type of which is an instantiation of the class type with type Any supplied as a type argument for each type parameter.
            // It is an error to explicitly declare a static property member with the name 'prototype'.
            const classType = <InterfaceType>getDeclaredTypeOfSymbol(getParentOfSymbol(prototype));
            return classType.typeParameters ? createTypeReference(<GenericType>classType, map(classType.typeParameters, _ => anyType)) : classType;
        }

        // Return the type of the given property in the given type, or undefined if no such property exists
        function getTypeOfPropertyOfType(type: Type, name: string): Type {
            const prop = getPropertyOfType(type, name);
            return prop ? getTypeOfSymbol(prop) : undefined;
        }

        function isTypeAny(type: Type) {
            return type && (type.flags & TypeFlags.Any) !== 0;
        }

        function isTypeNever(type: Type) {
            return type && (type.flags & TypeFlags.Never) !== 0;
        }

        // Return the type of a binding element parent. We check SymbolLinks first to see if a type has been
        // assigned by contextual typing.
        function getTypeForBindingElementParent(node: VariableLikeDeclaration) {
            const symbol = getSymbolOfNode(node);
            return symbol && getSymbolLinks(symbol).type || getTypeForVariableLikeDeclaration(node, /*includeOptionality*/ false);
        }

        function getTextOfPropertyName(name: PropertyName): string {
            switch (name.kind) {
                case SyntaxKind.Identifier:
                    return (<Identifier>name).text;
                case SyntaxKind.StringLiteral:
                case SyntaxKind.NumericLiteral:
                    return (<LiteralExpression>name).text;
                case SyntaxKind.ComputedPropertyName:
                    if (isStringOrNumericLiteral((<ComputedPropertyName>name).expression.kind)) {
                        return (<LiteralExpression>(<ComputedPropertyName>name).expression).text;
                    }
            }

            return undefined;
        }

        function isComputedNonLiteralName(name: PropertyName): boolean {
            return name.kind === SyntaxKind.ComputedPropertyName && !isStringOrNumericLiteral((<ComputedPropertyName>name).expression.kind);
        }

        /** Return the inferred type for a binding element */
        function getTypeForBindingElement(declaration: BindingElement): Type {
            const pattern = <BindingPattern>declaration.parent;
            const parentType = getTypeForBindingElementParent(<VariableLikeDeclaration>pattern.parent);
            // If parent has the unknown (error) type, then so does this binding element
            if (parentType === unknownType) {
                return unknownType;
            }
            // If no type was specified or inferred for parent, or if the specified or inferred type is any,
            // infer from the initializer of the binding element if one is present. Otherwise, go with the
            // undefined or any type of the parent.
            if (!parentType || isTypeAny(parentType)) {
                if (declaration.initializer) {
                    return checkExpressionCached(declaration.initializer);
                }
                return parentType;
            }

            let type: Type;
            if (pattern.kind === SyntaxKind.ObjectBindingPattern) {
                // Use explicitly specified property name ({ p: xxx } form), or otherwise the implied name ({ p } form)
                const name = declaration.propertyName || <Identifier>declaration.name;
                if (isComputedNonLiteralName(name)) {
                    // computed properties with non-literal names are treated as 'any'
                    return anyType;
                }
                if (declaration.initializer) {
                    getContextualType(declaration.initializer);
                }

                // Use type of the specified property, or otherwise, for a numeric name, the type of the numeric index signature,
                // or otherwise the type of the string index signature.
                const text = getTextOfPropertyName(name);

                type = getTypeOfPropertyOfType(parentType, text) ||
                    isNumericLiteralName(text) && getIndexTypeOfType(parentType, IndexKind.Number) ||
                    getIndexTypeOfType(parentType, IndexKind.String);
                if (!type) {
                    error(name, Diagnostics.Type_0_has_no_property_1_and_no_string_index_signature, typeToString(parentType), declarationNameToString(name));
                    return unknownType;
                }
            }
            else {
                // This elementType will be used if the specific property corresponding to this index is not
                // present (aka the tuple element property). This call also checks that the parentType is in
                // fact an iterable or array (depending on target language).
                const elementType = checkIteratedTypeOrElementType(parentType, pattern, /*allowStringInput*/ false);
                if (!declaration.dotDotDotToken) {
                    // Use specific property type when parent is a tuple or numeric index type when parent is an array
                    const propName = "" + indexOf(pattern.elements, declaration);
                    type = isTupleLikeType(parentType)
                        ? getTypeOfPropertyOfType(parentType, propName)
                        : elementType;
                    if (!type) {
                        if (isTupleType(parentType)) {
                            error(declaration, Diagnostics.Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2, typeToString(parentType), (<TupleType>parentType).elementTypes.length, pattern.elements.length);
                        }
                        else {
                            error(declaration, Diagnostics.Type_0_has_no_property_1, typeToString(parentType), propName);
                        }
                        return unknownType;
                    }
                }
                else {
                    // Rest element has an array type with the same element type as the parent type
                    type = createArrayType(elementType);
                }
            }
            // In strict null checking mode, if a default value of a non-undefined type is specified, remove
            // undefined from the final type.
            if (strictNullChecks && declaration.initializer && !(getFalsyFlags(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
                type = getTypeWithFacts(type, TypeFacts.NEUndefined);
            }
            return type;
        }

        function getTypeForVariableLikeDeclarationFromJSDocComment(declaration: VariableLikeDeclaration) {
            const jsDocType = getJSDocTypeForVariableLikeDeclarationFromJSDocComment(declaration);
            if (jsDocType) {
                return getTypeFromTypeNode(jsDocType);
            }
        }

        function getJSDocTypeForVariableLikeDeclarationFromJSDocComment(declaration: VariableLikeDeclaration): JSDocType {
            // First, see if this node has an @type annotation on it directly.
            const typeTag = getJSDocTypeTag(declaration);
            if (typeTag && typeTag.typeExpression) {
                return typeTag.typeExpression.type;
            }

            if (declaration.kind === SyntaxKind.VariableDeclaration &&
                declaration.parent.kind === SyntaxKind.VariableDeclarationList &&
                declaration.parent.parent.kind === SyntaxKind.VariableStatement) {

                // @type annotation might have been on the variable statement, try that instead.
                const annotation = getJSDocTypeTag(declaration.parent.parent);
                if (annotation && annotation.typeExpression) {
                    return annotation.typeExpression.type;
                }
            }
            else if (declaration.kind === SyntaxKind.Parameter) {
                // If it's a parameter, see if the parent has a jsdoc comment with an @param
                // annotation.
                const paramTag = getCorrespondingJSDocParameterTag(<ParameterDeclaration>declaration);
                if (paramTag && paramTag.typeExpression) {
                    return paramTag.typeExpression.type;
                }
            }

            return undefined;
        }

        function addOptionality(type: Type, optional: boolean): Type {
            return strictNullChecks && optional ? includeFalsyTypes(type, TypeFlags.Undefined) : type;
        }

        // Return the inferred type for a variable, parameter, or property declaration
        function getTypeForVariableLikeDeclaration(declaration: VariableLikeDeclaration, includeOptionality: boolean): Type {
            if (declaration.flags & NodeFlags.JavaScriptFile) {
                // If this is a variable in a JavaScript file, then use the JSDoc type (if it has
                // one as its type), otherwise fallback to the below standard TS codepaths to
                // try to figure it out.
                const type = getTypeForVariableLikeDeclarationFromJSDocComment(declaration);
                if (type && type !== unknownType) {
                    return type;
                }
            }

            // A variable declared in a for..in statement is always of type string
            if (declaration.parent.parent.kind === SyntaxKind.ForInStatement) {
                return stringType;
            }

            if (declaration.parent.parent.kind === SyntaxKind.ForOfStatement) {
                // checkRightHandSideOfForOf will return undefined if the for-of expression type was
                // missing properties/signatures required to get its iteratedType (like
                // [Symbol.iterator] or next). This may be because we accessed properties from anyType,
                // or it may have led to an error inside getElementTypeOfIterable.
                return checkRightHandSideOfForOf((<ForOfStatement>declaration.parent.parent).expression) || anyType;
            }

            if (isBindingPattern(declaration.parent)) {
                return getTypeForBindingElement(<BindingElement>declaration);
            }

            // Use type from type annotation if one is present
            if (declaration.type) {
                return addOptionality(getTypeFromTypeNode(declaration.type), /*optional*/ declaration.questionToken && includeOptionality);
            }

            if (declaration.kind === SyntaxKind.Parameter) {
                const func = <FunctionLikeDeclaration>declaration.parent;
                // For a parameter of a set accessor, use the type of the get accessor if one is present
                if (func.kind === SyntaxKind.SetAccessor && !hasDynamicName(func)) {
                    const getter = <AccessorDeclaration>getDeclarationOfKind(declaration.parent.symbol, SyntaxKind.GetAccessor);
                    if (getter) {
                        const getterSignature = getSignatureFromDeclaration(getter);
                        const thisParameter = getAccessorThisParameter(func as AccessorDeclaration);
                        if (thisParameter && declaration === thisParameter) {
                            // Use the type from the *getter*
                            Debug.assert(!thisParameter.type);
                            return getTypeOfSymbol(getterSignature.thisParameter);
                        }
                        return getReturnTypeOfSignature(getterSignature);
                    }
                }
                // Use contextual parameter type if one is available
                const type = declaration.symbol.name === "this"
                    ? getContextuallyTypedThisType(func)
                    : getContextuallyTypedParameterType(<ParameterDeclaration>declaration);
                if (type) {
                    return addOptionality(type, /*optional*/ declaration.questionToken && includeOptionality);
                }
            }

            // Use the type of the initializer expression if one is present
            if (declaration.initializer) {
                return addOptionality(checkExpressionCached(declaration.initializer), /*optional*/ declaration.questionToken && includeOptionality);
            }

            // If it is a short-hand property assignment, use the type of the identifier
            if (declaration.kind === SyntaxKind.ShorthandPropertyAssignment) {
                return checkIdentifier(<Identifier>declaration.name);
            }

            // If the declaration specifies a binding pattern, use the type implied by the binding pattern
            if (isBindingPattern(declaration.name)) {
                return getTypeFromBindingPattern(<BindingPattern>declaration.name, /*includePatternInType*/ false);
            }

            // No type specified and nothing can be inferred
            return undefined;
        }

        // Return the type implied by a binding pattern element. This is the type of the initializer of the element if
        // one is present. Otherwise, if the element is itself a binding pattern, it is the type implied by the binding
        // pattern. Otherwise, it is the type any.
        function getTypeFromBindingElement(element: BindingElement, includePatternInType?: boolean): Type {
            if (element.initializer) {
                const type = checkExpressionCached(element.initializer);
                reportErrorsFromWidening(element, type);
                return getWidenedType(type);
            }
            if (isBindingPattern(element.name)) {
                return getTypeFromBindingPattern(<BindingPattern>element.name, includePatternInType);
            }
            if (compilerOptions.noImplicitAny && !declarationBelongsToPrivateAmbientMember(element)) {
                reportImplicitAnyError(element, anyType);
            }
            return anyType;
        }

        // Return the type implied by an object binding pattern
        function getTypeFromObjectBindingPattern(pattern: BindingPattern, includePatternInType: boolean): Type {
            const members = createMap<Symbol>();
            let hasComputedProperties = false;
            forEach(pattern.elements, e => {
                const name = e.propertyName || <Identifier>e.name;
                if (isComputedNonLiteralName(name)) {
                    // do not include computed properties in the implied type
                    hasComputedProperties = true;
                    return;
                }

                const text = getTextOfPropertyName(name);
                const flags = SymbolFlags.Property | SymbolFlags.Transient | (e.initializer ? SymbolFlags.Optional : 0);
                const symbol = <TransientSymbol>createSymbol(flags, text);
                symbol.type = getTypeFromBindingElement(e, includePatternInType);
                symbol.bindingElement = e;
                members[symbol.name] = symbol;
            });
            const result = createAnonymousType(undefined, members, emptyArray, emptyArray, undefined, undefined);
            if (includePatternInType) {
                result.pattern = pattern;
            }
            if (hasComputedProperties) {
                result.flags |= TypeFlags.ObjectLiteralPatternWithComputedProperties;
            }
            return result;
        }

        // Return the type implied by an array binding pattern
        function getTypeFromArrayBindingPattern(pattern: BindingPattern, includePatternInType: boolean): Type {
            const elements = pattern.elements;
            if (elements.length === 0 || elements[elements.length - 1].dotDotDotToken) {
                return languageVersion >= ScriptTarget.ES6 ? createIterableType(anyType) : anyArrayType;
            }
            // If the pattern has at least one element, and no rest element, then it should imply a tuple type.
            const elementTypes = map(elements, e => e.kind === SyntaxKind.OmittedExpression ? anyType : getTypeFromBindingElement(e, includePatternInType));
            if (includePatternInType) {
                const result = createNewTupleType(elementTypes);
                result.pattern = pattern;
                return result;
            }
            return createTupleType(elementTypes);
        }

        // Return the type implied by a binding pattern. This is the type implied purely by the binding pattern itself
        // and without regard to its context (i.e. without regard any type annotation or initializer associated with the
        // declaration in which the binding pattern is contained). For example, the implied type of [x, y] is [any, any]
        // and the implied type of { x, y: z = 1 } is { x: any; y: number; }. The type implied by a binding pattern is
        // used as the contextual type of an initializer associated with the binding pattern. Also, for a destructuring
        // parameter with no type annotation or initializer, the type implied by the binding pattern becomes the type of
        // the parameter.
        function getTypeFromBindingPattern(pattern: BindingPattern, includePatternInType?: boolean): Type {
            return pattern.kind === SyntaxKind.ObjectBindingPattern
                ? getTypeFromObjectBindingPattern(pattern, includePatternInType)
                : getTypeFromArrayBindingPattern(pattern, includePatternInType);
        }

        // Return the type associated with a variable, parameter, or property declaration. In the simple case this is the type
        // specified in a type annotation or inferred from an initializer. However, in the case of a destructuring declaration it
        // is a bit more involved. For example:
        //
        //   var [x, s = ""] = [1, "one"];
        //
        // Here, the array literal [1, "one"] is contextually typed by the type [any, string], which is the implied type of the
        // binding pattern [x, s = ""]. Because the contextual type is a tuple type, the resulting type of [1, "one"] is the
        // tuple type [number, string]. Thus, the type inferred for 'x' is number and the type inferred for 's' is string.
        function getWidenedTypeForVariableLikeDeclaration(declaration: VariableLikeDeclaration, reportErrors?: boolean): Type {
            let type = getTypeForVariableLikeDeclaration(declaration, /*includeOptionality*/ true);
            if (type) {
                if (reportErrors) {
                    reportErrorsFromWidening(declaration, type);
                }
                // During a normal type check we'll never get to here with a property assignment (the check of the containing
                // object literal uses a different path). We exclude widening only so that language services and type verification
                // tools see the actual type.
                if (declaration.kind === SyntaxKind.PropertyAssignment) {
                    return type;
                }
                return getWidenedType(type);
            }

            // Rest parameters default to type any[], other parameters default to type any
            type = declaration.dotDotDotToken ? anyArrayType : anyType;

            // Report implicit any errors unless this is a private property within an ambient declaration
            if (reportErrors && compilerOptions.noImplicitAny) {
                if (!declarationBelongsToPrivateAmbientMember(declaration)) {
                    reportImplicitAnyError(declaration, type);
                }
            }
            return type;
        }

        function declarationBelongsToPrivateAmbientMember(declaration: VariableLikeDeclaration) {
            const root = getRootDeclaration(declaration);
            const memberDeclaration = root.kind === SyntaxKind.Parameter ? root.parent : root;
            return isPrivateWithinAmbient(memberDeclaration);
        }

        function getTypeOfVariableOrParameterOrProperty(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                // Handle prototype property
                if (symbol.flags & SymbolFlags.Prototype) {
                    return links.type = getTypeOfPrototypeProperty(symbol);
                }
                // Handle catch clause variables
                const declaration = symbol.valueDeclaration;
                if (declaration.parent.kind === SyntaxKind.CatchClause) {
                    return links.type = anyType;
                }
                // Handle export default expressions
                if (declaration.kind === SyntaxKind.ExportAssignment) {
                    return links.type = checkExpression((<ExportAssignment>declaration).expression);
                }
                if (declaration.flags & NodeFlags.JavaScriptFile && declaration.kind === SyntaxKind.JSDocPropertyTag && (<JSDocPropertyTag>declaration).typeExpression) {
                    return links.type = getTypeFromTypeNode((<JSDocPropertyTag>declaration).typeExpression.type);
                }
                // Handle variable, parameter or property
                if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                    return unknownType;
                }

                let type: Type;
                // Handle certain special assignment kinds, which happen to union across multiple declarations:
                // * module.exports = expr
                // * exports.p = expr
                // * this.p = expr
                // * className.prototype.method = expr
                if (declaration.kind === SyntaxKind.BinaryExpression ||
                    declaration.kind === SyntaxKind.PropertyAccessExpression && declaration.parent.kind === SyntaxKind.BinaryExpression) {
                    const declaredTypes = map(symbol.declarations,
                        decl => decl.kind === SyntaxKind.BinaryExpression ?
                            checkExpressionCached((<BinaryExpression>decl).right) :
                            checkExpressionCached((<BinaryExpression>decl.parent).right));
                    type = getUnionType(declaredTypes, /*subtypeReduction*/ true);
                }
                else {
                    type = getWidenedTypeForVariableLikeDeclaration(<VariableLikeDeclaration>declaration, /*reportErrors*/ true);
                }

                if (!popTypeResolution()) {
                    if ((<VariableLikeDeclaration>symbol.valueDeclaration).type) {
                        // Variable has type annotation that circularly references the variable itself
                        type = unknownType;
                        error(symbol.valueDeclaration, Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_type_annotation,
                            symbolToString(symbol));
                    }
                    else {
                        // Variable has initializer that circularly references the variable itself
                        type = anyType;
                        if (compilerOptions.noImplicitAny) {
                            error(symbol.valueDeclaration, Diagnostics._0_implicitly_has_type_any_because_it_does_not_have_a_type_annotation_and_is_referenced_directly_or_indirectly_in_its_own_initializer,
                                symbolToString(symbol));
                        }
                    }
                }
                links.type = type;
            }
            return links.type;
        }

        function getAnnotatedAccessorType(accessor: AccessorDeclaration): Type {
            if (accessor) {
                if (accessor.kind === SyntaxKind.GetAccessor) {
                    return accessor.type && getTypeFromTypeNode(accessor.type);
                }
                else {
                    const setterTypeAnnotation = getSetAccessorTypeAnnotationNode(accessor);
                    return setterTypeAnnotation && getTypeFromTypeNode(setterTypeAnnotation);
                }
            }
            return undefined;
        }

        function getAnnotatedAccessorThisParameter(accessor: AccessorDeclaration): Symbol | undefined {
            const parameter = getAccessorThisParameter(accessor);
            return parameter && parameter.symbol;
        }

        function getThisTypeOfDeclaration(declaration: SignatureDeclaration): Type | undefined {
            return getThisTypeOfSignature(getSignatureFromDeclaration(declaration));
        }

        function getTypeOfAccessors(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                const getter = <AccessorDeclaration>getDeclarationOfKind(symbol, SyntaxKind.GetAccessor);
                const setter = <AccessorDeclaration>getDeclarationOfKind(symbol, SyntaxKind.SetAccessor);

                if (getter && getter.flags & NodeFlags.JavaScriptFile) {
                    const jsDocType = getTypeForVariableLikeDeclarationFromJSDocComment(getter);
                    if (jsDocType) {
                        return links.type = jsDocType;
                    }
                }

                if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
                    return unknownType;
                }

                let type: Type;

                // First try to see if the user specified a return type on the get-accessor.
                const getterReturnType = getAnnotatedAccessorType(getter);
                if (getterReturnType) {
                    type = getterReturnType;
                }
                else {
                    // If the user didn't specify a return type, try to use the set-accessor's parameter type.
                    const setterParameterType = getAnnotatedAccessorType(setter);
                    if (setterParameterType) {
                        type = setterParameterType;
                    }
                    else {
                        // If there are no specified types, try to infer it from the body of the get accessor if it exists.
                        if (getter && getter.body) {
                            type = getReturnTypeFromBody(getter);
                        }
                        // Otherwise, fall back to 'any'.
                        else {
                            if (compilerOptions.noImplicitAny) {
                                error(setter, Diagnostics.Property_0_implicitly_has_type_any_because_its_set_accessor_lacks_a_type_annotation, symbolToString(symbol));
                            }
                            type = anyType;
                        }
                    }
                }
                if (!popTypeResolution()) {
                    type = anyType;
                    if (compilerOptions.noImplicitAny) {
                        const getter = <AccessorDeclaration>getDeclarationOfKind(symbol, SyntaxKind.GetAccessor);
                        error(getter, Diagnostics._0_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions, symbolToString(symbol));
                    }
                }
                links.type = type;
            }
            return links.type;
        }

        function getTypeOfFuncClassEnumModule(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                if (symbol.valueDeclaration.kind === SyntaxKind.ModuleDeclaration && isShorthandAmbientModule(<ModuleDeclaration>symbol.valueDeclaration)) {
                    links.type = anyType;
                }
                else {
                    const type = createObjectType(TypeFlags.Anonymous, symbol);
                    links.type = strictNullChecks && symbol.flags & SymbolFlags.Optional ?
                        includeFalsyTypes(type, TypeFlags.Undefined) : type;
                }
            }
            return links.type;
        }

        function getTypeOfEnumMember(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                links.type = getDeclaredTypeOfEnum(getParentOfSymbol(symbol));
            }
            return links.type;
        }

        function getTypeOfAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                const targetSymbol = resolveAlias(symbol);

                // It only makes sense to get the type of a value symbol. If the result of resolving
                // the alias is not a value, then it has no type. To get the type associated with a
                // type symbol, call getDeclaredTypeOfSymbol.
                // This check is important because without it, a call to getTypeOfSymbol could end
                // up recursively calling getTypeOfAlias, causing a stack overflow.
                links.type = targetSymbol.flags & SymbolFlags.Value
                    ? getTypeOfSymbol(targetSymbol)
                    : unknownType;
            }
            return links.type;
        }

        function getTypeOfInstantiatedSymbol(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.type) {
                links.type = instantiateType(getTypeOfSymbol(links.target), links.mapper);
            }
            return links.type;
        }

        function getTypeOfSymbol(symbol: Symbol): Type {
            if (symbol.flags & SymbolFlags.Instantiated) {
                return getTypeOfInstantiatedSymbol(symbol);
            }
            if (symbol.flags & (SymbolFlags.Variable | SymbolFlags.Property)) {
                return getTypeOfVariableOrParameterOrProperty(symbol);
            }
            if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.Enum | SymbolFlags.ValueModule)) {
                return getTypeOfFuncClassEnumModule(symbol);
            }
            if (symbol.flags & SymbolFlags.EnumMember) {
                return getTypeOfEnumMember(symbol);
            }
            if (symbol.flags & SymbolFlags.Accessor) {
                return getTypeOfAccessors(symbol);
            }
            if (symbol.flags & SymbolFlags.Alias) {
                return getTypeOfAlias(symbol);
            }
            return unknownType;
        }

        function getTargetType(type: ObjectType): Type {
            return type.flags & TypeFlags.Reference ? (<TypeReference>type).target : type;
        }

        function hasBaseType(type: InterfaceType, checkBase: InterfaceType) {
            return check(type);
            function check(type: InterfaceType): boolean {
                const target = <InterfaceType>getTargetType(type);
                return target === checkBase || forEach(getBaseTypes(target), check);
            }
        }

        // Appends the type parameters given by a list of declarations to a set of type parameters and returns the resulting set.
        // The function allocates a new array if the input type parameter set is undefined, but otherwise it modifies the set
        // in-place and returns the same array.
        function appendTypeParameters(typeParameters: TypeParameter[], declarations: TypeParameterDeclaration[]): TypeParameter[] {
            for (const declaration of declarations) {
                const tp = getDeclaredTypeOfTypeParameter(getSymbolOfNode(declaration));
                if (!typeParameters) {
                    typeParameters = [tp];
                }
                else if (!contains(typeParameters, tp)) {
                    typeParameters.push(tp);
                }
            }
            return typeParameters;
        }

        // Appends the outer type parameters of a node to a set of type parameters and returns the resulting set. The function
        // allocates a new array if the input type parameter set is undefined, but otherwise it modifies the set in-place and
        // returns the same array.
        function appendOuterTypeParameters(typeParameters: TypeParameter[], node: Node): TypeParameter[] {
            while (true) {
                node = node.parent;
                if (!node) {
                    return typeParameters;
                }
                if (node.kind === SyntaxKind.ClassDeclaration || node.kind === SyntaxKind.ClassExpression ||
                    node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.FunctionExpression ||
                    node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.ArrowFunction) {
                    const declarations = (<ClassLikeDeclaration | FunctionLikeDeclaration>node).typeParameters;
                    if (declarations) {
                        return appendTypeParameters(appendOuterTypeParameters(typeParameters, node), declarations);
                    }
                }
            }
        }

        // The outer type parameters are those defined by enclosing generic classes, methods, or functions.
        function getOuterTypeParametersOfClassOrInterface(symbol: Symbol): TypeParameter[] {
            const declaration = symbol.flags & SymbolFlags.Class ? symbol.valueDeclaration : getDeclarationOfKind(symbol, SyntaxKind.InterfaceDeclaration);
            return appendOuterTypeParameters(undefined, declaration);
        }

        // The local type parameters are the combined set of type parameters from all declarations of the class,
        // interface, or type alias.
        function getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol: Symbol): TypeParameter[] {
            let result: TypeParameter[];
            for (const node of symbol.declarations) {
                if (node.kind === SyntaxKind.InterfaceDeclaration || node.kind === SyntaxKind.ClassDeclaration ||
                    node.kind === SyntaxKind.ClassExpression || node.kind === SyntaxKind.TypeAliasDeclaration) {
                    const declaration = <InterfaceDeclaration | TypeAliasDeclaration>node;
                    if (declaration.typeParameters) {
                        result = appendTypeParameters(result, declaration.typeParameters);
                    }
                }
            }
            return result;
        }

        // The full set of type parameters for a generic class or interface type consists of its outer type parameters plus
        // its locally declared type parameters.
        function getTypeParametersOfClassOrInterface(symbol: Symbol): TypeParameter[] {
            return concatenate(getOuterTypeParametersOfClassOrInterface(symbol), getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol));
        }

        function isConstructorType(type: Type): boolean {
            return type.flags & TypeFlags.ObjectType && getSignaturesOfType(type, SignatureKind.Construct).length > 0;
        }

        function getBaseTypeNodeOfClass(type: InterfaceType): ExpressionWithTypeArguments {
            return getClassExtendsHeritageClauseElement(<ClassLikeDeclaration>type.symbol.valueDeclaration);
        }

        function getConstructorsForTypeArguments(type: ObjectType, typeArgumentNodes: TypeNode[]): Signature[] {
            const typeArgCount = typeArgumentNodes ? typeArgumentNodes.length : 0;
            return filter(getSignaturesOfType(type, SignatureKind.Construct),
                sig => (sig.typeParameters ? sig.typeParameters.length : 0) === typeArgCount);
        }

        function getInstantiatedConstructorsForTypeArguments(type: ObjectType, typeArgumentNodes: TypeNode[]): Signature[] {
            let signatures = getConstructorsForTypeArguments(type, typeArgumentNodes);
            if (typeArgumentNodes) {
                const typeArguments = map(typeArgumentNodes, getTypeFromTypeNode);
                signatures = map(signatures, sig => getSignatureInstantiation(sig, typeArguments));
            }
            return signatures;
        }

        // The base constructor of a class can resolve to
        // undefinedType if the class has no extends clause,
        // unknownType if an error occurred during resolution of the extends expression,
        // nullType if the extends expression is the null value, or
        // an object type with at least one construct signature.
        function getBaseConstructorTypeOfClass(type: InterfaceType): ObjectType {
            if (!type.resolvedBaseConstructorType) {
                const baseTypeNode = getBaseTypeNodeOfClass(type);
                if (!baseTypeNode) {
                    return type.resolvedBaseConstructorType = undefinedType;
                }
                if (!pushTypeResolution(type, TypeSystemPropertyName.ResolvedBaseConstructorType)) {
                    return unknownType;
                }
                const baseConstructorType = checkExpression(baseTypeNode.expression);
                if (baseConstructorType.flags & TypeFlags.ObjectType) {
                    // Resolving the members of a class requires us to resolve the base class of that class.
                    // We force resolution here such that we catch circularities now.
                    resolveStructuredTypeMembers(baseConstructorType);
                }
                if (!popTypeResolution()) {
                    error(type.symbol.valueDeclaration, Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_base_expression, symbolToString(type.symbol));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                if (baseConstructorType !== unknownType && baseConstructorType !== nullWideningType && !isConstructorType(baseConstructorType)) {
                    error(baseTypeNode.expression, Diagnostics.Type_0_is_not_a_constructor_function_type, typeToString(baseConstructorType));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                type.resolvedBaseConstructorType = baseConstructorType;
            }
            return type.resolvedBaseConstructorType;
        }

        function getBaseTypes(type: InterfaceType): ObjectType[] {
            const isClass = type.symbol.flags & SymbolFlags.Class;
            const isInterface = type.symbol.flags & SymbolFlags.Interface;
            if (!type.resolvedBaseTypes) {
                if (!isClass && !isInterface) {
                    Debug.fail("type must be class or interface");
                }
                if (isClass) {
                    resolveBaseTypesOfClass(type);
                }
                if (isInterface) {
                    resolveBaseTypesOfInterface(type);
                }
            }
            return type.resolvedBaseTypes;
        }

        function resolveBaseTypesOfClass(type: InterfaceType): void {
            type.resolvedBaseTypes = type.resolvedBaseTypes || emptyArray;
            const baseConstructorType = getBaseConstructorTypeOfClass(type);
            if (!(baseConstructorType.flags & TypeFlags.ObjectType)) {
                return;
            }
            const baseTypeNode = getBaseTypeNodeOfClass(type);
            let baseType: Type;
            const originalBaseType = baseConstructorType && baseConstructorType.symbol ? getDeclaredTypeOfSymbol(baseConstructorType.symbol) : undefined;
            if (baseConstructorType.symbol && baseConstructorType.symbol.flags & SymbolFlags.Class &&
                areAllOuterTypeParametersApplied(originalBaseType)) {
                // When base constructor type is a class with no captured type arguments we know that the constructors all have the same type parameters as the
                // class and all return the instance type of the class. There is no need for further checks and we can apply the
                // type arguments in the same manner as a type reference to get the same error reporting experience.
                baseType = getTypeFromClassOrInterfaceReference(baseTypeNode, baseConstructorType.symbol);
            }
            else {
                // The class derives from a "class-like" constructor function, check that we have at least one construct signature
                // with a matching number of type parameters and use the return type of the first instantiated signature. Elsewhere
                // we check that all instantiated signatures return the same type.
                const constructors = getInstantiatedConstructorsForTypeArguments(baseConstructorType, baseTypeNode.typeArguments);
                if (!constructors.length) {
                    error(baseTypeNode.expression, Diagnostics.No_base_constructor_has_the_specified_number_of_type_arguments);
                    return;
                }
                baseType = getReturnTypeOfSignature(constructors[0]);
            }
            if (baseType === unknownType) {
                return;
            }
            if (!(getTargetType(baseType).flags & (TypeFlags.Class | TypeFlags.Interface))) {
                error(baseTypeNode.expression, Diagnostics.Base_constructor_return_type_0_is_not_a_class_or_interface_type, typeToString(baseType));
                return;
            }
            if (type === baseType || hasBaseType(<InterfaceType>baseType, type)) {
                error(type.symbol.valueDeclaration, Diagnostics.Type_0_recursively_references_itself_as_a_base_type,
                    typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType));
                return;
            }
            if (type.resolvedBaseTypes === emptyArray) {
                type.resolvedBaseTypes = [baseType];
            }
            else {
                type.resolvedBaseTypes.push(baseType);
            }
        }

        function areAllOuterTypeParametersApplied(type: Type): boolean {
            // An unapplied type parameter has its symbol still the same as the matching argument symbol.
            // Since parameters are applied outer-to-inner, only the last outer parameter needs to be checked.
            const outerTypeParameters = (<InterfaceType>type).outerTypeParameters;
            if (outerTypeParameters) {
                const last = outerTypeParameters.length - 1;
                const typeArguments = (<TypeReference>type).typeArguments;
                return outerTypeParameters[last].symbol !== typeArguments[last].symbol;
            }
            return true;
        }

        function resolveBaseTypesOfInterface(type: InterfaceType): void {
            type.resolvedBaseTypes = type.resolvedBaseTypes || emptyArray;
            for (const declaration of type.symbol.declarations) {
                if (declaration.kind === SyntaxKind.InterfaceDeclaration && getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration)) {
                    for (const node of getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration)) {
                        const baseType = getTypeFromTypeNode(node);
                        if (baseType !== unknownType) {
                            if (getTargetType(baseType).flags & (TypeFlags.Class | TypeFlags.Interface)) {
                                if (type !== baseType && !hasBaseType(<InterfaceType>baseType, type)) {
                                    if (type.resolvedBaseTypes === emptyArray) {
                                        type.resolvedBaseTypes = [baseType];
                                    }
                                    else {
                                        type.resolvedBaseTypes.push(baseType);
                                    }
                                }
                                else {
                                    error(declaration, Diagnostics.Type_0_recursively_references_itself_as_a_base_type, typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType));
                                }
                            }
                            else {
                                error(node, Diagnostics.An_interface_may_only_extend_a_class_or_another_interface);
                            }
                        }
                    }
                }
            }
        }

        // Returns true if the interface given by the symbol is free of "this" references. Specifically, the result is
        // true if the interface itself contains no references to "this" in its body, if all base types are interfaces,
        // and if none of the base interfaces have a "this" type.
        function isIndependentInterface(symbol: Symbol): boolean {
            for (const declaration of symbol.declarations) {
                if (declaration.kind === SyntaxKind.InterfaceDeclaration) {
                    if (declaration.flags & NodeFlags.ContainsThis) {
                        return false;
                    }
                    const baseTypeNodes = getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration);
                    if (baseTypeNodes) {
                        for (const node of baseTypeNodes) {
                            if (isEntityNameExpression(node.expression)) {
                                const baseSymbol = resolveEntityName(node.expression, SymbolFlags.Type, /*ignoreErrors*/ true);
                                if (!baseSymbol || !(baseSymbol.flags & SymbolFlags.Interface) || getDeclaredTypeOfClassOrInterface(baseSymbol).thisType) {
                                    return false;
                                }
                            }
                        }
                    }
                }
            }
            return true;
        }

        function getDeclaredTypeOfClassOrInterface(symbol: Symbol): InterfaceType {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const kind = symbol.flags & SymbolFlags.Class ? TypeFlags.Class : TypeFlags.Interface;
                const type = links.declaredType = <InterfaceType>createObjectType(kind, symbol);
                const outerTypeParameters = getOuterTypeParametersOfClassOrInterface(symbol);
                const localTypeParameters = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol);
                // A class or interface is generic if it has type parameters or a "this" type. We always give classes a "this" type
                // because it is not feasible to analyze all members to determine if the "this" type escapes the class (in particular,
                // property types inferred from initializers and method return types inferred from return statements are very hard
                // to exhaustively analyze). We give interfaces a "this" type if we can't definitely determine that they are free of
                // "this" references.
                if (outerTypeParameters || localTypeParameters || kind === TypeFlags.Class || !isIndependentInterface(symbol)) {
                    type.flags |= TypeFlags.Reference;
                    type.typeParameters = concatenate(outerTypeParameters, localTypeParameters);
                    type.outerTypeParameters = outerTypeParameters;
                    type.localTypeParameters = localTypeParameters;
                    (<GenericType>type).instantiations = createMap<TypeReference>();
                    (<GenericType>type).instantiations[getTypeListId(type.typeParameters)] = <GenericType>type;
                    (<GenericType>type).target = <GenericType>type;
                    (<GenericType>type).typeArguments = type.typeParameters;
                    type.thisType = <TypeParameter>createType(TypeFlags.TypeParameter | TypeFlags.ThisType);
                    type.thisType.symbol = symbol;
                    type.thisType.constraint = type;
                }
            }
            return <InterfaceType>links.declaredType;
        }

        function getDeclaredTypeOfTypeAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                // Note that we use the links object as the target here because the symbol object is used as the unique
                // identity for resolution of the 'type' property in SymbolLinks.
                if (!pushTypeResolution(symbol, TypeSystemPropertyName.DeclaredType)) {
                    return unknownType;
                }

                const typeParameters = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(symbol);
                let declaration: JSDocTypedefTag | TypeAliasDeclaration = <JSDocTypedefTag>getDeclarationOfKind(symbol, SyntaxKind.JSDocTypedefTag);
                let type: Type;
                if (declaration) {
                    if (declaration.jsDocTypeLiteral) {
                        type = getTypeFromTypeNode(declaration.jsDocTypeLiteral);
                    }
                    else {
                        type = getTypeFromTypeNode(declaration.typeExpression.type);
                    }
                }
                else {
                    declaration = <TypeAliasDeclaration>getDeclarationOfKind(symbol, SyntaxKind.TypeAliasDeclaration);
                    type = getTypeFromTypeNode(declaration.type, symbol, typeParameters);
                }

                if (popTypeResolution()) {
                    links.typeParameters = typeParameters;
                    if (typeParameters) {
                        // Initialize the instantiation cache for generic type aliases. The declared type corresponds to
                        // an instantiation of the type alias with the type parameters supplied as type arguments.
                        links.instantiations = createMap<Type>();
                        links.instantiations[getTypeListId(links.typeParameters)] = type;
                    }
                }
                else {
                    type = unknownType;
                    error(declaration.name, Diagnostics.Type_alias_0_circularly_references_itself, symbolToString(symbol));
                }
                links.declaredType = type;
            }
            return links.declaredType;
        }

        function isLiteralEnumMember(symbol: Symbol, member: EnumMember) {
            const expr = member.initializer;
            if (!expr) {
                return !isInAmbientContext(member);
            }
            return expr.kind === SyntaxKind.NumericLiteral ||
                expr.kind === SyntaxKind.PrefixUnaryExpression && (<PrefixUnaryExpression>expr).operator === SyntaxKind.MinusToken &&
                (<PrefixUnaryExpression>expr).operand.kind === SyntaxKind.NumericLiteral ||
                expr.kind === SyntaxKind.Identifier && !!symbol.exports[(<Identifier>expr).text];
        }

        function enumHasLiteralMembers(symbol: Symbol) {
            for (const declaration of symbol.declarations) {
                if (declaration.kind === SyntaxKind.EnumDeclaration) {
                    for (const member of (<EnumDeclaration>declaration).members) {
                        if (!isLiteralEnumMember(symbol, member)) {
                            return false;
                        }
                    }
                }
            }
            return true;
        }

        function getDeclaredTypeOfEnum(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const enumType = links.declaredType = <EnumType>createType(TypeFlags.Enum);
                enumType.symbol = symbol;
                if (enumHasLiteralMembers(symbol)) {
                    const memberTypeList: Type[] = [];
                    const memberTypes = createMap<EnumLiteralType>();
                    for (const declaration of enumType.symbol.declarations) {
                        if (declaration.kind === SyntaxKind.EnumDeclaration) {
                            computeEnumMemberValues(<EnumDeclaration>declaration);
                            for (const member of (<EnumDeclaration>declaration).members) {
                                const memberSymbol = getSymbolOfNode(member);
                                const value = getEnumMemberValue(member);
                                if (!memberTypes[value]) {
                                    const memberType = memberTypes[value] = <EnumLiteralType>createType(TypeFlags.EnumLiteral);
                                    memberType.symbol = memberSymbol;
                                    memberType.baseType = <EnumType & UnionType>enumType;
                                    memberType.text = "" + value;
                                    memberTypeList.push(memberType);
                                }
                            }
                        }
                    }
                    enumType.memberTypes = memberTypes;
                    if (memberTypeList.length > 1) {
                        enumType.flags |= TypeFlags.Union;
                        (<EnumType & UnionType>enumType).types = memberTypeList;
                        unionTypes[getTypeListId(memberTypeList)] = <EnumType & UnionType>enumType;
                    }
                }
            }
            return links.declaredType;
        }

        function getDeclaredTypeOfEnumMember(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const enumType = <EnumType>getDeclaredTypeOfEnum(getParentOfSymbol(symbol));
                links.declaredType = enumType.flags & TypeFlags.Union ?
                    enumType.memberTypes[getEnumMemberValue(<EnumDeclaration>symbol.valueDeclaration)] :
                    enumType;
            }
            return links.declaredType;
        }

        function getDeclaredTypeOfTypeParameter(symbol: Symbol): TypeParameter {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                const type = <TypeParameter>createType(TypeFlags.TypeParameter);
                type.symbol = symbol;
                if (!(<TypeParameterDeclaration>getDeclarationOfKind(symbol, SyntaxKind.TypeParameter)).constraint) {
                    type.constraint = noConstraintType;
                }
                links.declaredType = type;
            }
            return <TypeParameter>links.declaredType;
        }

        function getDeclaredTypeOfAlias(symbol: Symbol): Type {
            const links = getSymbolLinks(symbol);
            if (!links.declaredType) {
                links.declaredType = getDeclaredTypeOfSymbol(resolveAlias(symbol));
            }
            return links.declaredType;
        }

        function getDeclaredTypeOfSymbol(symbol: Symbol): Type {
            Debug.assert((symbol.flags & SymbolFlags.Instantiated) === 0);
            if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                return getDeclaredTypeOfClassOrInterface(symbol);
            }
            if (symbol.flags & SymbolFlags.TypeAlias) {
                return getDeclaredTypeOfTypeAlias(symbol);
            }
            if (symbol.flags & SymbolFlags.TypeParameter) {
                return getDeclaredTypeOfTypeParameter(symbol);
            }
            if (symbol.flags & SymbolFlags.Enum) {
                return getDeclaredTypeOfEnum(symbol);
            }
            if (symbol.flags & SymbolFlags.EnumMember) {
                return getDeclaredTypeOfEnumMember(symbol);
            }
            if (symbol.flags & SymbolFlags.Alias) {
                return getDeclaredTypeOfAlias(symbol);
            }
            return unknownType;
        }

        // A type reference is considered independent if each type argument is considered independent.
        function isIndependentTypeReference(node: TypeReferenceNode): boolean {
            if (node.typeArguments) {
                for (const typeNode of node.typeArguments) {
                    if (!isIndependentType(typeNode)) {
                        return false;
                    }
                }
            }
            return true;
        }

        // A type is considered independent if it the any, string, number, boolean, symbol, or void keyword, a string
        // literal type, an array with an element type that is considered independent, or a type reference that is
        // considered independent.
        function isIndependentType(node: TypeNode): boolean {
            switch (node.kind) {
                case SyntaxKind.AnyKeyword:
                case SyntaxKind.StringKeyword:
                case SyntaxKind.NumberKeyword:
                case SyntaxKind.BooleanKeyword:
                case SyntaxKind.SymbolKeyword:
                case SyntaxKind.VoidKeyword:
                case SyntaxKind.UndefinedKeyword:
                case SyntaxKind.NullKeyword:
                case SyntaxKind.NeverKeyword:
                case SyntaxKind.LiteralType:
                    return true;
                case SyntaxKind.ArrayType:
                    return isIndependentType((<ArrayTypeNode>node).elementType);
                case SyntaxKind.TypeReference:
                    return isIndependentTypeReference(<TypeReferenceNode>node);
            }
            return false;
        }

        // A variable-like declaration is considered independent (free of this references) if it has a type annotation
        // that specifies an independent type, or if it has no type annotation and no initializer (and thus of type any).
        function isIndependentVariableLikeDeclaration(node: VariableLikeDeclaration): boolean {
            return node.type && isIndependentType(node.type) || !node.type && !node.initializer;
        }

        // A function-like declaration is considered independent (free of this references) if it has a return type
        // annotation that is considered independent and if each parameter is considered independent.
        function isIndependentFunctionLikeDeclaration(node: FunctionLikeDeclaration): boolean {
            if (node.kind !== SyntaxKind.Constructor && (!node.type || !isIndependentType(node.type))) {
                return false;
            }
            for (const parameter of node.parameters) {
                if (!isIndependentVariableLikeDeclaration(parameter)) {
                    return false;
                }
            }
            return true;
        }

        // Returns true if the class or interface member given by the symbol is free of "this" references. The
        // function may return false for symbols that are actually free of "this" references because it is not
        // feasible to perform a complete analysis in all cases. In particular, property members with types
        // inferred from their initializers and function members with inferred return types are conservatively
        // assumed not to be free of "this" references.
        function isIndependentMember(symbol: Symbol): boolean {
            if (symbol.declarations && symbol.declarations.length === 1) {
                const declaration = symbol.declarations[0];
                if (declaration) {
                    switch (declaration.kind) {
                        case SyntaxKind.PropertyDeclaration:
                        case SyntaxKind.PropertySignature:
                            return isIndependentVariableLikeDeclaration(<VariableLikeDeclaration>declaration);
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.MethodSignature:
                        case SyntaxKind.Constructor:
                            return isIndependentFunctionLikeDeclaration(<FunctionLikeDeclaration>declaration);
                    }
                }
            }
            return false;
        }

        function createSymbolTable(symbols: Symbol[]): SymbolTable {
            const result = createMap<Symbol>();
            for (const symbol of symbols) {
                result[symbol.name] = symbol;
            }
            return result;
        }

        // The mappingThisOnly flag indicates that the only type parameter being mapped is "this". When the flag is true,
        // we check symbols to see if we can quickly conclude they are free of "this" references, thus needing no instantiation.
        function createInstantiatedSymbolTable(symbols: Symbol[], mapper: TypeMapper, mappingThisOnly: boolean): SymbolTable {
            const result = createMap<Symbol>();
            for (const symbol of symbols) {
                result[symbol.name] = mappingThisOnly && isIndependentMember(symbol) ? symbol : instantiateSymbol(symbol, mapper);
            }
            return result;
        }

        function addInheritedMembers(symbols: SymbolTable, baseSymbols: Symbol[]) {
            for (const s of baseSymbols) {
                if (!symbols[s.name]) {
                    symbols[s.name] = s;
                }
            }
        }

        function resolveDeclaredMembers(type: InterfaceType): InterfaceTypeWithDeclaredMembers {
            if (!(<InterfaceTypeWithDeclaredMembers>type).declaredProperties) {
                const symbol = type.symbol;
                (<InterfaceTypeWithDeclaredMembers>type).declaredProperties = getNamedMembers(symbol.members);
                (<InterfaceTypeWithDeclaredMembers>type).declaredCallSignatures = getSignaturesOfSymbol(symbol.members["__call"]);
                (<InterfaceTypeWithDeclaredMembers>type).declaredConstructSignatures = getSignaturesOfSymbol(symbol.members["__new"]);
                (<InterfaceTypeWithDeclaredMembers>type).declaredStringIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.String);
                (<InterfaceTypeWithDeclaredMembers>type).declaredNumberIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.Number);
            }
            return <InterfaceTypeWithDeclaredMembers>type;
        }

        function getTypeWithThisArgument(type: ObjectType, thisArgument?: Type) {
            if (type.flags & TypeFlags.Reference) {
                return createTypeReference((<TypeReference>type).target,
                    concatenate((<TypeReference>type).typeArguments, [thisArgument || (<TypeReference>type).target.thisType]));
            }
            if (type.flags & TypeFlags.Tuple) {
                return createTupleType((type as TupleType).elementTypes, thisArgument);
            }
            return type;
        }

        function resolveObjectTypeMembers(type: ObjectType, source: InterfaceTypeWithDeclaredMembers, typeParameters: TypeParameter[], typeArguments: Type[]) {
            let mapper = identityMapper;
            let members = source.symbol.members;
            let callSignatures = source.declaredCallSignatures;
            let constructSignatures = source.declaredConstructSignatures;
            let stringIndexInfo = source.declaredStringIndexInfo;
            let numberIndexInfo = source.declaredNumberIndexInfo;
            if (!rangeEquals(typeParameters, typeArguments, 0, typeParameters.length)) {
                mapper = createTypeMapper(typeParameters, typeArguments);
                members = createInstantiatedSymbolTable(source.declaredProperties, mapper, /*mappingThisOnly*/ typeParameters.length === 1);
                callSignatures = instantiateList(source.declaredCallSignatures, mapper, instantiateSignature);
                constructSignatures = instantiateList(source.declaredConstructSignatures, mapper, instantiateSignature);
                stringIndexInfo = instantiateIndexInfo(source.declaredStringIndexInfo, mapper);
                numberIndexInfo = instantiateIndexInfo(source.declaredNumberIndexInfo, mapper);
            }
            const baseTypes = getBaseTypes(source);
            if (baseTypes.length) {
                if (members === source.symbol.members) {
                    members = createSymbolTable(source.declaredProperties);
                }
                const thisArgument = lastOrUndefined(typeArguments);
                for (const baseType of baseTypes) {
                    const instantiatedBaseType = thisArgument ? getTypeWithThisArgument(instantiateType(baseType, mapper), thisArgument) : baseType;
                    addInheritedMembers(members, getPropertiesOfObjectType(instantiatedBaseType));
                    callSignatures = concatenate(callSignatures, getSignaturesOfType(instantiatedBaseType, SignatureKind.Call));
                    constructSignatures = concatenate(constructSignatures, getSignaturesOfType(instantiatedBaseType, SignatureKind.Construct));
                    stringIndexInfo = stringIndexInfo || getIndexInfoOfType(instantiatedBaseType, IndexKind.String);
                    numberIndexInfo = numberIndexInfo || getIndexInfoOfType(instantiatedBaseType, IndexKind.Number);
                }
            }
            setObjectTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function resolveClassOrInterfaceMembers(type: InterfaceType): void {
            resolveObjectTypeMembers(type, resolveDeclaredMembers(type), emptyArray, emptyArray);
        }

        function resolveTypeReferenceMembers(type: TypeReference): void {
            const source = resolveDeclaredMembers(type.target);
            const typeParameters = concatenate(source.typeParameters, [source.thisType]);
            const typeArguments = type.typeArguments && type.typeArguments.length === typeParameters.length ?
                type.typeArguments : concatenate(type.typeArguments, [type]);
            resolveObjectTypeMembers(type, source, typeParameters, typeArguments);
        }

        function createSignature(declaration: SignatureDeclaration, typeParameters: TypeParameter[], thisParameter: Symbol | undefined, parameters: Symbol[],
            resolvedReturnType: Type, typePredicate: TypePredicate, minArgumentCount: number, hasRestParameter: boolean, hasLiteralTypes: boolean): Signature {
            const sig = new Signature(checker);
            sig.declaration = declaration;
            sig.typeParameters = typeParameters;
            sig.parameters = parameters;
            sig.thisParameter = thisParameter;
            sig.resolvedReturnType = resolvedReturnType;
            sig.typePredicate = typePredicate;
            sig.minArgumentCount = minArgumentCount;
            sig.hasRestParameter = hasRestParameter;
            sig.hasLiteralTypes = hasLiteralTypes;
            return sig;
        }

        function cloneSignature(sig: Signature): Signature {
            return createSignature(sig.declaration, sig.typeParameters, sig.thisParameter, sig.parameters, sig.resolvedReturnType,
                sig.typePredicate, sig.minArgumentCount, sig.hasRestParameter, sig.hasLiteralTypes);
        }

        function getDefaultConstructSignatures(classType: InterfaceType): Signature[] {
            const baseConstructorType = getBaseConstructorTypeOfClass(classType);
            const baseSignatures = getSignaturesOfType(baseConstructorType, SignatureKind.Construct);
            if (baseSignatures.length === 0) {
                return [createSignature(undefined, classType.localTypeParameters, undefined, emptyArray, classType, /*typePredicate*/ undefined, 0, /*hasRestParameter*/ false, /*hasLiteralTypes*/ false)];
            }
            const baseTypeNode = getBaseTypeNodeOfClass(classType);
            const typeArguments = map(baseTypeNode.typeArguments, getTypeFromTypeNode);
            const typeArgCount = typeArguments ? typeArguments.length : 0;
            const result: Signature[] = [];
            for (const baseSig of baseSignatures) {
                const typeParamCount = baseSig.typeParameters ? baseSig.typeParameters.length : 0;
                if (typeParamCount === typeArgCount) {
                    const sig = typeParamCount ? getSignatureInstantiation(baseSig, typeArguments) : cloneSignature(baseSig);
                    sig.typeParameters = classType.localTypeParameters;
                    sig.resolvedReturnType = classType;
                    result.push(sig);
                }
            }
            return result;
        }

        function createTupleTypeMemberSymbols(memberTypes: Type[]): SymbolTable {
            const members = createMap<Symbol>();
            for (let i = 0; i < memberTypes.length; i++) {
                const symbol = <TransientSymbol>createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "" + i);
                symbol.type = memberTypes[i];
                members[i] = symbol;
            }
            return members;
        }

        function resolveTupleTypeMembers(type: TupleType) {
            const arrayElementType = getUnionType(type.elementTypes);
            // Make the tuple type itself the 'this' type by including an extra type argument
            // (Unless it's provided in the case that the tuple is a type parameter constraint)
            const arrayType = resolveStructuredTypeMembers(createTypeFromGenericGlobalType(globalArrayType, [arrayElementType, type.thisType || type]));
            const members = createTupleTypeMemberSymbols(type.elementTypes);
            addInheritedMembers(members, arrayType.properties);
            setObjectTypeMembers(type, members, arrayType.callSignatures, arrayType.constructSignatures, arrayType.stringIndexInfo, arrayType.numberIndexInfo);
        }

        function findMatchingSignature(signatureList: Signature[], signature: Signature, partialMatch: boolean, ignoreThisTypes: boolean, ignoreReturnTypes: boolean): Signature {
            for (const s of signatureList) {
                if (compareSignaturesIdentical(s, signature, partialMatch, ignoreThisTypes, ignoreReturnTypes, compareTypesIdentical)) {
                    return s;
                }
            }
        }

        function findMatchingSignatures(signatureLists: Signature[][], signature: Signature, listIndex: number): Signature[] {
            if (signature.typeParameters) {
                // We require an exact match for generic signatures, so we only return signatures from the first
                // signature list and only if they have exact matches in the other signature lists.
                if (listIndex > 0) {
                    return undefined;
                }
                for (let i = 1; i < signatureLists.length; i++) {
                    if (!findMatchingSignature(signatureLists[i], signature, /*partialMatch*/ false, /*ignoreThisTypes*/ false, /*ignoreReturnTypes*/ false)) {
                        return undefined;
                    }
                }
                return [signature];
            }
            let result: Signature[] = undefined;
            for (let i = 0; i < signatureLists.length; i++) {
                // Allow matching non-generic signatures to have excess parameters and different return types
                const match = i === listIndex ? signature : findMatchingSignature(signatureLists[i], signature, /*partialMatch*/ true, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true);
                if (!match) {
                    return undefined;
                }
                if (!contains(result, match)) {
                    (result || (result = [])).push(match);
                }
            }
            return result;
        }

        // The signatures of a union type are those signatures that are present in each of the constituent types.
        // Generic signatures must match exactly, but non-generic signatures are allowed to have extra optional
        // parameters and may differ in return types. When signatures differ in return types, the resulting return
        // type is the union of the constituent return types.
        function getUnionSignatures(types: Type[], kind: SignatureKind): Signature[] {
            const signatureLists = map(types, t => getSignaturesOfType(t, kind));
            let result: Signature[] = undefined;
            for (let i = 0; i < signatureLists.length; i++) {
                for (const signature of signatureLists[i]) {
                    // Only process signatures with parameter lists that aren't already in the result list
                    if (!result || !findMatchingSignature(result, signature, /*partialMatch*/ false, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true)) {
                        const unionSignatures = findMatchingSignatures(signatureLists, signature, i);
                        if (unionSignatures) {
                            let s = signature;
                            // Union the result types when more than one signature matches
                            if (unionSignatures.length > 1) {
                                s = cloneSignature(signature);
                                if (forEach(unionSignatures, sig => sig.thisParameter)) {
                                    const thisType = getUnionType(map(unionSignatures, sig => getTypeOfSymbol(sig.thisParameter) || anyType), /*subtypeReduction*/ true);
                                    s.thisParameter = createTransientSymbol(signature.thisParameter, thisType);
                                }
                                // Clear resolved return type we possibly got from cloneSignature
                                s.resolvedReturnType = undefined;
                                s.unionSignatures = unionSignatures;
                            }
                            (result || (result = [])).push(s);
                        }
                    }
                }
            }
            return result || emptyArray;
        }

        function getUnionIndexInfo(types: Type[], kind: IndexKind): IndexInfo {
            const indexTypes: Type[] = [];
            let isAnyReadonly = false;
            for (const type of types) {
                const indexInfo = getIndexInfoOfType(type, kind);
                if (!indexInfo) {
                    return undefined;
                }
                indexTypes.push(indexInfo.type);
                isAnyReadonly = isAnyReadonly || indexInfo.isReadonly;
            }
            return createIndexInfo(getUnionType(indexTypes, /*subtypeReduction*/ true), isAnyReadonly);
        }

        function resolveUnionTypeMembers(type: UnionType) {
            // The members and properties collections are empty for union types. To get all properties of a union
            // type use getPropertiesOfType (only the language service uses this).
            const callSignatures = getUnionSignatures(type.types, SignatureKind.Call);
            const constructSignatures = getUnionSignatures(type.types, SignatureKind.Construct);
            const stringIndexInfo = getUnionIndexInfo(type.types, IndexKind.String);
            const numberIndexInfo = getUnionIndexInfo(type.types, IndexKind.Number);
            setObjectTypeMembers(type, emptySymbols, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function intersectTypes(type1: Type, type2: Type): Type {
            return !type1 ? type2 : !type2 ? type1 : getIntersectionType([type1, type2]);
        }

        function intersectIndexInfos(info1: IndexInfo, info2: IndexInfo): IndexInfo {
            return !info1 ? info2 : !info2 ? info1 : createIndexInfo(
                getIntersectionType([info1.type, info2.type]), info1.isReadonly && info2.isReadonly);
        }

        function resolveIntersectionTypeMembers(type: IntersectionType) {
            // The members and properties collections are empty for intersection types. To get all properties of an
            // intersection type use getPropertiesOfType (only the language service uses this).
            let callSignatures: Signature[] = emptyArray;
            let constructSignatures: Signature[] = emptyArray;
            let stringIndexInfo: IndexInfo = undefined;
            let numberIndexInfo: IndexInfo = undefined;
            for (const t of type.types) {
                callSignatures = concatenate(callSignatures, getSignaturesOfType(t, SignatureKind.Call));
                constructSignatures = concatenate(constructSignatures, getSignaturesOfType(t, SignatureKind.Construct));
                stringIndexInfo = intersectIndexInfos(stringIndexInfo, getIndexInfoOfType(t, IndexKind.String));
                numberIndexInfo = intersectIndexInfos(numberIndexInfo, getIndexInfoOfType(t, IndexKind.Number));
            }
            setObjectTypeMembers(type, emptySymbols, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
        }

        function resolveAnonymousTypeMembers(type: AnonymousType) {
            const symbol = type.symbol;
            if (type.target) {
                const members = createInstantiatedSymbolTable(getPropertiesOfObjectType(type.target), type.mapper, /*mappingThisOnly*/ false);
                const callSignatures = instantiateList(getSignaturesOfType(type.target, SignatureKind.Call), type.mapper, instantiateSignature);
                const constructSignatures = instantiateList(getSignaturesOfType(type.target, SignatureKind.Construct), type.mapper, instantiateSignature);
                const stringIndexInfo = instantiateIndexInfo(getIndexInfoOfType(type.target, IndexKind.String), type.mapper);
                const numberIndexInfo = instantiateIndexInfo(getIndexInfoOfType(type.target, IndexKind.Number), type.mapper);
                setObjectTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
            }
            else if (symbol.flags & SymbolFlags.TypeLiteral) {
                const members = symbol.members;
                const callSignatures = getSignaturesOfSymbol(members["__call"]);
                const constructSignatures = getSignaturesOfSymbol(members["__new"]);
                const stringIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.String);
                const numberIndexInfo = getIndexInfoOfSymbol(symbol, IndexKind.Number);
                setObjectTypeMembers(type, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
            }
            else {
                // Combinations of function, class, enum and module
                let members = emptySymbols;
                let constructSignatures: Signature[] = emptyArray;
                if (symbol.flags & SymbolFlags.HasExports) {
                    members = getExportsOfSymbol(symbol);
                }
                if (symbol.flags & SymbolFlags.Class) {
                    const classType = getDeclaredTypeOfClassOrInterface(symbol);
                    constructSignatures = getSignaturesOfSymbol(symbol.members["__constructor"]);
                    if (!constructSignatures.length) {
                        constructSignatures = getDefaultConstructSignatures(classType);
                    }
                    const baseConstructorType = getBaseConstructorTypeOfClass(classType);
                    if (baseConstructorType.flags & TypeFlags.ObjectType) {
                        members = createSymbolTable(getNamedMembers(members));
                        addInheritedMembers(members, getPropertiesOfObjectType(baseConstructorType));
                    }
                }
                const numberIndexInfo = symbol.flags & SymbolFlags.Enum ? enumNumberIndexInfo : undefined;
                setObjectTypeMembers(type, members, emptyArray, constructSignatures, undefined, numberIndexInfo);
                // We resolve the members before computing the signatures because a signature may use
                // typeof with a qualified name expression that circularly references the type we are
                // in the process of resolving (see issue #6072). The temporarily empty signature list
                // will never be observed because a qualified name can't reference signatures.
                if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method)) {
                    (<ResolvedType>type).callSignatures = getSignaturesOfSymbol(symbol);
                }
            }
        }

        function resolveStructuredTypeMembers(type: ObjectType): ResolvedType {
            if (!(<ResolvedType>type).members) {
                if (type.flags & TypeFlags.Reference) {
                    resolveTypeReferenceMembers(<TypeReference>type);
                }
                else if (type.flags & (TypeFlags.Class | TypeFlags.Interface)) {
                    resolveClassOrInterfaceMembers(<InterfaceType>type);
                }
                else if (type.flags & TypeFlags.Anonymous) {
                    resolveAnonymousTypeMembers(<AnonymousType>type);
                }
                else if (type.flags & TypeFlags.Tuple) {
                    resolveTupleTypeMembers(<TupleType>type);
                }
                else if (type.flags & TypeFlags.Union) {
                    resolveUnionTypeMembers(<UnionType>type);
                }
                else if (type.flags & TypeFlags.Intersection) {
                    resolveIntersectionTypeMembers(<IntersectionType>type);
                }
            }
            return <ResolvedType>type;
        }

        /** Return properties of an object type or an empty array for other types */
        function getPropertiesOfObjectType(type: Type): Symbol[] {
            if (type.flags & TypeFlags.ObjectType) {
                return resolveStructuredTypeMembers(<ObjectType>type).properties;
            }
            return emptyArray;
        }

        /** If the given type is an object type and that type has a property by the given name,
         * return the symbol for that property. Otherwise return undefined. */
        function getPropertyOfObjectType(type: Type, name: string): Symbol {
            if (type.flags & TypeFlags.ObjectType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                const symbol = resolved.members[name];
                if (symbol && symbolIsValue(symbol)) {
                    return symbol;
                }
            }
        }

        function getPropertiesOfUnionOrIntersectionType(type: UnionOrIntersectionType): Symbol[] {
            for (const current of type.types) {
                for (const prop of getPropertiesOfType(current)) {
                    getPropertyOfUnionOrIntersectionType(type, prop.name);
                }
                // The properties of a union type are those that are present in all constituent types, so
                // we only need to check the properties of the first type
                if (type.flags & TypeFlags.Union) {
                    break;
                }
            }
            return type.resolvedProperties ? symbolsToArray(type.resolvedProperties) : emptyArray;
        }

        function getPropertiesOfType(type: Type): Symbol[] {
            type = getApparentType(type);
            return type.flags & TypeFlags.UnionOrIntersection ? getPropertiesOfUnionOrIntersectionType(<UnionType>type) : getPropertiesOfObjectType(type);
        }

        /**
         * The apparent type of a type parameter is the base constraint instantiated with the type parameter
         * as the type argument for the 'this' type.
         */
        function getApparentTypeOfTypeParameter(type: TypeParameter) {
            if (!type.resolvedApparentType) {
                let constraintType = getConstraintOfTypeParameter(type);
                while (constraintType && constraintType.flags & TypeFlags.TypeParameter) {
                    constraintType = getConstraintOfTypeParameter(<TypeParameter>constraintType);
                }
                type.resolvedApparentType = getTypeWithThisArgument(constraintType || emptyObjectType, type);
            }
            return type.resolvedApparentType;
        }

        /**
         * For a type parameter, return the base constraint of the type parameter. For the string, number,
         * boolean, and symbol primitive types, return the corresponding object types. Otherwise return the
         * type itself. Note that the apparent type of a union type is the union type itself.
         */
        function getApparentType(type: Type): Type {
            if (type.flags & TypeFlags.TypeParameter) {
                type = getApparentTypeOfTypeParameter(<TypeParameter>type);
            }
            if (type.flags & TypeFlags.StringLike) {
                type = globalStringType;
            }
            else if (type.flags & TypeFlags.NumberLike) {
                type = globalNumberType;
            }
            else if (type.flags & TypeFlags.BooleanLike) {
                type = globalBooleanType;
            }
            else if (type.flags & TypeFlags.ESSymbol) {
                type = getGlobalESSymbolType();
            }
            return type;
        }

        function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: string): Symbol {
            const types = containingType.types;
            let props: Symbol[];
            // Flags we want to propagate to the result if they exist in all source symbols
            let commonFlags = (containingType.flags & TypeFlags.Intersection) ? SymbolFlags.Optional : SymbolFlags.None;
            let isReadonly = false;
            for (const current of types) {
                const type = getApparentType(current);
                if (type !== unknownType) {
                    const prop = getPropertyOfType(type, name);
                    if (prop && !(getDeclarationFlagsFromSymbol(prop) & (NodeFlags.Private | NodeFlags.Protected))) {
                        commonFlags &= prop.flags;
                        if (!props) {
                            props = [prop];
                        }
                        else if (!contains(props, prop)) {
                            props.push(prop);
                        }
                        if (isReadonlySymbol(prop)) {
                            isReadonly = true;
                        }
                    }
                    else if (containingType.flags & TypeFlags.Union) {
                        // A union type requires the property to be present in all constituent types
                        return undefined;
                    }
                }
            }
            if (!props) {
                return undefined;
            }
            if (props.length === 1) {
                return props[0];
            }
            const propTypes: Type[] = [];
            const declarations: Declaration[] = [];
            let commonType: Type = undefined;
            let hasCommonType = true;
            for (const prop of props) {
                if (prop.declarations) {
                    addRange(declarations, prop.declarations);
                }
                const type = getTypeOfSymbol(prop);
                if (!commonType) {
                    commonType = type;
                }
                else if (type !== commonType) {
                    hasCommonType = false;
                }
                propTypes.push(getTypeOfSymbol(prop));
            }
            const result = <TransientSymbol>createSymbol(
                SymbolFlags.Property |
                SymbolFlags.Transient |
                SymbolFlags.SyntheticProperty |
                commonFlags,
                name);
            result.containingType = containingType;
            result.hasCommonType = hasCommonType;
            result.declarations = declarations;
            result.isReadonly = isReadonly;
            result.type = containingType.flags & TypeFlags.Union ? getUnionType(propTypes) : getIntersectionType(propTypes);
            return result;
        }

        function getPropertyOfUnionOrIntersectionType(type: UnionOrIntersectionType, name: string): Symbol {
            const properties = type.resolvedProperties || (type.resolvedProperties = createMap<Symbol>());
            let property = properties[name];
            if (!property) {
                property = createUnionOrIntersectionProperty(type, name);
                if (property) {
                    properties[name] = property;
                }
            }
            return property;
        }

        /**
         * Return the symbol for the property with the given name in the given type. Creates synthetic union properties when
         * necessary, maps primitive types and type parameters are to their apparent types, and augments with properties from
         * Object and Function as appropriate.
         *
         * @param type a type to look up property from
         * @param name a name of property to look up in a given type
         */
        function getPropertyOfType(type: Type, name: string): Symbol {
            type = getApparentType(type);
            if (type.flags & TypeFlags.ObjectType) {
                const resolved = resolveStructuredTypeMembers(type);
                const symbol = resolved.members[name];
                if (symbol && symbolIsValue(symbol)) {
                    return symbol;
                }
                if (resolved === anyFunctionType || resolved.callSignatures.length || resolved.constructSignatures.length) {
                    const symbol = getPropertyOfObjectType(globalFunctionType, name);
                    if (symbol) {
                        return symbol;
                    }
                }
                return getPropertyOfObjectType(globalObjectType, name);
            }
            if (type.flags & TypeFlags.UnionOrIntersection) {
                return getPropertyOfUnionOrIntersectionType(<UnionOrIntersectionType>type, name);
            }
            return undefined;
        }

        function getSignaturesOfStructuredType(type: Type, kind: SignatureKind): Signature[] {
            if (type.flags & TypeFlags.StructuredType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return kind === SignatureKind.Call ? resolved.callSignatures : resolved.constructSignatures;
            }
            return emptyArray;
        }

        /**
         * Return the signatures of the given kind in the given type. Creates synthetic union signatures when necessary and
         * maps primitive types and type parameters are to their apparent types.
         */
        function getSignaturesOfType(type: Type, kind: SignatureKind): Signature[] {
            return getSignaturesOfStructuredType(getApparentType(type), kind);
        }

        function getIndexInfoOfStructuredType(type: Type, kind: IndexKind): IndexInfo {
            if (type.flags & TypeFlags.StructuredType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                return kind === IndexKind.String ? resolved.stringIndexInfo : resolved.numberIndexInfo;
            }
        }

        function getIndexTypeOfStructuredType(type: Type, kind: IndexKind): Type {
            const info = getIndexInfoOfStructuredType(type, kind);
            return info && info.type;
        }

        // Return the indexing info of the given kind in the given type. Creates synthetic union index types when necessary and
        // maps primitive types and type parameters are to their apparent types.
        function getIndexInfoOfType(type: Type, kind: IndexKind): IndexInfo {
            return getIndexInfoOfStructuredType(getApparentType(type), kind);
        }

        // Return the index type of the given kind in the given type. Creates synthetic union index types when necessary and
        // maps primitive types and type parameters are to their apparent types.
        function getIndexTypeOfType(type: Type, kind: IndexKind): Type {
            return getIndexTypeOfStructuredType(getApparentType(type), kind);
        }

        function getImplicitIndexTypeOfType(type: Type, kind: IndexKind): Type {
            if (isObjectLiteralType(type)) {
                const propTypes: Type[] = [];
                for (const prop of getPropertiesOfType(type)) {
                    if (kind === IndexKind.String || isNumericLiteralName(prop.name)) {
                        propTypes.push(getTypeOfSymbol(prop));
                    }
                }
                if (propTypes.length) {
                    return getUnionType(propTypes, /*subtypeReduction*/ true);
                }
            }
            return undefined;
        }

        function getTypeParametersFromJSDocTemplate(declaration: SignatureDeclaration): TypeParameter[] {
            if (declaration.flags & NodeFlags.JavaScriptFile) {
                const templateTag = getJSDocTemplateTag(declaration);
                if (templateTag) {
                    return getTypeParametersFromDeclaration(templateTag.typeParameters);
                }
            }

            return undefined;
        }

        // Return list of type parameters with duplicates removed (duplicate identifier errors are generated in the actual
        // type checking functions).
        function getTypeParametersFromDeclaration(typeParameterDeclarations: TypeParameterDeclaration[]): TypeParameter[] {
            const result: TypeParameter[] = [];
            forEach(typeParameterDeclarations, node => {
                const tp = getDeclaredTypeOfTypeParameter(node.symbol);
                if (!contains(result, tp)) {
                    result.push(tp);
                }
            });
            return result;
        }

        function symbolsToArray(symbols: SymbolTable): Symbol[] {
            const result: Symbol[] = [];
            for (const id in symbols) {
                if (!isReservedMemberName(id)) {
                    result.push(symbols[id]);
                }
            }
            return result;
        }

        function isJSDocOptionalParameter(node: ParameterDeclaration) {
            if (node.flags & NodeFlags.JavaScriptFile) {
                if (node.type && node.type.kind === SyntaxKind.JSDocOptionalType) {
                    return true;
                }

                const paramTag = getCorrespondingJSDocParameterTag(node);
                if (paramTag) {
                    if (paramTag.isBracketed) {
                        return true;
                    }

                    if (paramTag.typeExpression) {
                        return paramTag.typeExpression.type.kind === SyntaxKind.JSDocOptionalType;
                    }
                }
            }
        }

        function isOptionalParameter(node: ParameterDeclaration) {
            if (hasQuestionToken(node) || isJSDocOptionalParameter(node)) {
                return true;
            }

            if (node.initializer) {
                const signatureDeclaration = <SignatureDeclaration>node.parent;
                const signature = getSignatureFromDeclaration(signatureDeclaration);
                const parameterIndex = ts.indexOf(signatureDeclaration.parameters, node);
                Debug.assert(parameterIndex >= 0);
                return parameterIndex >= signature.minArgumentCount;
            }

            return false;
        }

        function createTypePredicateFromTypePredicateNode(node: TypePredicateNode): IdentifierTypePredicate | ThisTypePredicate {
            if (node.parameterName.kind === SyntaxKind.Identifier) {
                const parameterName = node.parameterName as Identifier;
                return {
                    kind: TypePredicateKind.Identifier,
                    parameterName: parameterName ? parameterName.text : undefined,
                    parameterIndex: parameterName ? getTypePredicateParameterIndex((node.parent as SignatureDeclaration).parameters, parameterName) : undefined,
                    type: getTypeFromTypeNode(node.type)
                } as IdentifierTypePredicate;
            }
            else {
                return {
                    kind: TypePredicateKind.This,
                    type: getTypeFromTypeNode(node.type)
                } as ThisTypePredicate;
            }
        }

        function getSignatureFromDeclaration(declaration: SignatureDeclaration): Signature {
            const links = getNodeLinks(declaration);
            if (!links.resolvedSignature) {
                const parameters: Symbol[] = [];
                let hasLiteralTypes = false;
                let minArgumentCount = -1;
                let thisParameter: Symbol = undefined;
                let hasThisParameter: boolean;
                const isJSConstructSignature = isJSDocConstructSignature(declaration);

                // If this is a JSDoc construct signature, then skip the first parameter in the
                // parameter list.  The first parameter represents the return type of the construct
                // signature.
                for (let i = isJSConstructSignature ? 1 : 0, n = declaration.parameters.length; i < n; i++) {
                    const param = declaration.parameters[i];

                    let paramSymbol = param.symbol;
                    // Include parameter symbol instead of property symbol in the signature
                    if (paramSymbol && !!(paramSymbol.flags & SymbolFlags.Property) && !isBindingPattern(param.name)) {
                        const resolvedSymbol = resolveName(param, paramSymbol.name, SymbolFlags.Value, undefined, undefined);
                        paramSymbol = resolvedSymbol;
                    }
                    if (i === 0 && paramSymbol.name === "this") {
                        hasThisParameter = true;
                        thisParameter = param.symbol;
                    }
                    else {
                        parameters.push(paramSymbol);
                    }

                    if (param.type && param.type.kind === SyntaxKind.LiteralType) {
                        hasLiteralTypes = true;
                    }

                    if (param.initializer || param.questionToken || param.dotDotDotToken || isJSDocOptionalParameter(param)) {
                        if (minArgumentCount < 0) {
                            minArgumentCount = i - (hasThisParameter ? 1 : 0);
                        }
                    }
                    else {
                        // If we see any required parameters, it means the prior ones were not in fact optional.
                        minArgumentCount = -1;
                    }
                }

                // If only one accessor includes a this-type annotation, the other behaves as if it had the same type annotation
                if ((declaration.kind === SyntaxKind.GetAccessor || declaration.kind === SyntaxKind.SetAccessor) &&
                    !hasDynamicName(declaration) &&
                    (!hasThisParameter || !thisParameter)) {
                    const otherKind = declaration.kind === SyntaxKind.GetAccessor ? SyntaxKind.SetAccessor : SyntaxKind.GetAccessor;
                    const other = <AccessorDeclaration>getDeclarationOfKind(declaration.symbol, otherKind);
                    if (other) {
                        thisParameter = getAnnotatedAccessorThisParameter(other);
                    }
                }

                if (minArgumentCount < 0) {
                    minArgumentCount = declaration.parameters.length - (hasThisParameter ? 1 : 0);
                }
                if (isJSConstructSignature) {
                    minArgumentCount--;
                }

                const classType = declaration.kind === SyntaxKind.Constructor ?
                    getDeclaredTypeOfClassOrInterface(getMergedSymbol((<ClassDeclaration>declaration.parent).symbol))
                    : undefined;
                const typeParameters = classType ? classType.localTypeParameters :
                    declaration.typeParameters ? getTypeParametersFromDeclaration(declaration.typeParameters) :
                        getTypeParametersFromJSDocTemplate(declaration);
                const returnType = getSignatureReturnTypeFromDeclaration(declaration, minArgumentCount, isJSConstructSignature, classType);
                const typePredicate = declaration.type && declaration.type.kind === SyntaxKind.TypePredicate ?
                    createTypePredicateFromTypePredicateNode(declaration.type as TypePredicateNode) :
                    undefined;

                links.resolvedSignature = createSignature(declaration, typeParameters, thisParameter, parameters, returnType, typePredicate, minArgumentCount, hasRestParameter(declaration), hasLiteralTypes);
            }
            return links.resolvedSignature;
        }

        function getSignatureReturnTypeFromDeclaration(declaration: SignatureDeclaration, minArgumentCount: number, isJSConstructSignature: boolean, classType: Type) {
            if (isJSConstructSignature) {
                return getTypeFromTypeNode(declaration.parameters[0].type);
            }
            else if (classType) {
                return classType;
            }
            else if (declaration.type) {
                return getTypeFromTypeNode(declaration.type);
            }

            if (declaration.flags & NodeFlags.JavaScriptFile) {
                const type = getReturnTypeFromJSDocComment(declaration);
                if (type && type !== unknownType) {
                    return type;
                }
            }

            // TypeScript 1.0 spec (April 2014):
            // If only one accessor includes a type annotation, the other behaves as if it had the same type annotation.
            if (declaration.kind === SyntaxKind.GetAccessor && !hasDynamicName(declaration)) {
                const setter = <AccessorDeclaration>getDeclarationOfKind(declaration.symbol, SyntaxKind.SetAccessor);
                return getAnnotatedAccessorType(setter);
            }

            if (nodeIsMissing((<FunctionLikeDeclaration>declaration).body)) {
                return anyType;
            }
        }

        function getSignaturesOfSymbol(symbol: Symbol): Signature[] {
            if (!symbol) return emptyArray;
            const result: Signature[] = [];
            for (let i = 0, len = symbol.declarations.length; i < len; i++) {
                const node = symbol.declarations[i];
                switch (node.kind) {
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.Constructor:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.IndexSignature:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.JSDocFunctionType:
                        // Don't include signature if node is the implementation of an overloaded function. A node is considered
                        // an implementation node if it has a body and the previous node is of the same kind and immediately
                        // precedes the implementation node (i.e. has the same parent and ends where the implementation starts).
                        if (i > 0 && (<FunctionLikeDeclaration>node).body) {
                            const previous = symbol.declarations[i - 1];
                            if (node.parent === previous.parent && node.kind === previous.kind && node.pos === previous.end) {
                                break;
                            }
                        }
                        result.push(getSignatureFromDeclaration(<SignatureDeclaration>node));
                }
            }
            return result;
        }

        function resolveExternalModuleTypeByLiteral(name: StringLiteral) {
            const moduleSym = resolveExternalModuleName(name, name);
            if (moduleSym) {
                const resolvedModuleSymbol = resolveExternalModuleSymbol(moduleSym);
                if (resolvedModuleSymbol) {
                    return getTypeOfSymbol(resolvedModuleSymbol);
                }
            }

            return anyType;
        }

        function getThisTypeOfSignature(signature: Signature): Type | undefined {
            if (signature.thisParameter) {
                return getTypeOfSymbol(signature.thisParameter);
            }
        }

        function getReturnTypeOfSignature(signature: Signature): Type {
            if (!signature.resolvedReturnType) {
                if (!pushTypeResolution(signature, TypeSystemPropertyName.ResolvedReturnType)) {
                    return unknownType;
                }
                let type: Type;
                if (signature.target) {
                    type = instantiateType(getReturnTypeOfSignature(signature.target), signature.mapper);
                }
                else if (signature.unionSignatures) {
                    type = getUnionType(map(signature.unionSignatures, getReturnTypeOfSignature), /*subtypeReduction*/ true);
                }
                else {
                    type = getReturnTypeFromBody(<FunctionLikeDeclaration>signature.declaration);
                }
                if (!popTypeResolution()) {
                    type = anyType;
                    if (compilerOptions.noImplicitAny) {
                        const declaration = <Declaration>signature.declaration;
                        if (declaration.name) {
                            error(declaration.name, Diagnostics._0_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions, declarationNameToString(declaration.name));
                        }
                        else {
                            error(declaration, Diagnostics.Function_implicitly_has_return_type_any_because_it_does_not_have_a_return_type_annotation_and_is_referenced_directly_or_indirectly_in_one_of_its_return_expressions);
                        }
                    }
                }
                signature.resolvedReturnType = type;
            }
            return signature.resolvedReturnType;
        }

        function getRestTypeOfSignature(signature: Signature): Type {
            if (signature.hasRestParameter) {
                const type = getTypeOfSymbol(lastOrUndefined(signature.parameters));
                if (type.flags & TypeFlags.Reference && (<TypeReference>type).target === globalArrayType) {
                    return (<TypeReference>type).typeArguments[0];
                }
            }
            return anyType;
        }

        function getSignatureInstantiation(signature: Signature, typeArguments: Type[]): Signature {
            return instantiateSignature(signature, createTypeMapper(signature.typeParameters, typeArguments), /*eraseTypeParameters*/ true);
        }

        function getErasedSignature(signature: Signature): Signature {
            if (!signature.typeParameters) return signature;
            if (!signature.erasedSignatureCache) {
                signature.erasedSignatureCache = instantiateSignature(signature, createTypeEraser(signature.typeParameters), /*eraseTypeParameters*/ true);
            }
            return signature.erasedSignatureCache;
        }

        function getOrCreateTypeFromSignature(signature: Signature): ObjectType {
            // There are two ways to declare a construct signature, one is by declaring a class constructor
            // using the constructor keyword, and the other is declaring a bare construct signature in an
            // object type literal or interface (using the new keyword). Each way of declaring a constructor
            // will result in a different declaration kind.
            if (!signature.isolatedSignatureType) {
                const isConstructor = signature.declaration.kind === SyntaxKind.Constructor || signature.declaration.kind === SyntaxKind.ConstructSignature;
                const type = <ResolvedType>createObjectType(TypeFlags.Anonymous);
                type.members = emptySymbols;
                type.properties = emptyArray;
                type.callSignatures = !isConstructor ? [signature] : emptyArray;
                type.constructSignatures = isConstructor ? [signature] : emptyArray;
                signature.isolatedSignatureType = type;
            }

            return signature.isolatedSignatureType;
        }

        function getIndexSymbol(symbol: Symbol): Symbol {
            return symbol.members["__index"];
        }

        function getIndexDeclarationOfSymbol(symbol: Symbol, kind: IndexKind): SignatureDeclaration {
            const syntaxKind = kind === IndexKind.Number ? SyntaxKind.NumberKeyword : SyntaxKind.StringKeyword;
            const indexSymbol = getIndexSymbol(symbol);
            if (indexSymbol) {
                for (const decl of indexSymbol.declarations) {
                    const node = <SignatureDeclaration>decl;
                    if (node.parameters.length === 1) {
                        const parameter = node.parameters[0];
                        if (parameter && parameter.type && parameter.type.kind === syntaxKind) {
                            return node;
                        }
                    }
                }
            }

            return undefined;
        }

        function createIndexInfo(type: Type, isReadonly: boolean, declaration?: SignatureDeclaration): IndexInfo {
            return { type, isReadonly, declaration };
        }

        function getIndexInfoOfSymbol(symbol: Symbol, kind: IndexKind): IndexInfo {
            const declaration = getIndexDeclarationOfSymbol(symbol, kind);
            if (declaration) {
                return createIndexInfo(declaration.type ? getTypeFromTypeNode(declaration.type) : anyType,
                    (declaration.flags & NodeFlags.Readonly) !== 0, declaration);
            }
            return undefined;
        }

        function getConstraintDeclaration(type: TypeParameter) {
            return (<TypeParameterDeclaration>getDeclarationOfKind(type.symbol, SyntaxKind.TypeParameter)).constraint;
        }

        function hasConstraintReferenceTo(type: Type, target: TypeParameter): boolean {
            let checked: Type[];
            while (type && !(type.flags & TypeFlags.ThisType) && type.flags & TypeFlags.TypeParameter && !contains(checked, type)) {
                if (type === target) {
                    return true;
                }
                (checked || (checked = [])).push(type);
                const constraintDeclaration = getConstraintDeclaration(<TypeParameter>type);
                type = constraintDeclaration && getTypeFromTypeNode(constraintDeclaration);
            }
            return false;
        }

        function getConstraintOfTypeParameter(typeParameter: TypeParameter): Type {
            if (!typeParameter.constraint) {
                if (typeParameter.target) {
                    const targetConstraint = getConstraintOfTypeParameter(typeParameter.target);
                    typeParameter.constraint = targetConstraint ? instantiateType(targetConstraint, typeParameter.mapper) : noConstraintType;
                }
                else {
                    const constraintDeclaration = getConstraintDeclaration(typeParameter);
                    let constraint = getTypeFromTypeNode(constraintDeclaration);
                    if (hasConstraintReferenceTo(constraint, typeParameter)) {
                        error(constraintDeclaration, Diagnostics.Type_parameter_0_has_a_circular_constraint, typeToString(typeParameter));
                        constraint = unknownType;
                    }
                    typeParameter.constraint = constraint;
                }
            }
            return typeParameter.constraint === noConstraintType ? undefined : typeParameter.constraint;
        }

        function getParentSymbolOfTypeParameter(typeParameter: TypeParameter): Symbol {
            return getSymbolOfNode(getDeclarationOfKind(typeParameter.symbol, SyntaxKind.TypeParameter).parent);
        }

        function getTypeListId(types: Type[]) {
            let result = "";
            if (types) {
                const length = types.length;
                let i = 0;
                while (i < length) {
                    const startId = types[i].id;
                    let count = 1;
                    while (i + count < length && types[i + count].id === startId + count) {
                        count++;
                    }
                    if (result.length) {
                        result += ",";
                    }
                    result += startId;
                    if (count > 1) {
                        result += ":" + count;
                    }
                    i += count;
                }
            }
            return result;
        }

        // This function is used to propagate certain flags when creating new object type references and union types.
        // It is only necessary to do so if a constituent type might be the undefined type, the null type, the type
        // of an object literal or the anyFunctionType. This is because there are operations in the type checker
        // that care about the presence of such types at arbitrary depth in a containing type.
        function getPropagatingFlagsOfTypes(types: Type[], excludeKinds: TypeFlags): TypeFlags {
            let result: TypeFlags = 0;
            for (const type of types) {
                if (!(type.flags & excludeKinds)) {
                    result |= type.flags;
                }
            }
            return result & TypeFlags.PropagatingFlags;
        }

        function createTypeReference(target: GenericType, typeArguments: Type[]): TypeReference {
            const id = getTypeListId(typeArguments);
            let type = target.instantiations[id];
            if (!type) {
                const propagatedFlags = typeArguments ? getPropagatingFlagsOfTypes(typeArguments, /*excludeKinds*/ 0) : 0;
                const flags = TypeFlags.Reference | propagatedFlags;
                type = target.instantiations[id] = <TypeReference>createObjectType(flags, target.symbol);
                type.target = target;
                type.typeArguments = typeArguments;
            }
            return type;
        }

        // Get type from reference to class or interface
        function getTypeFromClassOrInterfaceReference(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference, symbol: Symbol): Type {
            const type = <InterfaceType>getDeclaredTypeOfSymbol(getMergedSymbol(symbol));
            const typeParameters = type.localTypeParameters;
            if (typeParameters) {
                if (!node.typeArguments || node.typeArguments.length !== typeParameters.length) {
                    error(node, Diagnostics.Generic_type_0_requires_1_type_argument_s, typeToString(type, /*enclosingDeclaration*/ undefined, TypeFormatFlags.WriteArrayAsGenericType), typeParameters.length);
                    return unknownType;
                }
                // In a type reference, the outer type parameters of the referenced class or interface are automatically
                // supplied as type arguments and the type reference only specifies arguments for the local type parameters
                // of the class or interface.
                return createTypeReference(<GenericType>type, concatenate(type.outerTypeParameters, map(node.typeArguments, getTypeFromTypeNode)));
            }
            if (node.typeArguments) {
                error(node, Diagnostics.Type_0_is_not_generic, typeToString(type));
                return unknownType;
            }
            return type;
        }

        // Get type from reference to type alias. When a type alias is generic, the declared type of the type alias may include
        // references to the type parameters of the alias. We replace those with the actual type arguments by instantiating the
        // declared type. Instantiations are cached using the type identities of the type arguments as the key.
        function getTypeFromTypeAliasReference(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference, symbol: Symbol): Type {
            const type = getDeclaredTypeOfSymbol(symbol);
            const links = getSymbolLinks(symbol);
            const typeParameters = links.typeParameters;
            if (typeParameters) {
                if (!node.typeArguments || node.typeArguments.length !== typeParameters.length) {
                    error(node, Diagnostics.Generic_type_0_requires_1_type_argument_s, symbolToString(symbol), typeParameters.length);
                    return unknownType;
                }
                const typeArguments = map(node.typeArguments, getTypeFromTypeNode);
                const id = getTypeListId(typeArguments);
                return links.instantiations[id] || (links.instantiations[id] = instantiateType(type, createTypeMapper(typeParameters, typeArguments)));
            }
            if (node.typeArguments) {
                error(node, Diagnostics.Type_0_is_not_generic, symbolToString(symbol));
                return unknownType;
            }
            return type;
        }

        // Get type from reference to named type that cannot be generic (enum or type parameter)
        function getTypeFromNonGenericTypeReference(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference, symbol: Symbol): Type {
            if (node.typeArguments) {
                error(node, Diagnostics.Type_0_is_not_generic, symbolToString(symbol));
                return unknownType;
            }
            return getDeclaredTypeOfSymbol(symbol);
        }

        function getTypeReferenceName(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference): EntityNameOrEntityNameExpression | undefined {
            switch (node.kind) {
                case SyntaxKind.TypeReference:
                    return (<TypeReferenceNode>node).typeName;
                case SyntaxKind.JSDocTypeReference:
                    return (<JSDocTypeReference>node).name;
                case SyntaxKind.ExpressionWithTypeArguments:
                    // We only support expressions that are simple qualified names. For other
                    // expressions this produces undefined.
                    const expr = (<ExpressionWithTypeArguments>node).expression;
                    if (isEntityNameExpression(expr)) {
                        return expr;
                    }

                // fall through;
            }

            return undefined;
        }

        function resolveTypeReferenceName(
            node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference,
            typeReferenceName: EntityNameExpression | EntityName) {

            if (!typeReferenceName) {
                return unknownSymbol;
            }

            return resolveEntityName(typeReferenceName, SymbolFlags.Type) || unknownSymbol;
        }

        function getTypeReferenceType(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference, symbol: Symbol) {
            if (symbol === unknownSymbol) {
                return unknownType;
            }

            if (symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                return getTypeFromClassOrInterfaceReference(node, symbol);
            }

            if (symbol.flags & SymbolFlags.TypeAlias) {
                return getTypeFromTypeAliasReference(node, symbol);
            }

            if (symbol.flags & SymbolFlags.Value && node.kind === SyntaxKind.JSDocTypeReference) {
                // A JSDocTypeReference may have resolved to a value (as opposed to a type). In
                // that case, the type of this reference is just the type of the value we resolved
                // to.
                return getTypeOfSymbol(symbol);
            }

            return getTypeFromNonGenericTypeReference(node, symbol);
        }

        function getTypeFromTypeReference(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                let symbol: Symbol;
                let type: Type;
                if (node.kind === SyntaxKind.JSDocTypeReference) {
                    const typeReferenceName = getTypeReferenceName(node);
                    symbol = resolveTypeReferenceName(node, typeReferenceName);
                    type = getTypeReferenceType(node, symbol);
                }
                else {
                    // We only support expressions that are simple qualified names. For other expressions this produces undefined.
                    const typeNameOrExpression: EntityNameOrEntityNameExpression = node.kind === SyntaxKind.TypeReference
                        ? (<TypeReferenceNode>node).typeName
                        : isEntityNameExpression((<ExpressionWithTypeArguments>node).expression)
                            ? <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression
                            : undefined;
                    symbol = typeNameOrExpression && resolveEntityName(typeNameOrExpression, SymbolFlags.Type) || unknownSymbol;
                    type = symbol === unknownSymbol ? unknownType :
                        symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface) ? getTypeFromClassOrInterfaceReference(node, symbol) :
                            symbol.flags & SymbolFlags.TypeAlias ? getTypeFromTypeAliasReference(node, symbol) :
                                getTypeFromNonGenericTypeReference(node, symbol);
                }
                // Cache both the resolved symbol and the resolved type. The resolved symbol is needed in when we check the
                // type reference in checkTypeReferenceOrExpressionWithTypeArguments.
                links.resolvedSymbol = symbol;
                links.resolvedType = type;
            }
            return links.resolvedType;
        }

        function getTypeFromTypeQueryNode(node: TypeQueryNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                // TypeScript 1.0 spec (April 2014): 3.6.3
                // The expression is processed as an identifier expression (section 4.3)
                // or property access expression(section 4.10),
                // the widened type(section 3.9) of which becomes the result.
                links.resolvedType = getWidenedType(checkExpression(node.exprName));
            }
            return links.resolvedType;
        }

        function getTypeOfGlobalSymbol(symbol: Symbol, arity: number): ObjectType {

            function getTypeDeclaration(symbol: Symbol): Declaration {
                const declarations = symbol.declarations;
                for (const declaration of declarations) {
                    switch (declaration.kind) {
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.InterfaceDeclaration:
                        case SyntaxKind.EnumDeclaration:
                            return declaration;
                    }
                }
            }

            if (!symbol) {
                return arity ? emptyGenericType : emptyObjectType;
            }
            const type = getDeclaredTypeOfSymbol(symbol);
            if (!(type.flags & TypeFlags.ObjectType)) {
                error(getTypeDeclaration(symbol), Diagnostics.Global_type_0_must_be_a_class_or_interface_type, symbol.name);
                return arity ? emptyGenericType : emptyObjectType;
            }
            if (((<InterfaceType>type).typeParameters ? (<InterfaceType>type).typeParameters.length : 0) !== arity) {
                error(getTypeDeclaration(symbol), Diagnostics.Global_type_0_must_have_1_type_parameter_s, symbol.name, arity);
                return arity ? emptyGenericType : emptyObjectType;
            }
            return <ObjectType>type;
        }

        function getGlobalValueSymbol(name: string): Symbol {
            return getGlobalSymbol(name, SymbolFlags.Value, Diagnostics.Cannot_find_global_value_0);
        }

        function getGlobalTypeSymbol(name: string): Symbol {
            return getGlobalSymbol(name, SymbolFlags.Type, Diagnostics.Cannot_find_global_type_0);
        }

        function getGlobalSymbol(name: string, meaning: SymbolFlags, diagnostic: DiagnosticMessage): Symbol {
            return resolveName(undefined, name, meaning, diagnostic, name);
        }

        function getGlobalType(name: string, arity = 0): ObjectType {
            return getTypeOfGlobalSymbol(getGlobalTypeSymbol(name), arity);
        }

        /**
         * Returns a type that is inside a namespace at the global scope, e.g.
         * getExportedTypeFromNamespace('JSX', 'Element') returns the JSX.Element type
         */
        function getExportedTypeFromNamespace(namespace: string, name: string): Type {
            const namespaceSymbol = getGlobalSymbol(namespace, SymbolFlags.Namespace, /*diagnosticMessage*/ undefined);
            const typeSymbol = namespaceSymbol && getSymbol(namespaceSymbol.exports, name, SymbolFlags.Type);
            return typeSymbol && getDeclaredTypeOfSymbol(typeSymbol);
        }

        /**
          * Creates a TypeReference for a generic `TypedPropertyDescriptor<T>`.
          */
        function createTypedPropertyDescriptorType(propertyType: Type): Type {
            const globalTypedPropertyDescriptorType = getGlobalTypedPropertyDescriptorType();
            return globalTypedPropertyDescriptorType !== emptyGenericType
                ? createTypeReference(<GenericType>globalTypedPropertyDescriptorType, [propertyType])
                : emptyObjectType;
        }

        /**
         * Instantiates a global type that is generic with some element type, and returns that instantiation.
         */
        function createTypeFromGenericGlobalType(genericGlobalType: GenericType, typeArguments: Type[]): Type {
            return genericGlobalType !== emptyGenericType ? createTypeReference(genericGlobalType, typeArguments) : emptyObjectType;
        }

        function createIterableType(elementType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalIterableType(), [elementType]);
        }

        function createIterableIteratorType(elementType: Type): Type {
            return createTypeFromGenericGlobalType(getGlobalIterableIteratorType(), [elementType]);
        }

        function createArrayType(elementType: Type): Type {
            return createTypeFromGenericGlobalType(globalArrayType, [elementType]);
        }

        function getTypeFromArrayTypeNode(node: ArrayTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = createArrayType(getTypeFromTypeNode(node.elementType));
            }
            return links.resolvedType;
        }

        function createTupleType(elementTypes: Type[], thisType?: Type) {
            const id = getTypeListId(elementTypes) + "," + (thisType ? thisType.id : 0);
            return tupleTypes[id] || (tupleTypes[id] = createNewTupleType(elementTypes, thisType));
        }

        function createNewTupleType(elementTypes: Type[], thisType?: Type) {
            const propagatedFlags = getPropagatingFlagsOfTypes(elementTypes, /*excludeKinds*/ 0);
            const type = <TupleType>createObjectType(TypeFlags.Tuple | propagatedFlags);
            type.elementTypes = elementTypes;
            type.thisType = thisType;
            return type;
        }

        function getTypeFromTupleTypeNode(node: TupleTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = createTupleType(map(node.elementTypes, getTypeFromTypeNode));
            }
            return links.resolvedType;
        }

        interface TypeSet extends Array<Type> {
            containsAny?: boolean;
            containsUndefined?: boolean;
            containsNull?: boolean;
            containsNonWideningType?: boolean;
        }

        function binarySearchTypes(types: Type[], type: Type): number {
            let low = 0;
            let high = types.length - 1;
            const typeId = type.id;
            while (low <= high) {
                const middle = low + ((high - low) >> 1);
                const id = types[middle].id;
                if (id === typeId) {
                    return middle;
                }
                else if (id > typeId) {
                    high = middle - 1;
                }
                else {
                    low = middle + 1;
                }
            }
            return ~low;
        }

        function containsType(types: Type[], type: Type): boolean {
            return binarySearchTypes(types, type) >= 0;
        }

        function addTypeToUnion(typeSet: TypeSet, type: Type) {
            if (type.flags & TypeFlags.Union) {
                addTypesToUnion(typeSet, (<UnionType>type).types);
            }
            else if (type.flags & TypeFlags.Any) {
                typeSet.containsAny = true;
            }
            else if (!strictNullChecks && type.flags & TypeFlags.Nullable) {
                if (type.flags & TypeFlags.Undefined) typeSet.containsUndefined = true;
                if (type.flags & TypeFlags.Null) typeSet.containsNull = true;
                if (!(type.flags & TypeFlags.ContainsWideningType)) typeSet.containsNonWideningType = true;
            }
            else if (!(type.flags & TypeFlags.Never)) {
                const len = typeSet.length;
                const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearchTypes(typeSet, type);
                if (index < 0) {
                    if (!(type.flags & TypeFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method) && containsIdenticalType(typeSet, type))) {
                        typeSet.splice(~index, 0, type);
                    }
                }
            }
        }

        // Add the given types to the given type set. Order is preserved, duplicates are removed,
        // and nested types of the given kind are flattened into the set.
        function addTypesToUnion(typeSet: TypeSet, types: Type[]) {
            for (const type of types) {
                addTypeToUnion(typeSet, type);
            }
        }

        function containsIdenticalType(types: Type[], type: Type) {
            for (const t of types) {
                if (isTypeIdenticalTo(t, type)) {
                    return true;
                }
            }
            return false;
        }

        function isSubtypeOfAny(candidate: Type, types: Type[]): boolean {
            for (let i = 0, len = types.length; i < len; i++) {
                if (candidate !== types[i] && isTypeSubtypeOf(candidate, types[i])) {
                    return true;
                }
            }
            return false;
        }

        function removeSubtypes(types: Type[]) {
            let i = types.length;
            while (i > 0) {
                i--;
                if (isSubtypeOfAny(types[i], types)) {
                    types.splice(i, 1);
                }
            }
        }

        // We sort and deduplicate the constituent types based on object identity. If the subtypeReduction
        // flag is specified we also reduce the constituent type set to only include types that aren't subtypes
        // of other types. Subtype reduction is expensive for large union types and is possible only when union
        // types are known not to circularly reference themselves (as is the case with union types created by
        // expression constructs such as array literals and the || and ?: operators). Named types can
        // circularly reference themselves and therefore cannot be subtype reduced during their declaration.
        // For example, "type Item = string | (() => Item" is a named type that circularly references itself.
        function getUnionType(types: Type[], subtypeReduction?: boolean, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            if (types.length === 0) {
                return neverType;
            }
            if (types.length === 1) {
                return types[0];
            }
            const typeSet = [] as TypeSet;
            addTypesToUnion(typeSet, types);
            if (typeSet.containsAny) {
                return anyType;
            }
            if (subtypeReduction) {
                removeSubtypes(typeSet);
            }
            if (typeSet.length === 0) {
                return typeSet.containsNull ? typeSet.containsNonWideningType ? nullType : nullWideningType :
                    typeSet.containsUndefined ? typeSet.containsNonWideningType ? undefinedType : undefinedWideningType :
                        neverType;
            }
            return getUnionTypeFromSortedList(typeSet, aliasSymbol, aliasTypeArguments);
        }

        // This function assumes the constituent type list is sorted and deduplicated.
        function getUnionTypeFromSortedList(types: Type[], aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            if (types.length === 0) {
                return neverType;
            }
            if (types.length === 1) {
                return types[0];
            }
            const id = getTypeListId(types);
            let type = unionTypes[id];
            if (!type) {
                const propagatedFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
                type = unionTypes[id] = <UnionType>createObjectType(TypeFlags.Union | propagatedFlags);
                type.types = types;
                type.aliasSymbol = aliasSymbol;
                type.aliasTypeArguments = aliasTypeArguments;
            }
            return type;
        }

        function getTypeFromUnionTypeNode(node: UnionTypeNode, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getUnionType(map(node.types, getTypeFromTypeNode), /*subtypeReduction*/ false, aliasSymbol, aliasTypeArguments);
            }
            return links.resolvedType;
        }

        function addTypeToIntersection(typeSet: TypeSet, type: Type) {
            if (type.flags & TypeFlags.Intersection) {
                addTypesToIntersection(typeSet, (<IntersectionType>type).types);
            }
            else if (type.flags & TypeFlags.Any) {
                typeSet.containsAny = true;
            }
            else if (!(type.flags & TypeFlags.Never) && (strictNullChecks || !(type.flags & TypeFlags.Nullable)) && !contains(typeSet, type)) {
                typeSet.push(type);
            }
        }

        // Add the given types to the given type set. Order is preserved, duplicates are removed,
        // and nested types of the given kind are flattened into the set.
        function addTypesToIntersection(typeSet: TypeSet, types: Type[]) {
            for (const type of types) {
                addTypeToIntersection(typeSet, type);
            }
        }

        // We do not perform structural deduplication on intersection types. Intersection types are created only by the &
        // type operator and we can't reduce those because we want to support recursive intersection types. For example,
        // a type alias of the form "type List<T> = T & { next: List<T> }" cannot be reduced during its declaration.
        // Also, unlike union types, the order of the constituent types is preserved in order that overload resolution
        // for intersections of types with signatures can be deterministic.
        function getIntersectionType(types: Type[], aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            if (types.length === 0) {
                return emptyObjectType;
            }
            const typeSet = [] as TypeSet;
            addTypesToIntersection(typeSet, types);
            if (typeSet.containsAny) {
                return anyType;
            }
            if (typeSet.length === 1) {
                return typeSet[0];
            }
            const id = getTypeListId(typeSet);
            let type = intersectionTypes[id];
            if (!type) {
                const propagatedFlags = getPropagatingFlagsOfTypes(typeSet, /*excludeKinds*/ TypeFlags.Nullable);
                type = intersectionTypes[id] = <IntersectionType>createObjectType(TypeFlags.Intersection | propagatedFlags);
                type.types = typeSet;
                type.aliasSymbol = aliasSymbol;
                type.aliasTypeArguments = aliasTypeArguments;
            }
            return type;
        }

        function getTypeFromIntersectionTypeNode(node: IntersectionTypeNode, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getIntersectionType(map(node.types, getTypeFromTypeNode), aliasSymbol, aliasTypeArguments);
            }
            return links.resolvedType;
        }

        function getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node: Node, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                // Deferred resolution of members is handled by resolveObjectTypeMembers
                const type = createObjectType(TypeFlags.Anonymous, node.symbol);
                type.aliasSymbol = aliasSymbol;
                type.aliasTypeArguments = aliasTypeArguments;
                links.resolvedType = type;
            }
            return links.resolvedType;
        }

        function createLiteralType(flags: TypeFlags, text: string) {
            const type = <LiteralType>createType(flags);
            type.text = text;
            return type;
        }

        function getLiteralTypeForText(flags: TypeFlags, text: string) {
            const map = flags & TypeFlags.StringLiteral ? stringLiteralTypes : numericLiteralTypes;
            return map[text] || (map[text] = createLiteralType(flags, text));
        }

        function getTypeFromLiteralTypeNode(node: LiteralTypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = checkExpression(node.literal);
            }
            return links.resolvedType;
        }

        function getTypeFromJSDocVariadicType(node: JSDocVariadicType): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const type = getTypeFromTypeNode(node.type);
                links.resolvedType = type ? createArrayType(type) : unknownType;
            }
            return links.resolvedType;
        }

        function getTypeFromJSDocTupleType(node: JSDocTupleType): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                const types = map(node.types, getTypeFromTypeNode);
                links.resolvedType = createTupleType(types);
            }
            return links.resolvedType;
        }

        function getThisType(node: Node): Type {
            const container = getThisContainer(node, /*includeArrowFunctions*/ false);
            const parent = container && container.parent;
            if (parent && (isClassLike(parent) || parent.kind === SyntaxKind.InterfaceDeclaration)) {
                if (!(container.flags & NodeFlags.Static) &&
                    (container.kind !== SyntaxKind.Constructor || isNodeDescendentOf(node, (<ConstructorDeclaration>container).body))) {
                    return getDeclaredTypeOfClassOrInterface(getSymbolOfNode(parent)).thisType;
                }
            }
            error(node, Diagnostics.A_this_type_is_available_only_in_a_non_static_member_of_a_class_or_interface);
            return unknownType;
        }

        function getTypeFromThisTypeNode(node: TypeNode): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                links.resolvedType = getThisType(node);
            }
            return links.resolvedType;
        }

        function getTypeFromTypeNode(node: TypeNode, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
            switch (node.kind) {
                case SyntaxKind.AnyKeyword:
                case SyntaxKind.JSDocAllType:
                case SyntaxKind.JSDocUnknownType:
                    return anyType;
                case SyntaxKind.StringKeyword:
                    return stringType;
                case SyntaxKind.NumberKeyword:
                    return numberType;
                case SyntaxKind.BooleanKeyword:
                    return booleanType;
                case SyntaxKind.SymbolKeyword:
                    return esSymbolType;
                case SyntaxKind.VoidKeyword:
                    return voidType;
                case SyntaxKind.UndefinedKeyword:
                    return undefinedType;
                case SyntaxKind.NullKeyword:
                    return nullType;
                case SyntaxKind.NeverKeyword:
                    return neverType;
                case SyntaxKind.ThisType:
                case SyntaxKind.ThisKeyword:
                    return getTypeFromThisTypeNode(node);
                case SyntaxKind.LiteralType:
                    return getTypeFromLiteralTypeNode(<LiteralTypeNode>node);
                case SyntaxKind.TypeReference:
                case SyntaxKind.JSDocTypeReference:
                    return getTypeFromTypeReference(<TypeReferenceNode>node);
                case SyntaxKind.TypePredicate:
                    return booleanType;
                case SyntaxKind.ExpressionWithTypeArguments:
                    return getTypeFromTypeReference(<ExpressionWithTypeArguments>node);
                case SyntaxKind.TypeQuery:
                    return getTypeFromTypeQueryNode(<TypeQueryNode>node);
                case SyntaxKind.ArrayType:
                case SyntaxKind.JSDocArrayType:
                    return getTypeFromArrayTypeNode(<ArrayTypeNode>node);
                case SyntaxKind.TupleType:
                    return getTypeFromTupleTypeNode(<TupleTypeNode>node);
                case SyntaxKind.UnionType:
                case SyntaxKind.JSDocUnionType:
                    return getTypeFromUnionTypeNode(<UnionTypeNode>node, aliasSymbol, aliasTypeArguments);
                case SyntaxKind.IntersectionType:
                    return getTypeFromIntersectionTypeNode(<IntersectionTypeNode>node, aliasSymbol, aliasTypeArguments);
                case SyntaxKind.ParenthesizedType:
                case SyntaxKind.JSDocNullableType:
                case SyntaxKind.JSDocNonNullableType:
                case SyntaxKind.JSDocConstructorType:
                case SyntaxKind.JSDocThisType:
                case SyntaxKind.JSDocOptionalType:
                    return getTypeFromTypeNode((<ParenthesizedTypeNode | JSDocTypeReferencingNode>node).type);
                case SyntaxKind.FunctionType:
                case SyntaxKind.ConstructorType:
                case SyntaxKind.TypeLiteral:
                case SyntaxKind.JSDocTypeLiteral:
                case SyntaxKind.JSDocFunctionType:
                case SyntaxKind.JSDocRecordType:
                    return getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node, aliasSymbol, aliasTypeArguments);
                // This function assumes that an identifier or qualified name is a type expression
                // Callers should first ensure this by calling isTypeNode
                case SyntaxKind.Identifier:
                case SyntaxKind.QualifiedName:
                    const symbol = getSymbolAtLocation(node);
                    return symbol && getDeclaredTypeOfSymbol(symbol);
                case SyntaxKind.JSDocTupleType:
                    return getTypeFromJSDocTupleType(<JSDocTupleType>node);
                case SyntaxKind.JSDocVariadicType:
                    return getTypeFromJSDocVariadicType(<JSDocVariadicType>node);
                default:
                    return unknownType;
            }
        }

        function instantiateList<T>(items: T[], mapper: TypeMapper, instantiator: (item: T, mapper: TypeMapper) => T): T[] {
            if (items && items.length) {
                const result: T[] = [];
                for (const v of items) {
                    result.push(instantiator(v, mapper));
                }
                return result;
            }
            return items;
        }

        function createUnaryTypeMapper(source: Type, target: Type): TypeMapper {
            return t => t === source ? target : t;
        }

        function createBinaryTypeMapper(source1: Type, target1: Type, source2: Type, target2: Type): TypeMapper {
            return t => t === source1 ? target1 : t === source2 ? target2 : t;
        }

        function createArrayTypeMapper(sources: Type[], targets: Type[]): TypeMapper {
            return t => {
                for (let i = 0; i < sources.length; i++) {
                    if (t === sources[i]) {
                        return targets ? targets[i] : anyType;
                    }
                }
                return t;
            };
        }

        function createTypeMapper(sources: Type[], targets: Type[]): TypeMapper {
            const count = sources.length;
            const mapper: TypeMapper =
                count == 1 ? createUnaryTypeMapper(sources[0], targets ? targets[0] : anyType) :
                    count == 2 ? createBinaryTypeMapper(sources[0], targets ? targets[0] : anyType, sources[1], targets ? targets[1] : anyType) :
                        createArrayTypeMapper(sources, targets);
            mapper.mappedTypes = sources;
            mapper.targetTypes = targets;
            return mapper;
        }

        function createTypeEraser(sources: Type[]): TypeMapper {
            return createTypeMapper(sources, undefined);
        }

        function getInferenceMapper(context: InferenceContext): TypeMapper {
            if (!context.mapper) {
                const mapper: TypeMapper = t => {
                    const typeParameters = context.typeParameters;
                    for (let i = 0; i < typeParameters.length; i++) {
                        if (t === typeParameters[i]) {
                            context.inferences[i].isFixed = true;
                            return getInferredType(context, i);
                        }
                    }
                    return t;
                };
                mapper.mappedTypes = context.typeParameters;
                mapper.context = context;
                context.mapper = mapper;
            }
            return context.mapper;
        }

        function identityMapper(type: Type): Type {
            return type;
        }

        function combineTypeMappers(mapper1: TypeMapper, mapper2: TypeMapper): TypeMapper {
            const mapper: TypeMapper = t => instantiateType(mapper1(t), mapper2);
            mapper.mappedTypes = mapper1.mappedTypes;
            return mapper;
        }

        function cloneTypeParameter(typeParameter: TypeParameter): TypeParameter {
            const result = <TypeParameter>createType(TypeFlags.TypeParameter);
            result.symbol = typeParameter.symbol;
            result.target = typeParameter;
            return result;
        }

        function cloneTypePredicate(predicate: TypePredicate, mapper: TypeMapper): ThisTypePredicate | IdentifierTypePredicate {
            if (isIdentifierTypePredicate(predicate)) {
                return {
                    kind: TypePredicateKind.Identifier,
                    parameterName: predicate.parameterName,
                    parameterIndex: predicate.parameterIndex,
                    type: instantiateType(predicate.type, mapper)
                } as IdentifierTypePredicate;
            }
            else {
                return {
                    kind: TypePredicateKind.This,
                    type: instantiateType(predicate.type, mapper)
                } as ThisTypePredicate;
            }
        }

        function instantiateSignature(signature: Signature, mapper: TypeMapper, eraseTypeParameters?: boolean): Signature {
            let freshTypeParameters: TypeParameter[];
            let freshTypePredicate: TypePredicate;
            if (signature.typeParameters && !eraseTypeParameters) {
                // First create a fresh set of type parameters, then include a mapping from the old to the
                // new type parameters in the mapper function. Finally store this mapper in the new type
                // parameters such that we can use it when instantiating constraints.
                freshTypeParameters = map(signature.typeParameters, cloneTypeParameter);
                mapper = combineTypeMappers(createTypeMapper(signature.typeParameters, freshTypeParameters), mapper);
                for (const tp of freshTypeParameters) {
                    tp.mapper = mapper;
                }
            }
            if (signature.typePredicate) {
                freshTypePredicate = cloneTypePredicate(signature.typePredicate, mapper);
            }
            const result = createSignature(signature.declaration, freshTypeParameters,
                signature.thisParameter && instantiateSymbol(signature.thisParameter, mapper),
                instantiateList(signature.parameters, mapper, instantiateSymbol),
                instantiateType(signature.resolvedReturnType, mapper),
                freshTypePredicate,
                signature.minArgumentCount, signature.hasRestParameter, signature.hasLiteralTypes);
            result.target = signature;
            result.mapper = mapper;
            return result;
        }

        function instantiateSymbol(symbol: Symbol, mapper: TypeMapper): Symbol {
            if (symbol.flags & SymbolFlags.Instantiated) {
                const links = getSymbolLinks(symbol);
                // If symbol being instantiated is itself a instantiation, fetch the original target and combine the
                // type mappers. This ensures that original type identities are properly preserved and that aliases
                // always reference a non-aliases.
                symbol = links.target;
                mapper = combineTypeMappers(links.mapper, mapper);
            }

            // Keep the flags from the symbol we're instantiating.  Mark that is instantiated, and
            // also transient so that we can just store data on it directly.
            const result = <TransientSymbol>createSymbol(SymbolFlags.Instantiated | SymbolFlags.Transient | symbol.flags, symbol.name);
            result.declarations = symbol.declarations;
            result.parent = symbol.parent;
            result.target = symbol;
            result.mapper = mapper;
            if (symbol.valueDeclaration) {
                result.valueDeclaration = symbol.valueDeclaration;
            }

            return result;
        }

        function instantiateAnonymousType(type: AnonymousType, mapper: TypeMapper): ObjectType {
            if (mapper.instantiations) {
                const cachedType = mapper.instantiations[type.id];
                if (cachedType) {
                    return cachedType;
                }
            }
            else {
                mapper.instantiations = [];
            }
            // Mark the anonymous type as instantiated such that our infinite instantiation detection logic can recognize it
            const result = <AnonymousType>createObjectType(TypeFlags.Anonymous | TypeFlags.Instantiated, type.symbol);
            result.target = type;
            result.mapper = mapper;
            result.aliasSymbol = type.aliasSymbol;
            result.aliasTypeArguments = mapper.targetTypes;
            mapper.instantiations[type.id] = result;
            return result;
        }

        function isSymbolInScopeOfMappedTypeParameter(symbol: Symbol, mapper: TypeMapper) {
            const mappedTypes = mapper.mappedTypes;
            // Starting with the parent of the symbol's declaration, check if the mapper maps any of
            // the type parameters introduced by enclosing declarations. We just pick the first
            // declaration since multiple declarations will all have the same parent anyway.
            let node = symbol.declarations[0].parent;
            while (node) {
                switch (node.kind) {
                    case SyntaxKind.FunctionType:
                    case SyntaxKind.ConstructorType:
                    case SyntaxKind.FunctionDeclaration:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                    case SyntaxKind.Constructor:
                    case SyntaxKind.CallSignature:
                    case SyntaxKind.ConstructSignature:
                    case SyntaxKind.IndexSignature:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.TypeAliasDeclaration:
                        const declaration = <DeclarationWithTypeParameters>node;
                        if (declaration.typeParameters) {
                            for (const d of declaration.typeParameters) {
                                if (contains(mappedTypes, getDeclaredTypeOfTypeParameter(getSymbolOfNode(d)))) {
                                    return true;
                                }
                            }
                        }
                        if (isClassLike(node) || node.kind === SyntaxKind.InterfaceDeclaration) {
                            const thisType = getDeclaredTypeOfClassOrInterface(getSymbolOfNode(node)).thisType;
                            if (thisType && contains(mappedTypes, thisType)) {
                                return true;
                            }
                        }
                        break;
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.SourceFile:
                        return false;
                }
                node = node.parent;
            }
            return false;
        }

        function instantiateType(type: Type, mapper: TypeMapper): Type {
            if (type && mapper !== identityMapper) {
                if (type.flags & TypeFlags.TypeParameter) {
                    return mapper(<TypeParameter>type);
                }
                if (type.flags & TypeFlags.Anonymous) {
                    // If the anonymous type originates in a declaration of a function, method, class, or
                    // interface, in an object type literal, or in an object literal expression, we may need
                    // to instantiate the type because it might reference a type parameter. We skip instantiation
                    // if none of the type parameters that are in scope in the type's declaration are mapped by
                    // the given mapper, however we can only do that analysis if the type isn't itself an
                    // instantiation.
                    return type.symbol &&
                        type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.TypeLiteral | SymbolFlags.ObjectLiteral) &&
                        (type.flags & TypeFlags.Instantiated || isSymbolInScopeOfMappedTypeParameter(type.symbol, mapper)) ?
                        instantiateAnonymousType(<AnonymousType>type, mapper) : type;
                }
                if (type.flags & TypeFlags.Reference) {
                    return createTypeReference((<TypeReference>type).target, instantiateList((<TypeReference>type).typeArguments, mapper, instantiateType));
                }
                if (type.flags & TypeFlags.Tuple) {
                    return createTupleType(instantiateList((<TupleType>type).elementTypes, mapper, instantiateType));
                }
                if (type.flags & TypeFlags.Union && !(type.flags & TypeFlags.Primitive)) {
                    return getUnionType(instantiateList((<UnionType>type).types, mapper, instantiateType), /*subtypeReduction*/ false, type.aliasSymbol, mapper.targetTypes);
                }
                if (type.flags & TypeFlags.Intersection) {
                    return getIntersectionType(instantiateList((<IntersectionType>type).types, mapper, instantiateType), type.aliasSymbol, mapper.targetTypes);
                }
            }
            return type;
        }

        function instantiateIndexInfo(info: IndexInfo, mapper: TypeMapper): IndexInfo {
            return info && createIndexInfo(instantiateType(info.type, mapper), info.isReadonly, info.declaration);
        }

        // Returns true if the given expression contains (at any level of nesting) a function or arrow expression
        // that is subject to contextual typing.
        function isContextSensitive(node: Expression | MethodDeclaration | ObjectLiteralElement): boolean {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));
            switch (node.kind) {
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    return isContextSensitiveFunctionLikeDeclaration(<FunctionExpression>node);
                case SyntaxKind.ObjectLiteralExpression:
                    return forEach((<ObjectLiteralExpression>node).properties, isContextSensitive);
                case SyntaxKind.ArrayLiteralExpression:
                    return forEach((<ArrayLiteralExpression>node).elements, isContextSensitive);
                case SyntaxKind.ConditionalExpression:
                    return isContextSensitive((<ConditionalExpression>node).whenTrue) ||
                        isContextSensitive((<ConditionalExpression>node).whenFalse);
                case SyntaxKind.BinaryExpression:
                    return (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken &&
                        (isContextSensitive((<BinaryExpression>node).left) || isContextSensitive((<BinaryExpression>node).right));
                case SyntaxKind.PropertyAssignment:
                    return isContextSensitive((<PropertyAssignment>node).initializer);
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                    return isContextSensitiveFunctionLikeDeclaration(<MethodDeclaration>node);
                case SyntaxKind.ParenthesizedExpression:
                    return isContextSensitive((<ParenthesizedExpression>node).expression);
            }

            return false;
        }

        function isContextSensitiveFunctionLikeDeclaration(node: FunctionLikeDeclaration) {
            const areAllParametersUntyped = !forEach(node.parameters, p => p.type);
            const isNullaryArrow = node.kind === SyntaxKind.ArrowFunction && !node.parameters.length;
            return !node.typeParameters && areAllParametersUntyped && !isNullaryArrow;
        }

        function isContextSensitiveFunctionOrObjectLiteralMethod(func: Node): func is FunctionExpression | MethodDeclaration {
            return (isFunctionExpressionOrArrowFunction(func) || isObjectLiteralMethod(func)) && isContextSensitiveFunctionLikeDeclaration(func);
        }

        function getTypeWithoutSignatures(type: Type): Type {
            if (type.flags & TypeFlags.ObjectType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                if (resolved.constructSignatures.length) {
                    const result = <ResolvedType>createObjectType(TypeFlags.Anonymous, type.symbol);
                    result.members = resolved.members;
                    result.properties = resolved.properties;
                    result.callSignatures = emptyArray;
                    result.constructSignatures = emptyArray;
                    type = result;
                }
            }
            return type;
        }

        // TYPE CHECKING

        function isTypeIdenticalTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, identityRelation);
        }

        function compareTypesIdentical(source: Type, target: Type): Ternary {
            return isTypeRelatedTo(source, target, identityRelation) ? Ternary.True : Ternary.False;
        }

        function compareTypesAssignable(source: Type, target: Type): Ternary {
            return isTypeRelatedTo(source, target, assignableRelation) ? Ternary.True : Ternary.False;
        }

        function isTypeSubtypeOf(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, subtypeRelation);
        }

        function isTypeAssignableTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, assignableRelation);
        }

        // A type S is considered to be an instance of a type T if S and T are the same type or if S is a
        // subtype of T but not structurally identical to T. This specifically means that two distinct but
        // structurally identical types (such as two classes) are not considered instances of each other.
        function isTypeInstanceOf(source: Type, target: Type): boolean {
            return source === target || isTypeSubtypeOf(source, target) && !isTypeIdenticalTo(source, target);
        }

        /**
         * This is *not* a bi-directional relationship.
         * If one needs to check both directions for comparability, use a second call to this function or 'checkTypeComparableTo'.
         */
        function isTypeComparableTo(source: Type, target: Type): boolean {
            return isTypeRelatedTo(source, target, comparableRelation);
        }

        function areTypesComparable(type1: Type, type2: Type): boolean {
            return isTypeComparableTo(type1, type2) || isTypeComparableTo(type2, type1);
        }

        function checkTypeSubtypeOf(source: Type, target: Type, errorNode: Node, headMessage?: DiagnosticMessage, containingMessageChain?: DiagnosticMessageChain): boolean {
            return checkTypeRelatedTo(source, target, subtypeRelation, errorNode, headMessage, containingMessageChain);
        }

        function checkTypeAssignableTo(source: Type, target: Type, errorNode: Node, headMessage?: DiagnosticMessage, containingMessageChain?: DiagnosticMessageChain): boolean {
            return checkTypeRelatedTo(source, target, assignableRelation, errorNode, headMessage, containingMessageChain);
        }

        /**
         * This is *not* a bi-directional relationship.
         * If one needs to check both directions for comparability, use a second call to this function or 'isTypeComparableTo'.
         */
        function checkTypeComparableTo(source: Type, target: Type, errorNode: Node, headMessage?: DiagnosticMessage, containingMessageChain?: DiagnosticMessageChain): boolean {
            return checkTypeRelatedTo(source, target, comparableRelation, errorNode, headMessage, containingMessageChain);
        }

        function isSignatureAssignableTo(source: Signature,
            target: Signature,
            ignoreReturnTypes: boolean): boolean {
            return compareSignaturesRelated(source, target, ignoreReturnTypes, /*reportErrors*/ false, /*errorReporter*/ undefined, compareTypesAssignable) !== Ternary.False;
        }

        type ErrorReporter = (message: DiagnosticMessage, arg0?: string, arg1?: string) => void;

        /**
         * See signatureRelatedTo, compareSignaturesIdentical
         */
        function compareSignaturesRelated(source: Signature,
            target: Signature,
            ignoreReturnTypes: boolean,
            reportErrors: boolean,
            errorReporter: ErrorReporter,
            compareTypes: (s: Type, t: Type, reportErrors?: boolean) => Ternary): Ternary {
            // TODO (drosen): De-duplicate code between related functions.
            if (source === target) {
                return Ternary.True;
            }
            if (!target.hasRestParameter && source.minArgumentCount > target.parameters.length) {
                return Ternary.False;
            }

            // Spec 1.0 Section 3.8.3 & 3.8.4:
            // M and N (the signatures) are instantiated using type Any as the type argument for all type parameters declared by M and N
            source = getErasedSignature(source);
            target = getErasedSignature(target);

            let result = Ternary.True;

            const sourceThisType = getThisTypeOfSignature(source);
            if (sourceThisType && sourceThisType !== voidType) {
                const targetThisType = getThisTypeOfSignature(target);
                if (targetThisType) {
                    // void sources are assignable to anything.
                    const related = compareTypes(sourceThisType, targetThisType, /*reportErrors*/ false)
                        || compareTypes(targetThisType, sourceThisType, reportErrors);
                    if (!related) {
                        if (reportErrors) {
                            errorReporter(Diagnostics.The_this_types_of_each_signature_are_incompatible);
                        }
                        return Ternary.False;
                    }
                    result &= related;
                }
            }

            const sourceMax = getNumNonRestParameters(source);
            const targetMax = getNumNonRestParameters(target);
            const checkCount = getNumParametersToCheckForSignatureRelatability(source, sourceMax, target, targetMax);
            const sourceParams = source.parameters;
            const targetParams = target.parameters;
            for (let i = 0; i < checkCount; i++) {
                const s = i < sourceMax ? getTypeOfParameter(sourceParams[i]) : getRestTypeOfSignature(source);
                const t = i < targetMax ? getTypeOfParameter(targetParams[i]) : getRestTypeOfSignature(target);
                const related = compareTypes(s, t, /*reportErrors*/ false) || compareTypes(t, s, reportErrors);
                if (!related) {
                    if (reportErrors) {
                        errorReporter(Diagnostics.Types_of_parameters_0_and_1_are_incompatible,
                            sourceParams[i < sourceMax ? i : sourceMax].name,
                            targetParams[i < targetMax ? i : targetMax].name);
                    }
                    return Ternary.False;
                }
                result &= related;
            }

            if (!ignoreReturnTypes) {
                const targetReturnType = getReturnTypeOfSignature(target);
                if (targetReturnType === voidType) {
                    return result;
                }
                const sourceReturnType = getReturnTypeOfSignature(source);

                // The following block preserves behavior forbidding boolean returning functions from being assignable to type guard returning functions
                if (target.typePredicate) {
                    if (source.typePredicate) {
                        result &= compareTypePredicateRelatedTo(source.typePredicate, target.typePredicate, reportErrors, errorReporter, compareTypes);
                    }
                    else if (isIdentifierTypePredicate(target.typePredicate)) {
                        if (reportErrors) {
                            errorReporter(Diagnostics.Signature_0_must_have_a_type_predicate, signatureToString(source));
                        }
                        return Ternary.False;
                    }
                }
                else {
                    result &= compareTypes(sourceReturnType, targetReturnType, reportErrors);
                }

            }

            return result;
        }

        function compareTypePredicateRelatedTo(source: TypePredicate,
            target: TypePredicate,
            reportErrors: boolean,
            errorReporter: ErrorReporter,
            compareTypes: (s: Type, t: Type, reportErrors?: boolean) => Ternary): Ternary {
            if (source.kind !== target.kind) {
                if (reportErrors) {
                    errorReporter(Diagnostics.A_this_based_type_guard_is_not_compatible_with_a_parameter_based_type_guard);
                    errorReporter(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
                }
                return Ternary.False;
            }

            if (source.kind === TypePredicateKind.Identifier) {
                const sourceIdentifierPredicate = source as IdentifierTypePredicate;
                const targetIdentifierPredicate = target as IdentifierTypePredicate;
                if (sourceIdentifierPredicate.parameterIndex !== targetIdentifierPredicate.parameterIndex) {
                    if (reportErrors) {
                        errorReporter(Diagnostics.Parameter_0_is_not_in_the_same_position_as_parameter_1, sourceIdentifierPredicate.parameterName, targetIdentifierPredicate.parameterName);
                        errorReporter(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
                    }
                    return Ternary.False;
                }
            }

            const related = compareTypes(source.type, target.type, reportErrors);
            if (related === Ternary.False && reportErrors) {
                errorReporter(Diagnostics.Type_predicate_0_is_not_assignable_to_1, typePredicateToString(source), typePredicateToString(target));
            }
            return related;
        }

        function isImplementationCompatibleWithOverload(implementation: Signature, overload: Signature): boolean {
            const erasedSource = getErasedSignature(implementation);
            const erasedTarget = getErasedSignature(overload);

            // First see if the return types are compatible in either direction.
            const sourceReturnType = getReturnTypeOfSignature(erasedSource);
            const targetReturnType = getReturnTypeOfSignature(erasedTarget);
            if (targetReturnType === voidType
                || isTypeRelatedTo(targetReturnType, sourceReturnType, assignableRelation)
                || isTypeRelatedTo(sourceReturnType, targetReturnType, assignableRelation)) {

                return isSignatureAssignableTo(erasedSource, erasedTarget, /*ignoreReturnTypes*/ true);
            }

            return false;
        }

        function getNumNonRestParameters(signature: Signature) {
            const numParams = signature.parameters.length;
            return signature.hasRestParameter ?
                numParams - 1 :
                numParams;
        }

        function getNumParametersToCheckForSignatureRelatability(source: Signature, sourceNonRestParamCount: number, target: Signature, targetNonRestParamCount: number) {
            if (source.hasRestParameter === target.hasRestParameter) {
                if (source.hasRestParameter) {
                    // If both have rest parameters, get the max and add 1 to
                    // compensate for the rest parameter.
                    return Math.max(sourceNonRestParamCount, targetNonRestParamCount) + 1;
                }
                else {
                    return Math.min(sourceNonRestParamCount, targetNonRestParamCount);
                }
            }
            else {
                // Return the count for whichever signature doesn't have rest parameters.
                return source.hasRestParameter ?
                    targetNonRestParamCount :
                    sourceNonRestParamCount;
            }
        }

        function isEnumTypeRelatedTo(source: EnumType, target: EnumType, errorReporter?: ErrorReporter) {
            if (source === target) {
                return true;
            }
            if (source.symbol.name !== target.symbol.name || !(source.symbol.flags & SymbolFlags.RegularEnum) || !(target.symbol.flags & SymbolFlags.RegularEnum)) {
                return false;
            }
            const targetEnumType = getTypeOfSymbol(target.symbol);
            for (const property of getPropertiesOfType(getTypeOfSymbol(source.symbol))) {
                if (property.flags & SymbolFlags.EnumMember) {
                    const targetProperty = getPropertyOfType(targetEnumType, property.name);
                    if (!targetProperty || !(targetProperty.flags & SymbolFlags.EnumMember)) {
                        if (errorReporter) {
                            errorReporter(Diagnostics.Property_0_is_missing_in_type_1, property.name,
                                typeToString(target, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType));
                        }
                        return false;
                    }
                }
            }
            return true;
        }

        function isSimpleTypeRelatedTo(source: Type, target: Type, relation: Map<RelationComparisonResult>, errorReporter?: ErrorReporter) {
            if (target.flags & TypeFlags.Never) return false;
            if (target.flags & TypeFlags.Any || source.flags & TypeFlags.Never) return true;
            if (source.flags & TypeFlags.StringLike && target.flags & TypeFlags.String) return true;
            if (source.flags & TypeFlags.NumberLike && target.flags & TypeFlags.Number) return true;
            if (source.flags & TypeFlags.BooleanLike && target.flags & TypeFlags.Boolean) return true;
            if (source.flags & TypeFlags.EnumLiteral && target.flags & TypeFlags.Enum && (<EnumLiteralType>source).baseType === target) return true;
            if (source.flags & TypeFlags.Enum && target.flags & TypeFlags.Enum && isEnumTypeRelatedTo(<EnumType>source, <EnumType>target, errorReporter)) return true;
            if (source.flags & TypeFlags.Undefined && (!strictNullChecks || target.flags & (TypeFlags.Undefined | TypeFlags.Void))) return true;
            if (source.flags & TypeFlags.Null && (!strictNullChecks || target.flags & TypeFlags.Null)) return true;
            if (relation === assignableRelation || relation === comparableRelation) {
                if (source.flags & TypeFlags.Any) return true;
                if (source.flags & (TypeFlags.Number | TypeFlags.NumberLiteral) && target.flags & TypeFlags.Enum) return true;
                if (source.flags & TypeFlags.NumberLiteral && target.flags & TypeFlags.EnumLiteral && (<LiteralType>source).text === (<LiteralType>target).text) return true;
            }
            return false;
        }

        function isTypeRelatedTo(source: Type, target: Type, relation: Map<RelationComparisonResult>) {
            if (source === target || relation !== identityRelation && isSimpleTypeRelatedTo(source, target, relation)) {
                return true;
            }
            if (source.flags & TypeFlags.ObjectType && target.flags & TypeFlags.ObjectType) {
                const id = relation !== identityRelation || source.id < target.id ? source.id + "," + target.id : target.id + "," + source.id;
                const related = relation[id];
                if (related !== undefined) {
                    return related === RelationComparisonResult.Succeeded;
                }
            }
            if (source.flags & TypeFlags.StructuredOrTypeParameter || target.flags & TypeFlags.StructuredOrTypeParameter) {
                return checkTypeRelatedTo(source, target, relation, undefined, undefined, undefined);
            }
            return false;
        }

        /**
         * Checks if 'source' is related to 'target' (e.g.: is a assignable to).
         * @param source The left-hand-side of the relation.
         * @param target The right-hand-side of the relation.
         * @param relation The relation considered. One of 'identityRelation', 'subtypeRelation', 'assignableRelation', or 'comparableRelation'.
         * Used as both to determine which checks are performed and as a cache of previously computed results.
         * @param errorNode The suggested node upon which all errors will be reported, if defined. This may or may not be the actual node used.
         * @param headMessage If the error chain should be prepended by a head message, then headMessage will be used.
         * @param containingMessageChain A chain of errors to prepend any new errors found.
         */
        function checkTypeRelatedTo(
            source: Type,
            target: Type,
            relation: Map<RelationComparisonResult>,
            errorNode: Node,
            headMessage?: DiagnosticMessage,
            containingMessageChain?: DiagnosticMessageChain): boolean {

            let errorInfo: DiagnosticMessageChain;
            let sourceStack: ObjectType[];
            let targetStack: ObjectType[];
            let maybeStack: Map<RelationComparisonResult>[];
            let expandingFlags: number;
            let depth = 0;
            let overflow = false;

            Debug.assert(relation !== identityRelation || !errorNode, "no error reporting in identity checking");

            const result = isRelatedTo(source, target, /*reportErrors*/ !!errorNode, headMessage);
            if (overflow) {
                error(errorNode, Diagnostics.Excessive_stack_depth_comparing_types_0_and_1, typeToString(source), typeToString(target));
            }
            else if (errorInfo) {
                if (containingMessageChain) {
                    errorInfo = concatenateDiagnosticMessageChains(containingMessageChain, errorInfo);
                }

                diagnostics.add(createDiagnosticForNodeFromMessageChain(errorNode, errorInfo));
            }
            return result !== Ternary.False;

            function reportError(message: DiagnosticMessage, arg0?: string, arg1?: string, arg2?: string): void {
                Debug.assert(!!errorNode);
                errorInfo = chainDiagnosticMessages(errorInfo, message, arg0, arg1, arg2);
            }

            function reportRelationError(message: DiagnosticMessage, source: Type, target: Type) {
                let sourceType = typeToString(source);
                let targetType = typeToString(target);
                if (sourceType === targetType) {
                    sourceType = typeToString(source, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType);
                    targetType = typeToString(target, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType);
                }

                if (!message) {
                    message = relation === comparableRelation ?
                        Diagnostics.Type_0_is_not_comparable_to_type_1 :
                        Diagnostics.Type_0_is_not_assignable_to_type_1;
                }

                reportError(message, sourceType, targetType);
            }

            // Compare two types and return
            // Ternary.True if they are related with no assumptions,
            // Ternary.Maybe if they are related with assumptions of other relationships, or
            // Ternary.False if they are not related.
            function isRelatedTo(source: Type, target: Type, reportErrors?: boolean, headMessage?: DiagnosticMessage): Ternary {
                let result: Ternary;
                // both types are the same - covers 'they are the same primitive type or both are Any' or the same type parameter cases
                if (source === target) return Ternary.True;

                if (relation === identityRelation) {
                    return isIdenticalTo(source, target);
                }

                if (isSimpleTypeRelatedTo(source, target, relation, reportErrors ? reportError : undefined)) return Ternary.True;

                if (source.flags & TypeFlags.FreshObjectLiteral) {
                    if (hasExcessProperties(<FreshObjectLiteralType>source, target, reportErrors)) {
                        if (reportErrors) {
                            reportRelationError(headMessage, source, target);
                        }
                        return Ternary.False;
                    }
                    // Above we check for excess properties with respect to the entire target type. When union
                    // and intersection types are further deconstructed on the target side, we don't want to
                    // make the check again (as it might fail for a partial target type). Therefore we obtain
                    // the regular source type and proceed with that.
                    if (target.flags & TypeFlags.UnionOrIntersection) {
                        source = getRegularTypeOfObjectLiteral(source);
                    }
                }

                const saveErrorInfo = errorInfo;

                // Note that these checks are specifically ordered to produce correct results.
                if (source.flags & TypeFlags.Union) {
                    if (relation === comparableRelation) {
                        result = someTypeRelatedToType(source as UnionType, target, reportErrors && !(source.flags & TypeFlags.Primitive));
                    }
                    else {
                        result = eachTypeRelatedToType(source as UnionType, target, reportErrors && !(source.flags & TypeFlags.Primitive));
                    }

                    if (result) {
                        return result;
                    }
                }
                else if (target.flags & TypeFlags.Intersection) {
                    result = typeRelatedToEachType(source, target as IntersectionType, reportErrors);

                    if (result) {
                        return result;
                    }
                }
                else {
                    // It is necessary to try these "some" checks on both sides because there may be nested "each" checks
                    // on either side that need to be prioritized. For example, A | B = (A | B) & (C | D) or
                    // A & B = (A & B) | (C & D).
                    if (source.flags & TypeFlags.Intersection) {
                        // Check to see if any constituents of the intersection are immediately related to the target.
                        //
                        // Don't report errors though. Checking whether a constituent is related to the source is not actually
                        // useful and leads to some confusing error messages. Instead it is better to let the below checks
                        // take care of this, or to not elaborate at all. For instance,
                        //
                        //    - For an object type (such as 'C = A & B'), users are usually more interested in structural errors.
                        //
                        //    - For a union type (such as '(A | B) = (C & D)'), it's better to hold onto the whole intersection
                        //          than to report that 'D' is not assignable to 'A' or 'B'.
                        //
                        //    - For a primitive type or type parameter (such as 'number = A & B') there is no point in
                        //          breaking the intersection apart.
                        if (result = someTypeRelatedToType(<IntersectionType>source, target, /*reportErrors*/ false)) {
                            return result;
                        }
                    }
                    if (target.flags & TypeFlags.Union) {
                        if (result = typeRelatedToSomeType(source, <UnionType>target, reportErrors && !(source.flags & TypeFlags.Primitive) && !(target.flags & TypeFlags.Primitive))) {
                            return result;
                        }
                    }
                }

                if (source.flags & TypeFlags.TypeParameter) {
                    let constraint = getConstraintOfTypeParameter(<TypeParameter>source);

                    if (!constraint || constraint.flags & TypeFlags.Any) {
                        constraint = emptyObjectType;
                    }

                    // The constraint may need to be further instantiated with its 'this' type.
                    constraint = getTypeWithThisArgument(constraint, source);

                    // Report constraint errors only if the constraint is not the empty object type
                    const reportConstraintErrors = reportErrors && constraint !== emptyObjectType;
                    if (result = isRelatedTo(constraint, target, reportConstraintErrors)) {
                        errorInfo = saveErrorInfo;
                        return result;
                    }
                }
                else {
                    if (source.flags & TypeFlags.Reference && target.flags & TypeFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                        // We have type references to same target type, see if relationship holds for all type arguments
                        if (result = typeArgumentsRelatedTo(<TypeReference>source, <TypeReference>target, reportErrors)) {
                            return result;
                        }
                    }
                    // Even if relationship doesn't hold for unions, intersections, or generic type references,
                    // it may hold in a structural comparison.
                    const apparentSource = getApparentType(source);
                    // In a check of the form X = A & B, we will have previously checked if A relates to X or B relates
                    // to X. Failing both of those we want to check if the aggregation of A and B's members structurally
                    // relates to X. Thus, we include intersection types on the source side here.
                    if (apparentSource.flags & (TypeFlags.ObjectType | TypeFlags.Intersection) && target.flags & TypeFlags.ObjectType) {
                        // Report structural errors only if we haven't reported any errors yet
                        const reportStructuralErrors = reportErrors && errorInfo === saveErrorInfo && !(source.flags & TypeFlags.Primitive);
                        if (result = objectTypeRelatedTo(apparentSource, source, target, reportStructuralErrors)) {
                            errorInfo = saveErrorInfo;
                            return result;
                        }
                    }
                }

                if (reportErrors) {
                    reportRelationError(headMessage, source, target);
                }
                return Ternary.False;
            }

            function isIdenticalTo(source: Type, target: Type): Ternary {
                let result: Ternary;
                if (source.flags & TypeFlags.ObjectType && target.flags & TypeFlags.ObjectType) {
                    if (source.flags & TypeFlags.Reference && target.flags & TypeFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                        // We have type references to same target type, see if all type arguments are identical
                        if (result = typeArgumentsRelatedTo(<TypeReference>source, <TypeReference>target, /*reportErrors*/ false)) {
                            return result;
                        }
                    }
                    return objectTypeRelatedTo(source, source, target, /*reportErrors*/ false);
                }
                if (source.flags & TypeFlags.Union && target.flags & TypeFlags.Union ||
                    source.flags & TypeFlags.Intersection && target.flags & TypeFlags.Intersection) {
                    if (result = eachTypeRelatedToSomeType(<UnionOrIntersectionType>source, <UnionOrIntersectionType>target, /*reportErrors*/ false)) {
                        if (result &= eachTypeRelatedToSomeType(<UnionOrIntersectionType>target, <UnionOrIntersectionType>source, /*reportErrors*/ false)) {
                            return result;
                        }
                    }
                }
                return Ternary.False;
            }

            // Check if a property with the given name is known anywhere in the given type. In an object type, a property
            // is considered known if the object type is empty and the check is for assignability, if the object type has
            // index signatures, or if the property is actually declared in the object type. In a union or intersection
            // type, a property is considered known if it is known in any constituent type.
            function isKnownProperty(type: Type, name: string): boolean {
                if (type.flags & TypeFlags.ObjectType) {
                    const resolved = resolveStructuredTypeMembers(type);
                    if ((relation === assignableRelation || relation === comparableRelation) && (type === globalObjectType || isEmptyObjectType(resolved)) ||
                        resolved.stringIndexInfo ||
                        (resolved.numberIndexInfo && isNumericLiteralName(name)) ||
                        getPropertyOfType(type, name)) {
                        return true;
                    }
                }
                else if (type.flags & TypeFlags.UnionOrIntersection) {
                    for (const t of (<UnionOrIntersectionType>type).types) {
                        if (isKnownProperty(t, name)) {
                            return true;
                        }
                    }
                }
                return false;
            }

            function isEmptyObjectType(t: ResolvedType) {
                return t.properties.length === 0 &&
                    t.callSignatures.length === 0 &&
                    t.constructSignatures.length === 0 &&
                    !t.stringIndexInfo &&
                    !t.numberIndexInfo;
            }

            function hasExcessProperties(source: FreshObjectLiteralType, target: Type, reportErrors: boolean): boolean {
                if (!(target.flags & TypeFlags.ObjectLiteralPatternWithComputedProperties) && maybeTypeOfKind(target, TypeFlags.ObjectType)) {
                    for (const prop of getPropertiesOfObjectType(source)) {
                        if (!isKnownProperty(target, prop.name)) {
                            if (reportErrors) {
                                // We know *exactly* where things went wrong when comparing the types.
                                // Use this property as the error node as this will be more helpful in
                                // reasoning about what went wrong.
                                Debug.assert(!!errorNode);
                                errorNode = prop.valueDeclaration;
                                reportError(Diagnostics.Object_literal_may_only_specify_known_properties_and_0_does_not_exist_in_type_1,
                                    symbolToString(prop), typeToString(target));
                            }
                            return true;
                        }
                    }
                }
                return false;
            }

            function eachTypeRelatedToSomeType(source: UnionOrIntersectionType, target: UnionOrIntersectionType, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                const sourceTypes = source.types;
                for (const sourceType of sourceTypes) {
                    const related = typeRelatedToSomeType(sourceType, target, /*reportErrors*/ false);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function typeRelatedToSomeType(source: Type, target: UnionOrIntersectionType, reportErrors: boolean): Ternary {
                const targetTypes = target.types;
                if (target.flags & TypeFlags.Union && containsType(targetTypes, source)) {
                    return Ternary.True;
                }
                const len = targetTypes.length;
                for (let i = 0; i < len; i++) {
                    const related = isRelatedTo(source, targetTypes[i], reportErrors && i === len - 1);
                    if (related) {
                        return related;
                    }
                }
                return Ternary.False;
            }

            function typeRelatedToEachType(source: Type, target: UnionOrIntersectionType, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                const targetTypes = target.types;
                for (const targetType of targetTypes) {
                    const related = isRelatedTo(source, targetType, reportErrors);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function someTypeRelatedToType(source: UnionOrIntersectionType, target: Type, reportErrors: boolean): Ternary {
                const sourceTypes = source.types;
                if (source.flags & TypeFlags.Union && containsType(sourceTypes, target)) {
                    return Ternary.True;
                }
                const len = sourceTypes.length;
                for (let i = 0; i < len; i++) {
                    const related = isRelatedTo(sourceTypes[i], target, reportErrors && i === len - 1);
                    if (related) {
                        return related;
                    }
                }
                return Ternary.False;
            }

            function eachTypeRelatedToType(source: UnionOrIntersectionType, target: Type, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                const sourceTypes = source.types;
                for (const sourceType of sourceTypes) {
                    const related = isRelatedTo(sourceType, target, reportErrors);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function typeArgumentsRelatedTo(source: TypeReference, target: TypeReference, reportErrors: boolean): Ternary {
                const sources = source.typeArguments || emptyArray;
                const targets = target.typeArguments || emptyArray;
                if (sources.length !== targets.length && relation === identityRelation) {
                    return Ternary.False;
                }
                const length = sources.length <= targets.length ? sources.length : targets.length;
                let result = Ternary.True;
                for (let i = 0; i < length; i++) {
                    const related = isRelatedTo(sources[i], targets[i], reportErrors);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            // Determine if two object types are related by structure. First, check if the result is already available in the global cache.
            // Second, check if we have already started a comparison of the given two types in which case we assume the result to be true.
            // Third, check if both types are part of deeply nested chains of generic type instantiations and if so assume the types are
            // equal and infinitely expanding. Fourth, if we have reached a depth of 100 nested comparisons, assume we have runaway recursion
            // and issue an error. Otherwise, actually compare the structure of the two types.
            function objectTypeRelatedTo(source: Type, originalSource: Type, target: Type, reportErrors: boolean): Ternary {
                if (overflow) {
                    return Ternary.False;
                }
                const id = relation !== identityRelation || source.id < target.id ? source.id + "," + target.id : target.id + "," + source.id;
                const related = relation[id];
                if (related !== undefined) {
                    if (reportErrors && related === RelationComparisonResult.Failed) {
                        // We are elaborating errors and the cached result is an unreported failure. Record the result as a reported
                        // failure and continue computing the relation such that errors get reported.
                        relation[id] = RelationComparisonResult.FailedAndReported;
                    }
                    else {
                        return related === RelationComparisonResult.Succeeded ? Ternary.True : Ternary.False;
                    }
                }
                if (depth > 0) {
                    for (let i = 0; i < depth; i++) {
                        // If source and target are already being compared, consider them related with assumptions
                        if (maybeStack[i][id]) {
                            return Ternary.Maybe;
                        }
                    }
                    if (depth === 100) {
                        overflow = true;
                        return Ternary.False;
                    }
                }
                else {
                    sourceStack = [];
                    targetStack = [];
                    maybeStack = [];
                    expandingFlags = 0;
                }
                sourceStack[depth] = source;
                targetStack[depth] = target;
                maybeStack[depth] = createMap<RelationComparisonResult>();
                maybeStack[depth][id] = RelationComparisonResult.Succeeded;
                depth++;
                const saveExpandingFlags = expandingFlags;
                if (!(expandingFlags & 1) && isDeeplyNestedGeneric(source, sourceStack, depth)) expandingFlags |= 1;
                if (!(expandingFlags & 2) && isDeeplyNestedGeneric(target, targetStack, depth)) expandingFlags |= 2;
                let result: Ternary;
                if (expandingFlags === 3) {
                    result = Ternary.Maybe;
                }
                else {
                    result = propertiesRelatedTo(source, target, reportErrors);
                    if (result) {
                        result &= signaturesRelatedTo(source, target, SignatureKind.Call, reportErrors);
                        if (result) {
                            result &= signaturesRelatedTo(source, target, SignatureKind.Construct, reportErrors);
                            if (result) {
                                result &= indexTypesRelatedTo(source, originalSource, target, IndexKind.String, reportErrors);
                                if (result) {
                                    result &= indexTypesRelatedTo(source, originalSource, target, IndexKind.Number, reportErrors);
                                }
                            }
                        }
                    }
                }
                expandingFlags = saveExpandingFlags;
                depth--;
                if (result) {
                    const maybeCache = maybeStack[depth];
                    // If result is definitely true, copy assumptions to global cache, else copy to next level up
                    const destinationCache = (result === Ternary.True || depth === 0) ? relation : maybeStack[depth - 1];
                    copyProperties(maybeCache, destinationCache);
                }
                else {
                    // A false result goes straight into global cache (when something is false under assumptions it
                    // will also be false without assumptions)
                    relation[id] = reportErrors ? RelationComparisonResult.FailedAndReported : RelationComparisonResult.Failed;
                }
                return result;
            }

            function propertiesRelatedTo(source: Type, target: Type, reportErrors: boolean): Ternary {
                if (relation === identityRelation) {
                    return propertiesIdenticalTo(source, target);
                }
                let result = Ternary.True;
                const properties = getPropertiesOfObjectType(target);
                const requireOptionalProperties = relation === subtypeRelation && !(source.flags & TypeFlags.ObjectLiteral);
                for (const targetProp of properties) {
                    const sourceProp = getPropertyOfType(source, targetProp.name);

                    if (sourceProp !== targetProp) {
                        if (!sourceProp) {
                            if (!(targetProp.flags & SymbolFlags.Optional) || requireOptionalProperties) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_missing_in_type_1, symbolToString(targetProp), typeToString(source));
                                }
                                return Ternary.False;
                            }
                        }
                        else if (!(targetProp.flags & SymbolFlags.Prototype)) {
                            const sourcePropFlags = getDeclarationFlagsFromSymbol(sourceProp);
                            const targetPropFlags = getDeclarationFlagsFromSymbol(targetProp);
                            if (sourcePropFlags & NodeFlags.Private || targetPropFlags & NodeFlags.Private) {
                                if (sourceProp.valueDeclaration !== targetProp.valueDeclaration) {
                                    if (reportErrors) {
                                        if (sourcePropFlags & NodeFlags.Private && targetPropFlags & NodeFlags.Private) {
                                            reportError(Diagnostics.Types_have_separate_declarations_of_a_private_property_0, symbolToString(targetProp));
                                        }
                                        else {
                                            reportError(Diagnostics.Property_0_is_private_in_type_1_but_not_in_type_2, symbolToString(targetProp),
                                                typeToString(sourcePropFlags & NodeFlags.Private ? source : target),
                                                typeToString(sourcePropFlags & NodeFlags.Private ? target : source));
                                        }
                                    }
                                    return Ternary.False;
                                }
                            }
                            else if (targetPropFlags & NodeFlags.Protected) {
                                const sourceDeclaredInClass = sourceProp.parent && sourceProp.parent.flags & SymbolFlags.Class;
                                const sourceClass = sourceDeclaredInClass ? <InterfaceType>getDeclaredTypeOfSymbol(getParentOfSymbol(sourceProp)) : undefined;
                                const targetClass = <InterfaceType>getDeclaredTypeOfSymbol(getParentOfSymbol(targetProp));
                                if (!sourceClass || !hasBaseType(sourceClass, targetClass)) {
                                    if (reportErrors) {
                                        reportError(Diagnostics.Property_0_is_protected_but_type_1_is_not_a_class_derived_from_2,
                                            symbolToString(targetProp), typeToString(sourceClass || source), typeToString(targetClass));
                                    }
                                    return Ternary.False;
                                }
                            }
                            else if (sourcePropFlags & NodeFlags.Protected) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_protected_in_type_1_but_public_in_type_2,
                                        symbolToString(targetProp), typeToString(source), typeToString(target));
                                }
                                return Ternary.False;
                            }
                            const related = isRelatedTo(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp), reportErrors);
                            if (!related) {
                                if (reportErrors) {
                                    reportError(Diagnostics.Types_of_property_0_are_incompatible, symbolToString(targetProp));
                                }
                                return Ternary.False;
                            }
                            result &= related;
                            if (sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
                                // TypeScript 1.0 spec (April 2014): 3.8.3
                                // S is a subtype of a type T, and T is a supertype of S if ...
                                // S' and T are object types and, for each member M in T..
                                // M is a property and S' contains a property N where
                                // if M is a required property, N is also a required property
                                // (M - property in T)
                                // (N - property in S)
                                if (reportErrors) {
                                    reportError(Diagnostics.Property_0_is_optional_in_type_1_but_required_in_type_2,
                                        symbolToString(targetProp), typeToString(source), typeToString(target));
                                }
                                return Ternary.False;
                            }
                        }
                    }
                }
                return result;
            }

            function propertiesIdenticalTo(source: Type, target: Type): Ternary {
                if (!(source.flags & TypeFlags.ObjectType && target.flags & TypeFlags.ObjectType)) {
                    return Ternary.False;
                }
                const sourceProperties = getPropertiesOfObjectType(source);
                const targetProperties = getPropertiesOfObjectType(target);
                if (sourceProperties.length !== targetProperties.length) {
                    return Ternary.False;
                }
                let result = Ternary.True;
                for (const sourceProp of sourceProperties) {
                    const targetProp = getPropertyOfObjectType(target, sourceProp.name);
                    if (!targetProp) {
                        return Ternary.False;
                    }
                    const related = compareProperties(sourceProp, targetProp, isRelatedTo);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function signaturesRelatedTo(source: Type, target: Type, kind: SignatureKind, reportErrors: boolean): Ternary {
                if (relation === identityRelation) {
                    return signaturesIdenticalTo(source, target, kind);
                }
                if (target === anyFunctionType || source === anyFunctionType) {
                    return Ternary.True;
                }

                const sourceSignatures = getSignaturesOfType(source, kind);
                const targetSignatures = getSignaturesOfType(target, kind);
                if (kind === SignatureKind.Construct && sourceSignatures.length && targetSignatures.length) {
                    if (isAbstractConstructorType(source) && !isAbstractConstructorType(target)) {
                        // An abstract constructor type is not assignable to a non-abstract constructor type
                        // as it would otherwise be possible to new an abstract class. Note that the assignability
                        // check we perform for an extends clause excludes construct signatures from the target,
                        // so this check never proceeds.
                        if (reportErrors) {
                            reportError(Diagnostics.Cannot_assign_an_abstract_constructor_type_to_a_non_abstract_constructor_type);
                        }
                        return Ternary.False;
                    }
                    if (!constructorVisibilitiesAreCompatible(sourceSignatures[0], targetSignatures[0], reportErrors)) {
                        return Ternary.False;
                    }
                }

                let result = Ternary.True;
                const saveErrorInfo = errorInfo;

                outer: for (const t of targetSignatures) {
                    // Only elaborate errors from the first failure
                    let shouldElaborateErrors = reportErrors;
                    for (const s of sourceSignatures) {
                        const related = signatureRelatedTo(s, t, shouldElaborateErrors);
                        if (related) {
                            result &= related;
                            errorInfo = saveErrorInfo;
                            continue outer;
                        }
                        shouldElaborateErrors = false;
                    }

                    if (shouldElaborateErrors) {
                        reportError(Diagnostics.Type_0_provides_no_match_for_the_signature_1,
                            typeToString(source),
                            signatureToString(t, /*enclosingDeclaration*/ undefined, /*flags*/ undefined, kind));
                    }
                    return Ternary.False;
                }
                return result;
            }

            /**
             * See signatureAssignableTo, compareSignaturesIdentical
             */
            function signatureRelatedTo(source: Signature, target: Signature, reportErrors: boolean): Ternary {
                return compareSignaturesRelated(source, target, /*ignoreReturnTypes*/ false, reportErrors, reportError, isRelatedTo);
            }

            function signaturesIdenticalTo(source: Type, target: Type, kind: SignatureKind): Ternary {
                const sourceSignatures = getSignaturesOfType(source, kind);
                const targetSignatures = getSignaturesOfType(target, kind);
                if (sourceSignatures.length !== targetSignatures.length) {
                    return Ternary.False;
                }
                let result = Ternary.True;
                for (let i = 0, len = sourceSignatures.length; i < len; i++) {
                    const related = compareSignaturesIdentical(sourceSignatures[i], targetSignatures[i], /*partialMatch*/ false, /*ignoreThisTypes*/ false, /*ignoreReturnTypes*/ false, isRelatedTo);
                    if (!related) {
                        return Ternary.False;
                    }
                    result &= related;
                }
                return result;
            }

            function eachPropertyRelatedTo(source: Type, target: Type, kind: IndexKind, reportErrors: boolean): Ternary {
                let result = Ternary.True;
                for (const prop of getPropertiesOfObjectType(source)) {
                    if (kind === IndexKind.String || isNumericLiteralName(prop.name)) {
                        const related = isRelatedTo(getTypeOfSymbol(prop), target, reportErrors);
                        if (!related) {
                            if (reportErrors) {
                                reportError(Diagnostics.Property_0_is_incompatible_with_index_signature, symbolToString(prop));
                            }
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
                return result;
            }

            function indexInfoRelatedTo(sourceInfo: IndexInfo, targetInfo: IndexInfo, reportErrors: boolean) {
                const related = isRelatedTo(sourceInfo.type, targetInfo.type, reportErrors);
                if (!related && reportErrors) {
                    reportError(Diagnostics.Index_signatures_are_incompatible);
                }
                return related;
            }

            function indexTypesRelatedTo(source: Type, originalSource: Type, target: Type, kind: IndexKind, reportErrors: boolean) {
                if (relation === identityRelation) {
                    return indexTypesIdenticalTo(source, target, kind);
                }
                const targetInfo = getIndexInfoOfType(target, kind);
                if (!targetInfo || ((targetInfo.type.flags & TypeFlags.Any) && !(originalSource.flags & TypeFlags.Primitive))) {
                    // Index signature of type any permits assignment from everything but primitives
                    return Ternary.True;
                }
                const sourceInfo = getIndexInfoOfType(source, kind) ||
                    kind === IndexKind.Number && getIndexInfoOfType(source, IndexKind.String);
                if (sourceInfo) {
                    return indexInfoRelatedTo(sourceInfo, targetInfo, reportErrors);
                }
                if (isObjectLiteralType(source)) {
                    let related = Ternary.True;
                    if (kind === IndexKind.String) {
                        const sourceNumberInfo = getIndexInfoOfType(source, IndexKind.Number);
                        if (sourceNumberInfo) {
                            related = indexInfoRelatedTo(sourceNumberInfo, targetInfo, reportErrors);
                        }
                    }
                    if (related) {
                        related &= eachPropertyRelatedTo(source, targetInfo.type, kind, reportErrors);
                    }
                    return related;
                }
                if (reportErrors) {
                    reportError(Diagnostics.Index_signature_is_missing_in_type_0, typeToString(source));
                }
                return Ternary.False;
            }

            function indexTypesIdenticalTo(source: Type, target: Type, indexKind: IndexKind): Ternary {
                const targetInfo = getIndexInfoOfType(target, indexKind);
                const sourceInfo = getIndexInfoOfType(source, indexKind);
                if (!sourceInfo && !targetInfo) {
                    return Ternary.True;
                }
                if (sourceInfo && targetInfo && sourceInfo.isReadonly === targetInfo.isReadonly) {
                    return isRelatedTo(sourceInfo.type, targetInfo.type);
                }
                return Ternary.False;
            }

            function constructorVisibilitiesAreCompatible(sourceSignature: Signature, targetSignature: Signature, reportErrors: boolean) {
                if (!sourceSignature.declaration || !targetSignature.declaration) {
                    return true;
                }

                const sourceAccessibility = sourceSignature.declaration.flags & (NodeFlags.Private | NodeFlags.Protected);
                const targetAccessibility = targetSignature.declaration.flags & (NodeFlags.Private | NodeFlags.Protected);

                // A public, protected and private signature is assignable to a private signature.
                if (targetAccessibility === NodeFlags.Private) {
                    return true;
                }

                // A public and protected signature is assignable to a protected signature.
                if (targetAccessibility === NodeFlags.Protected && sourceAccessibility !== NodeFlags.Private) {
                    return true;
                }

                // Only a public signature is assignable to public signature.
                if (targetAccessibility !== NodeFlags.Protected && !sourceAccessibility) {
                    return true;
                }

                if (reportErrors) {
                    reportError(Diagnostics.Cannot_assign_a_0_constructor_type_to_a_1_constructor_type, visibilityToString(sourceAccessibility), visibilityToString(targetAccessibility));
                }

                return false;
            }
        }

        // Return true if the given type is the constructor type for an abstract class
        function isAbstractConstructorType(type: Type) {
            if (type.flags & TypeFlags.Anonymous) {
                const symbol = type.symbol;
                if (symbol && symbol.flags & SymbolFlags.Class) {
                    const declaration = getClassLikeDeclarationOfSymbol(symbol);
                    if (declaration && declaration.flags & NodeFlags.Abstract) {
                        return true;
                    }
                }
            }
            return false;
        }

        // Return true if the given type is part of a deeply nested chain of generic instantiations. We consider this to be the case
        // when structural type comparisons have been started for 10 or more instantiations of the same generic type. It is possible,
        // though highly unlikely, for this test to be true in a situation where a chain of instantiations is not infinitely expanding.
        // Effectively, we will generate a false positive when two types are structurally equal to at least 10 levels, but unequal at
        // some level beyond that.
        function isDeeplyNestedGeneric(type: Type, stack: Type[], depth: number): boolean {
            // We track type references (created by createTypeReference) and instantiated types (created by instantiateType)
            if (type.flags & (TypeFlags.Reference | TypeFlags.Instantiated) && depth >= 5) {
                const symbol = type.symbol;
                let count = 0;
                for (let i = 0; i < depth; i++) {
                    const t = stack[i];
                    if (t.flags & (TypeFlags.Reference | TypeFlags.Instantiated) && t.symbol === symbol) {
                        count++;
                        if (count >= 5) return true;
                    }
                }
            }
            return false;
        }

        function isPropertyIdenticalTo(sourceProp: Symbol, targetProp: Symbol): boolean {
            return compareProperties(sourceProp, targetProp, compareTypesIdentical) !== Ternary.False;
        }

        function compareProperties(sourceProp: Symbol, targetProp: Symbol, compareTypes: (source: Type, target: Type) => Ternary): Ternary {
            // Two members are considered identical when
            // - they are public properties with identical names, optionality, and types,
            // - they are private or protected properties originating in the same declaration and having identical types
            if (sourceProp === targetProp) {
                return Ternary.True;
            }
            const sourcePropAccessibility = getDeclarationFlagsFromSymbol(sourceProp) & (NodeFlags.Private | NodeFlags.Protected);
            const targetPropAccessibility = getDeclarationFlagsFromSymbol(targetProp) & (NodeFlags.Private | NodeFlags.Protected);
            if (sourcePropAccessibility !== targetPropAccessibility) {
                return Ternary.False;
            }
            if (sourcePropAccessibility) {
                if (getTargetSymbol(sourceProp) !== getTargetSymbol(targetProp)) {
                    return Ternary.False;
                }
            }
            else {
                if ((sourceProp.flags & SymbolFlags.Optional) !== (targetProp.flags & SymbolFlags.Optional)) {
                    return Ternary.False;
                }
            }
            if (isReadonlySymbol(sourceProp) !== isReadonlySymbol(targetProp)) {
                return Ternary.False;
            }
            return compareTypes(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp));
        }

        function isMatchingSignature(source: Signature, target: Signature, partialMatch: boolean) {
            // A source signature matches a target signature if the two signatures have the same number of required,
            // optional, and rest parameters.
            if (source.parameters.length === target.parameters.length &&
                source.minArgumentCount === target.minArgumentCount &&
                source.hasRestParameter === target.hasRestParameter) {
                return true;
            }
            // A source signature partially matches a target signature if the target signature has no fewer required
            // parameters and no more overall parameters than the source signature (where a signature with a rest
            // parameter is always considered to have more overall parameters than one without).
            const sourceRestCount = source.hasRestParameter ? 1 : 0;
            const targetRestCount = target.hasRestParameter ? 1 : 0;
            if (partialMatch && source.minArgumentCount <= target.minArgumentCount && (
                sourceRestCount > targetRestCount ||
                sourceRestCount === targetRestCount && source.parameters.length >= target.parameters.length)) {
                return true;
            }
            return false;
        }

        /**
         * See signatureRelatedTo, compareSignaturesIdentical
         */
        function compareSignaturesIdentical(source: Signature, target: Signature, partialMatch: boolean, ignoreThisTypes: boolean, ignoreReturnTypes: boolean, compareTypes: (s: Type, t: Type) => Ternary): Ternary {
            // TODO (drosen): De-duplicate code between related functions.
            if (source === target) {
                return Ternary.True;
            }
            if (!(isMatchingSignature(source, target, partialMatch))) {
                return Ternary.False;
            }
            // Check that the two signatures have the same number of type parameters. We might consider
            // also checking that any type parameter constraints match, but that would require instantiating
            // the constraints with a common set of type arguments to get relatable entities in places where
            // type parameters occur in the constraints. The complexity of doing that doesn't seem worthwhile,
            // particularly as we're comparing erased versions of the signatures below.
            if ((source.typeParameters ? source.typeParameters.length : 0) !== (target.typeParameters ? target.typeParameters.length : 0)) {
                return Ternary.False;
            }
            // Spec 1.0 Section 3.8.3 & 3.8.4:
            // M and N (the signatures) are instantiated using type Any as the type argument for all type parameters declared by M and N
            source = getErasedSignature(source);
            target = getErasedSignature(target);
            let result = Ternary.True;

            if (!ignoreThisTypes) {
                const sourceThisType = getThisTypeOfSignature(source);
                if (sourceThisType) {
                    const targetThisType = getThisTypeOfSignature(target);
                    if (targetThisType) {
                        const related = compareTypes(sourceThisType, targetThisType);
                        if (!related) {
                            return Ternary.False;
                        }
                        result &= related;
                    }
                }
            }

            const targetLen = target.parameters.length;
            for (let i = 0; i < targetLen; i++) {
                const s = isRestParameterIndex(source, i) ? getRestTypeOfSignature(source) : getTypeOfParameter(source.parameters[i]);
                const t = isRestParameterIndex(target, i) ? getRestTypeOfSignature(target) : getTypeOfParameter(target.parameters[i]);
                const related = compareTypes(s, t);
                if (!related) {
                    return Ternary.False;
                }
                result &= related;
            }
            if (!ignoreReturnTypes) {
                result &= compareTypes(getReturnTypeOfSignature(source), getReturnTypeOfSignature(target));
            }
            return result;
        }

        function isRestParameterIndex(signature: Signature, parameterIndex: number) {
            return signature.hasRestParameter && parameterIndex >= signature.parameters.length - 1;
        }

        function isSupertypeOfEach(candidate: Type, types: Type[]): boolean {
            for (const t of types) {
                if (candidate !== t && !isTypeSubtypeOf(t, candidate)) return false;
            }
            return true;
        }

        function getCommonSupertype(types: Type[]): Type {
            if (!strictNullChecks) {
                return forEach(types, t => isSupertypeOfEach(t, types) ? t : undefined);
            }
            const primaryTypes = filter(types, t => !(t.flags & TypeFlags.Nullable));
            if (!primaryTypes.length) {
                return getUnionType(types, /*subtypeReduction*/ true);
            }
            const supertype = forEach(primaryTypes, t => isSupertypeOfEach(t, primaryTypes) ? t : undefined);
            return supertype && includeFalsyTypes(supertype, getFalsyFlagsOfTypes(types) & TypeFlags.Nullable);
        }

        function reportNoCommonSupertypeError(types: Type[], errorLocation: Node, errorMessageChainHead: DiagnosticMessageChain): void {
            // The downfallType/bestSupertypeDownfallType is the first type that caused a particular candidate
            // to not be the common supertype. So if it weren't for this one downfallType (and possibly others),
            // the type in question could have been the common supertype.
            let bestSupertype: Type;
            let bestSupertypeDownfallType: Type;
            let bestSupertypeScore = 0;

            for (let i = 0; i < types.length; i++) {
                let score = 0;
                let downfallType: Type = undefined;
                for (let j = 0; j < types.length; j++) {
                    if (isTypeSubtypeOf(types[j], types[i])) {
                        score++;
                    }
                    else if (!downfallType) {
                        downfallType = types[j];
                    }
                }

                Debug.assert(!!downfallType, "If there is no common supertype, each type should have a downfallType");

                if (score > bestSupertypeScore) {
                    bestSupertype = types[i];
                    bestSupertypeDownfallType = downfallType;
                    bestSupertypeScore = score;
                }

                // types.length - 1 is the maximum score, given that getCommonSupertype returned false
                if (bestSupertypeScore === types.length - 1) {
                    break;
                }
            }

            // In the following errors, the {1} slot is before the {0} slot because checkTypeSubtypeOf supplies the
            // subtype as the first argument to the error
            checkTypeSubtypeOf(bestSupertypeDownfallType, bestSupertype, errorLocation,
                Diagnostics.Type_argument_candidate_1_is_not_a_valid_type_argument_because_it_is_not_a_supertype_of_candidate_0,
                errorMessageChainHead);
        }

        function isArrayType(type: Type): boolean {
            return type.flags & TypeFlags.Reference && (<TypeReference>type).target === globalArrayType;
        }

        function isArrayLikeType(type: Type): boolean {
            // A type is array-like if it is a reference to the global Array or global ReadonlyArray type,
            // or if it is not the undefined or null type and if it is assignable to ReadonlyArray<any>
            return type.flags & TypeFlags.Reference && ((<TypeReference>type).target === globalArrayType || (<TypeReference>type).target === globalReadonlyArrayType) ||
                !(type.flags & TypeFlags.Nullable) && isTypeAssignableTo(type, anyReadonlyArrayType);
        }

        function isTupleLikeType(type: Type): boolean {
            return !!getPropertyOfType(type, "0");
        }

        function isUnitType(type: Type): boolean {
            return (type.flags & (TypeFlags.Literal | TypeFlags.Undefined | TypeFlags.Null)) !== 0;
        }

        function isUnitUnionType(type: Type): boolean {
            return type.flags & TypeFlags.Boolean ? true :
                type.flags & TypeFlags.Union ? type.flags & TypeFlags.Enum ? true : !forEach((<UnionType>type).types, t => !isUnitType(t)) :
                isUnitType(type);
        }

        function getBaseTypeOfUnitType(type: Type): Type {
            return type.flags & TypeFlags.StringLiteral ? stringType :
                type.flags & TypeFlags.NumberLiteral ? numberType :
                type.flags & TypeFlags.BooleanLiteral ? booleanType :
                type.flags & TypeFlags.EnumLiteral ? (<EnumLiteralType>type).baseType :
                type.flags & TypeFlags.Union && !(type.flags & TypeFlags.Enum) ? getUnionType(map((<UnionType>type).types, getBaseTypeOfUnitType)) :
                type;
        }

        /**
         * Check if a Type was written as a tuple type literal.
         * Prefer using isTupleLikeType() unless the use of `elementTypes` is required.
         */
        function isTupleType(type: Type): type is TupleType {
            return !!(type.flags & TypeFlags.Tuple);
        }

        function getFalsyFlagsOfTypes(types: Type[]): TypeFlags {
            let result: TypeFlags = 0;
            for (const t of types) {
                result |= getFalsyFlags(t);
            }
            return result;
        }

        // Returns the String, Number, Boolean, StringLiteral, NumberLiteral, BooleanLiteral, Void, Undefined, or Null
        // flags for the string, number, boolean, "", 0, false, void, undefined, or null types respectively. Returns
        // no flags for all other types (including non-falsy literal types).
        function getFalsyFlags(type: Type): TypeFlags {
            return type.flags & TypeFlags.Union ? getFalsyFlagsOfTypes((<UnionType>type).types) :
                type.flags & TypeFlags.StringLiteral ? type === emptyStringType ? TypeFlags.StringLiteral : 0 :
                type.flags & TypeFlags.NumberLiteral ? type === zeroType ? TypeFlags.NumberLiteral : 0 :
                type.flags & TypeFlags.BooleanLiteral ? type === falseType ? TypeFlags.BooleanLiteral : 0 :
                type.flags & TypeFlags.PossiblyFalsy;
        }

        function includeFalsyTypes(type: Type, flags: TypeFlags) {
            if ((getFalsyFlags(type) & flags) === flags) {
                return type;
            }
            const types = [type];
            if (flags & TypeFlags.StringLike) types.push(emptyStringType);
            if (flags & TypeFlags.NumberLike) types.push(zeroType);
            if (flags & TypeFlags.BooleanLike) types.push(falseType);
            if (flags & TypeFlags.Void) types.push(voidType);
            if (flags & TypeFlags.Undefined) types.push(undefinedType);
            if (flags & TypeFlags.Null) types.push(nullType);
            return getUnionType(types, /*subtypeReduction*/ true);
        }

        function removeDefinitelyFalsyTypes(type: Type): Type {
            return getFalsyFlags(type) & TypeFlags.DefinitelyFalsy ?
                filterType(type, t => !(getFalsyFlags(t) & TypeFlags.DefinitelyFalsy)) :
                type;
        }

        function getNonNullableType(type: Type): Type {
            return strictNullChecks ? getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull) : type;
        }

        /**
         * Return true if type was inferred from an object literal or written as an object type literal
         * with no call or construct signatures.
         */
        function isObjectLiteralType(type: Type) {
            return type.symbol && (type.symbol.flags & (SymbolFlags.ObjectLiteral | SymbolFlags.TypeLiteral)) !== 0 &&
                getSignaturesOfType(type, SignatureKind.Call).length === 0 &&
                getSignaturesOfType(type, SignatureKind.Construct).length === 0;
        }

        function createTransientSymbol(source: Symbol, type: Type) {
            const symbol = <TransientSymbol>createSymbol(source.flags | SymbolFlags.Transient, source.name);
            symbol.declarations = source.declarations;
            symbol.parent = source.parent;
            symbol.type = type;
            symbol.target = source;
            if (source.valueDeclaration) {
                symbol.valueDeclaration = source.valueDeclaration;
            }
            return symbol;
        }

        function transformTypeOfMembers(type: Type, f: (propertyType: Type) => Type) {
            const members = createMap<Symbol>();
            for (const property of getPropertiesOfObjectType(type)) {
                const original = getTypeOfSymbol(property);
                const updated = f(original);
                members[property.name] = updated === original ? property : createTransientSymbol(property, updated);
            };
            return members;
        }

        /**
         * If the the provided object literal is subject to the excess properties check,
         * create a new that is exempt. Recursively mark object literal members as exempt.
         * Leave signatures alone since they are not subject to the check.
         */
        function getRegularTypeOfObjectLiteral(type: Type): Type {
            if (!(type.flags & TypeFlags.FreshObjectLiteral)) {
                return type;
            }
            const regularType = (<FreshObjectLiteralType>type).regularType;
            if (regularType) {
                return regularType;
            }

            const resolved = <ResolvedType>type;
            const members = transformTypeOfMembers(type, getRegularTypeOfObjectLiteral);
            const regularNew = createAnonymousType(resolved.symbol,
                members,
                resolved.callSignatures,
                resolved.constructSignatures,
                resolved.stringIndexInfo,
                resolved.numberIndexInfo);
            regularNew.flags = resolved.flags & ~TypeFlags.FreshObjectLiteral;
            (<FreshObjectLiteralType>type).regularType = regularNew;
            return regularNew;
        }

        function getWidenedTypeOfObjectLiteral(type: Type): Type {
            const members = transformTypeOfMembers(type, prop => {
                const widened = getWidenedType(prop);
                return prop === widened ? prop : widened;
            });
            const stringIndexInfo = getIndexInfoOfType(type, IndexKind.String);
            const numberIndexInfo = getIndexInfoOfType(type, IndexKind.Number);
            return createAnonymousType(type.symbol, members, emptyArray, emptyArray,
                stringIndexInfo && createIndexInfo(getWidenedType(stringIndexInfo.type), stringIndexInfo.isReadonly),
                numberIndexInfo && createIndexInfo(getWidenedType(numberIndexInfo.type), numberIndexInfo.isReadonly));
        }

        function getWidenedConstituentType(type: Type): Type {
            return type.flags & TypeFlags.Nullable ? type : getWidenedType(type);
        }

        function getWidenedType(type: Type): Type {
            if (type.flags & TypeFlags.RequiresWidening) {
                if (type.flags & TypeFlags.Nullable) {
                    return anyType;
                }
                if (type.flags & TypeFlags.ObjectLiteral) {
                    return getWidenedTypeOfObjectLiteral(type);
                }
                if (type.flags & TypeFlags.Union) {
                    return getUnionType(map((<UnionType>type).types, getWidenedConstituentType));
                }
                if (isArrayType(type)) {
                    return createArrayType(getWidenedType((<TypeReference>type).typeArguments[0]));
                }
                if (isTupleType(type)) {
                    return createTupleType(map(type.elementTypes, getWidenedType));
                }
            }
            return type;
        }

        /**
         * Reports implicit any errors that occur as a result of widening 'null' and 'undefined'
         * to 'any'. A call to reportWideningErrorsInType is normally accompanied by a call to
         * getWidenedType. But in some cases getWidenedType is called without reporting errors
         * (type argument inference is an example).
         *
         * The return value indicates whether an error was in fact reported. The particular circumstances
         * are on a best effort basis. Currently, if the null or undefined that causes widening is inside
         * an object literal property (arbitrarily deeply), this function reports an error. If no error is
         * reported, reportImplicitAnyError is a suitable fallback to report a general error.
         */
        function reportWideningErrorsInType(type: Type): boolean {
            let errorReported = false;
            if (type.flags & TypeFlags.Union) {
                for (const t of (<UnionType>type).types) {
                    if (reportWideningErrorsInType(t)) {
                        errorReported = true;
                    }
                }
            }
            if (isArrayType(type)) {
                return reportWideningErrorsInType((<TypeReference>type).typeArguments[0]);
            }
            if (isTupleType(type)) {
                for (const t of type.elementTypes) {
                    if (reportWideningErrorsInType(t)) {
                        errorReported = true;
                    }
                }
            }
            if (type.flags & TypeFlags.ObjectLiteral) {
                for (const p of getPropertiesOfObjectType(type)) {
                    const t = getTypeOfSymbol(p);
                    if (t.flags & TypeFlags.ContainsWideningType) {
                        if (!reportWideningErrorsInType(t)) {
                            error(p.valueDeclaration, Diagnostics.Object_literal_s_property_0_implicitly_has_an_1_type, p.name, typeToString(getWidenedType(t)));
                        }
                        errorReported = true;
                    }
                }
            }
            return errorReported;
        }

        function reportImplicitAnyError(declaration: Declaration, type: Type) {
            const typeAsString = typeToString(getWidenedType(type));
            let diagnostic: DiagnosticMessage;
            switch (declaration.kind) {
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    diagnostic = Diagnostics.Member_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.Parameter:
                    diagnostic = (<ParameterDeclaration>declaration).dotDotDotToken ?
                        Diagnostics.Rest_parameter_0_implicitly_has_an_any_type :
                        Diagnostics.Parameter_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.BindingElement:
                    diagnostic = Diagnostics.Binding_element_0_implicitly_has_an_1_type;
                    break;
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    if (!declaration.name) {
                        error(declaration, Diagnostics.Function_expression_which_lacks_return_type_annotation_implicitly_has_an_0_return_type, typeAsString);
                        return;
                    }
                    diagnostic = Diagnostics._0_which_lacks_return_type_annotation_implicitly_has_an_1_return_type;
                    break;
                default:
                    diagnostic = Diagnostics.Variable_0_implicitly_has_an_1_type;
            }
            error(declaration, diagnostic, declarationNameToString(declaration.name), typeAsString);
        }

        function reportErrorsFromWidening(declaration: Declaration, type: Type) {
            if (produceDiagnostics && compilerOptions.noImplicitAny && type.flags & TypeFlags.ContainsWideningType) {
                // Report implicit any error within type if possible, otherwise report error on declaration
                if (!reportWideningErrorsInType(type)) {
                    reportImplicitAnyError(declaration, type);
                }
            }
        }

        function forEachMatchingParameterType(source: Signature, target: Signature, callback: (s: Type, t: Type) => void) {
            const sourceMax = source.parameters.length;
            const targetMax = target.parameters.length;
            let count: number;
            if (source.hasRestParameter && target.hasRestParameter) {
                count = Math.max(sourceMax, targetMax);
            }
            else if (source.hasRestParameter) {
                count = targetMax;
            }
            else if (target.hasRestParameter) {
                count = sourceMax;
            }
            else {
                count = Math.min(sourceMax, targetMax);
            }
            for (let i = 0; i < count; i++) {
                callback(getTypeAtPosition(source, i), getTypeAtPosition(target, i));
            }
        }

        function createInferenceContext(typeParameters: TypeParameter[], inferUnionTypes: boolean): InferenceContext {
            const inferences = map(typeParameters, createTypeInferencesObject);

            return {
                typeParameters,
                inferUnionTypes,
                inferences,
                inferredTypes: new Array(typeParameters.length),
            };
        }

        function createTypeInferencesObject(): TypeInferences {
            return {
                primary: undefined,
                secondary: undefined,
                isFixed: false,
            };
        }

        // Return true if the given type could possibly reference a type parameter for which
        // we perform type inference (i.e. a type parameter of a generic function). We cache
        // results for union and intersection types for performance reasons.
        function couldContainTypeParameters(type: Type): boolean {
            return !!(type.flags & TypeFlags.TypeParameter ||
                type.flags & TypeFlags.Reference && forEach((<TypeReference>type).typeArguments, couldContainTypeParameters) ||
                type.flags & TypeFlags.Tuple && forEach((<TupleType>type).elementTypes, couldContainTypeParameters) ||
                type.flags & TypeFlags.Anonymous && type.symbol && type.symbol.flags & (SymbolFlags.Method | SymbolFlags.TypeLiteral | SymbolFlags.Class) ||
                type.flags & TypeFlags.UnionOrIntersection && couldUnionOrIntersectionContainTypeParameters(<UnionOrIntersectionType>type));
        }

        function couldUnionOrIntersectionContainTypeParameters(type: UnionOrIntersectionType): boolean {
            if (type.couldContainTypeParameters === undefined) {
                type.couldContainTypeParameters = forEach(type.types, couldContainTypeParameters);
            }
            return type.couldContainTypeParameters;
        }

        function inferTypes(context: InferenceContext, source: Type, target: Type) {
            let sourceStack: Type[];
            let targetStack: Type[];
            let depth = 0;
            let inferiority = 0;
            const visited = createMap<boolean>();
            inferFromTypes(source, target);

            function isInProcess(source: Type, target: Type) {
                for (let i = 0; i < depth; i++) {
                    if (source === sourceStack[i] && target === targetStack[i]) {
                        return true;
                    }
                }
                return false;
            }

            function inferFromTypes(source: Type, target: Type) {
                if (!couldContainTypeParameters(target)) {
                    return;
                }
                if (source.flags & TypeFlags.Union && target.flags & TypeFlags.Union && !(source.flags & TypeFlags.Enum && target.flags & TypeFlags.Enum) ||
                    source.flags & TypeFlags.Intersection && target.flags & TypeFlags.Intersection) {
                    // Source and target are both unions or both intersections. If source and target
                    // are the same type, just relate each constituent type to itself.
                    if (source === target) {
                        for (const t of (<UnionOrIntersectionType>source).types) {
                            inferFromTypes(t, t);
                        }
                        return;
                    }
                    // Find each target constituent type that has an identically matching source
                    // constituent type, and for each such target constituent type infer from the type to
                    // itself. When inferring from a type to itself we effectively find all type parameter
                    // occurrences within that type and infer themselves as their type arguments.
                    let matchingTypes: Type[];
                    for (const t of (<UnionOrIntersectionType>target).types) {
                        if (typeIdenticalToSomeType(t, (<UnionOrIntersectionType>source).types)) {
                            (matchingTypes || (matchingTypes = [])).push(t);
                            inferFromTypes(t, t);
                        }
                    }
                    // Next, to improve the quality of inferences, reduce the source and target types by
                    // removing the identically matched constituents. For example, when inferring from
                    // 'string | string[]' to 'string | T' we reduce the types to 'string[]' and 'T'.
                    if (matchingTypes) {
                        source = removeTypesFromUnionOrIntersection(<UnionOrIntersectionType>source, matchingTypes);
                        target = removeTypesFromUnionOrIntersection(<UnionOrIntersectionType>target, matchingTypes);
                    }
                }
                if (target.flags & TypeFlags.TypeParameter) {
                    // If target is a type parameter, make an inference, unless the source type contains
                    // the anyFunctionType (the wildcard type that's used to avoid contextually typing functions).
                    // Because the anyFunctionType is internal, it should not be exposed to the user by adding
                    // it as an inference candidate. Hopefully, a better candidate will come along that does
                    // not contain anyFunctionType when we come back to this argument for its second round
                    // of inference.
                    if (source.flags & TypeFlags.ContainsAnyFunctionType) {
                        return;
                    }
                    const typeParameters = context.typeParameters;
                    for (let i = 0; i < typeParameters.length; i++) {
                        if (target === typeParameters[i]) {
                            const inferences = context.inferences[i];
                            if (!inferences.isFixed) {
                                // Any inferences that are made to a type parameter in a union type are inferior
                                // to inferences made to a flat (non-union) type. This is because if we infer to
                                // T | string[], we really don't know if we should be inferring to T or not (because
                                // the correct constituent on the target side could be string[]). Therefore, we put
                                // such inferior inferences into a secondary bucket, and only use them if the primary
                                // bucket is empty.
                                const candidates = inferiority ?
                                    inferences.secondary || (inferences.secondary = []) :
                                    inferences.primary || (inferences.primary = []);
                                if (!contains(candidates, source)) {
                                    candidates.push(source);
                                }
                            }
                            return;
                        }
                    }
                }
                else if (source.flags & TypeFlags.Reference && target.flags & TypeFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                    // If source and target are references to the same generic type, infer from type arguments
                    const sourceTypes = (<TypeReference>source).typeArguments || emptyArray;
                    const targetTypes = (<TypeReference>target).typeArguments || emptyArray;
                    const count = sourceTypes.length < targetTypes.length ? sourceTypes.length : targetTypes.length;
                    for (let i = 0; i < count; i++) {
                        inferFromTypes(sourceTypes[i], targetTypes[i]);
                    }
                }
                else if (source.flags & TypeFlags.Tuple && target.flags & TypeFlags.Tuple && (<TupleType>source).elementTypes.length === (<TupleType>target).elementTypes.length) {
                    // If source and target are tuples of the same size, infer from element types
                    const sourceTypes = (<TupleType>source).elementTypes;
                    const targetTypes = (<TupleType>target).elementTypes;
                    for (let i = 0; i < sourceTypes.length; i++) {
                        inferFromTypes(sourceTypes[i], targetTypes[i]);
                    }
                }
                else if (target.flags & TypeFlags.UnionOrIntersection) {
                    const targetTypes = (<UnionOrIntersectionType>target).types;
                    let typeParameterCount = 0;
                    let typeParameter: TypeParameter;
                    // First infer to each type in union or intersection that isn't a type parameter
                    for (const t of targetTypes) {
                        if (t.flags & TypeFlags.TypeParameter && contains(context.typeParameters, t)) {
                            typeParameter = <TypeParameter>t;
                            typeParameterCount++;
                        }
                        else {
                            inferFromTypes(source, t);
                        }
                    }
                    // Next, if target containings a single naked type parameter, make a secondary inference to that type
                    // parameter. This gives meaningful results for union types in co-variant positions and intersection
                    // types in contra-variant positions (such as callback parameters).
                    if (typeParameterCount === 1) {
                        inferiority++;
                        inferFromTypes(source, typeParameter);
                        inferiority--;
                    }
                }
                else if (source.flags & TypeFlags.UnionOrIntersection) {
                    // Source is a union or intersection type, infer from each constituent type
                    const sourceTypes = (<UnionOrIntersectionType>source).types;
                    for (const sourceType of sourceTypes) {
                        inferFromTypes(sourceType, target);
                    }
                }
                else {
                    source = getApparentType(source);
                    if (source.flags & TypeFlags.ObjectType) {
                        if (isInProcess(source, target)) {
                            return;
                        }
                        if (isDeeplyNestedGeneric(source, sourceStack, depth) && isDeeplyNestedGeneric(target, targetStack, depth)) {
                            return;
                        }
                        const key = source.id + "," + target.id;
                        if (visited[key]) {
                            return;
                        }
                        visited[key] = true;
                        if (depth === 0) {
                            sourceStack = [];
                            targetStack = [];
                        }
                        sourceStack[depth] = source;
                        targetStack[depth] = target;
                        depth++;
                        inferFromProperties(source, target);
                        inferFromSignatures(source, target, SignatureKind.Call);
                        inferFromSignatures(source, target, SignatureKind.Construct);
                        inferFromIndexTypes(source, target);
                        depth--;
                    }
                }
            }

            function inferFromProperties(source: Type, target: Type) {
                const properties = getPropertiesOfObjectType(target);
                for (const targetProp of properties) {
                    const sourceProp = getPropertyOfObjectType(source, targetProp.name);
                    if (sourceProp) {
                        inferFromTypes(getTypeOfSymbol(sourceProp), getTypeOfSymbol(targetProp));
                    }
                }
            }

            function inferFromSignatures(source: Type, target: Type, kind: SignatureKind) {
                const sourceSignatures = getSignaturesOfType(source, kind);
                const targetSignatures = getSignaturesOfType(target, kind);
                const sourceLen = sourceSignatures.length;
                const targetLen = targetSignatures.length;
                const len = sourceLen < targetLen ? sourceLen : targetLen;
                for (let i = 0; i < len; i++) {
                    inferFromSignature(getErasedSignature(sourceSignatures[sourceLen - len + i]), getErasedSignature(targetSignatures[targetLen - len + i]));
                }
            }

            function inferFromSignature(source: Signature, target: Signature) {
                forEachMatchingParameterType(source, target, inferFromTypes);

                if (source.typePredicate && target.typePredicate && source.typePredicate.kind === target.typePredicate.kind) {
                    inferFromTypes(source.typePredicate.type, target.typePredicate.type);
                }
                else {
                    inferFromTypes(getReturnTypeOfSignature(source), getReturnTypeOfSignature(target));
                }
            }

            function inferFromIndexTypes(source: Type, target: Type) {
                const targetStringIndexType = getIndexTypeOfType(target, IndexKind.String);
                if (targetStringIndexType) {
                    const sourceIndexType = getIndexTypeOfType(source, IndexKind.String) ||
                        getImplicitIndexTypeOfType(source, IndexKind.String);
                    if (sourceIndexType) {
                        inferFromTypes(sourceIndexType, targetStringIndexType);
                    }
                }
                const targetNumberIndexType = getIndexTypeOfType(target, IndexKind.Number);
                if (targetNumberIndexType) {
                    const sourceIndexType = getIndexTypeOfType(source, IndexKind.Number) ||
                        getIndexTypeOfType(source, IndexKind.String) ||
                        getImplicitIndexTypeOfType(source, IndexKind.Number);
                    if (sourceIndexType) {
                        inferFromTypes(sourceIndexType, targetNumberIndexType);
                    }
                }
            }
        }

        function typeIdenticalToSomeType(type: Type, types: Type[]): boolean {
            for (const t of types) {
                if (isTypeIdenticalTo(t, type)) {
                    return true;
                }
            }
            return false;
        }

        /**
         * Return a new union or intersection type computed by removing a given set of types
         * from a given union or intersection type.
         */
        function removeTypesFromUnionOrIntersection(type: UnionOrIntersectionType, typesToRemove: Type[]) {
            const reducedTypes: Type[] = [];
            for (const t of type.types) {
                if (!typeIdenticalToSomeType(t, typesToRemove)) {
                    reducedTypes.push(t);
                }
            }
            return type.flags & TypeFlags.Union ? getUnionType(reducedTypes) : getIntersectionType(reducedTypes);
        }

        function getInferenceCandidates(context: InferenceContext, index: number): Type[] {
            const inferences = context.inferences[index];
            return inferences.primary || inferences.secondary || emptyArray;
        }

        function getInferredType(context: InferenceContext, index: number): Type {
            let inferredType = context.inferredTypes[index];
            let inferenceSucceeded: boolean;
            if (!inferredType) {
                const inferences = getInferenceCandidates(context, index);
                if (inferences.length) {
                    // Infer widened union or supertype, or the unknown type for no common supertype
                    const unionOrSuperType = context.inferUnionTypes ? getUnionType(inferences, /*subtypeReduction*/ true) : getCommonSupertype(inferences);
                    inferredType = unionOrSuperType ? getWidenedType(unionOrSuperType) : unknownType;
                    inferenceSucceeded = !!unionOrSuperType;
                }
                else {
                    // Infer the empty object type when no inferences were made. It is important to remember that
                    // in this case, inference still succeeds, meaning there is no error for not having inference
                    // candidates. An inference error only occurs when there are *conflicting* candidates, i.e.
                    // candidates with no common supertype.
                    inferredType = emptyObjectType;
                    inferenceSucceeded = true;
                }
                context.inferredTypes[index] = inferredType;

                // Only do the constraint check if inference succeeded (to prevent cascading errors)
                if (inferenceSucceeded) {
                    const constraint = getConstraintOfTypeParameter(context.typeParameters[index]);
                    if (constraint) {
                        const instantiatedConstraint = instantiateType(constraint, getInferenceMapper(context));
                        if (!isTypeAssignableTo(inferredType, getTypeWithThisArgument(instantiatedConstraint, inferredType))) {
                            context.inferredTypes[index] = inferredType = instantiatedConstraint;
                        }
                    }
                }
                else if (context.failedTypeParameterIndex === undefined || context.failedTypeParameterIndex > index) {
                    // If inference failed, it is necessary to record the index of the failed type parameter (the one we are on).
                    // It might be that inference has already failed on a later type parameter on a previous call to inferTypeArguments.
                    // So if this failure is on preceding type parameter, this type parameter is the new failure index.
                    context.failedTypeParameterIndex = index;
                }
            }
            return inferredType;
        }

        function getInferredTypes(context: InferenceContext): Type[] {
            for (let i = 0; i < context.inferredTypes.length; i++) {
                getInferredType(context, i);
            }
            return context.inferredTypes;
        }

        // EXPRESSION TYPE CHECKING

        function getResolvedSymbol(node: Identifier): Symbol {
            const links = getNodeLinks(node);
            if (!links.resolvedSymbol) {
                links.resolvedSymbol = !nodeIsMissing(node) && resolveName(node, node.text, SymbolFlags.Value | SymbolFlags.ExportValue, Diagnostics.Cannot_find_name_0, node) || unknownSymbol;
            }
            return links.resolvedSymbol;
        }

        function isInTypeQuery(node: Node): boolean {
            // TypeScript 1.0 spec (April 2014): 3.6.3
            // A type query consists of the keyword typeof followed by an expression.
            // The expression is restricted to a single identifier or a sequence of identifiers separated by periods
            while (node) {
                switch (node.kind) {
                    case SyntaxKind.TypeQuery:
                        return true;
                    case SyntaxKind.Identifier:
                    case SyntaxKind.QualifiedName:
                        node = node.parent;
                        continue;
                    default:
                        return false;
                }
            }
            Debug.fail("should not get here");
        }

        // Return the flow cache key for a "dotted name" (i.e. a sequence of identifiers
        // separated by dots). The key consists of the id of the symbol referenced by the
        // leftmost identifier followed by zero or more property names separated by dots.
        // The result is undefined if the reference isn't a dotted name.
        function getFlowCacheKey(node: Node): string {
            if (node.kind === SyntaxKind.Identifier) {
                const symbol = getResolvedSymbol(<Identifier>node);
                return symbol !== unknownSymbol ? "" + getSymbolId(symbol) : undefined;
            }
            if (node.kind === SyntaxKind.ThisKeyword) {
                return "0";
            }
            if (node.kind === SyntaxKind.PropertyAccessExpression) {
                const key = getFlowCacheKey((<PropertyAccessExpression>node).expression);
                return key && key + "." + (<PropertyAccessExpression>node).name.text;
            }
            return undefined;
        }

        function getLeftmostIdentifierOrThis(node: Node): Node {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.ThisKeyword:
                    return node;
                case SyntaxKind.PropertyAccessExpression:
                    return getLeftmostIdentifierOrThis((<PropertyAccessExpression>node).expression);
            }
            return undefined;
        }

        function isMatchingReference(source: Node, target: Node): boolean {
            switch (source.kind) {
                case SyntaxKind.Identifier:
                    return target.kind === SyntaxKind.Identifier && getResolvedSymbol(<Identifier>source) === getResolvedSymbol(<Identifier>target) ||
                        (target.kind === SyntaxKind.VariableDeclaration || target.kind === SyntaxKind.BindingElement) &&
                        getExportSymbolOfValueSymbolIfExported(getResolvedSymbol(<Identifier>source)) === getSymbolOfNode(target);
                case SyntaxKind.ThisKeyword:
                    return target.kind === SyntaxKind.ThisKeyword;
                case SyntaxKind.PropertyAccessExpression:
                    return target.kind === SyntaxKind.PropertyAccessExpression &&
                        (<PropertyAccessExpression>source).name.text === (<PropertyAccessExpression>target).name.text &&
                        isMatchingReference((<PropertyAccessExpression>source).expression, (<PropertyAccessExpression>target).expression);
            }
            return false;
        }

        function containsMatchingReference(source: Node, target: Node) {
            while (source.kind === SyntaxKind.PropertyAccessExpression) {
                source = (<PropertyAccessExpression>source).expression;
                if (isMatchingReference(source, target)) {
                    return true;
                }
            }
            return false;
        }

        // Return true if target is a property access xxx.yyy, source is a property access xxx.zzz, the declared
        // type of xxx is a union type, and yyy is a property that is possibly a discriminant. We consider a property
        // a possible discriminant if its type differs in the constituents of containing union type, and if every
        // choice is a unit type or a union of unit types.
        function containsMatchingReferenceDiscriminant(source: Node, target: Node) {
            return target.kind === SyntaxKind.PropertyAccessExpression &&
                containsMatchingReference(source, (<PropertyAccessExpression>target).expression) &&
                isDiscriminantProperty(getDeclaredTypeOfReference((<PropertyAccessExpression>target).expression), (<PropertyAccessExpression>target).name.text);
        }

        function getDeclaredTypeOfReference(expr: Node): Type {
            if (expr.kind === SyntaxKind.Identifier) {
                return getTypeOfSymbol(getResolvedSymbol(<Identifier>expr));
            }
            if (expr.kind === SyntaxKind.PropertyAccessExpression) {
                const type = getDeclaredTypeOfReference((<PropertyAccessExpression>expr).expression);
                return type && getTypeOfPropertyOfType(type, (<PropertyAccessExpression>expr).name.text);
            }
            return undefined;
        }

        function isDiscriminantProperty(type: Type, name: string) {
            if (type && type.flags & TypeFlags.Union) {
                let prop = getPropertyOfType(type, name);
                if (!prop) {
                    // The type may be a union that includes nullable or primitive types. If filtering
                    // those out produces a different type, get the property from that type instead.
                    // Effectively, we're checking if this *could* be a discriminant property once nullable
                    // and primitive types are removed by other type guards.
                    const filteredType = getTypeWithFacts(type, TypeFacts.Discriminatable);
                    if (filteredType !== type && filteredType.flags & TypeFlags.Union) {
                        prop = getPropertyOfType(filteredType, name);
                    }
                }
                if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
                        (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
                            isUnitUnionType(getTypeOfSymbol(prop));
                    }
                    return (<TransientSymbol>prop).isDiscriminantProperty;
                }
            }
            return false;
        }

        function isOrContainsMatchingReference(source: Node, target: Node) {
            return isMatchingReference(source, target) || containsMatchingReference(source, target);
        }

        function hasMatchingArgument(callExpression: CallExpression, reference: Node) {
            if (callExpression.arguments) {
                for (const argument of callExpression.arguments) {
                    if (isOrContainsMatchingReference(reference, argument)) {
                        return true;
                    }
                }
            }
            if (callExpression.expression.kind === SyntaxKind.PropertyAccessExpression &&
                isOrContainsMatchingReference(reference, (<PropertyAccessExpression>callExpression.expression).expression)) {
                return true;
            }
            return false;
        }

        function getFlowNodeId(flow: FlowNode): number {
            if (!flow.id) {
                flow.id = nextFlowId;
                nextFlowId++;
            }
            return flow.id;
        }

        function typeMaybeAssignableTo(source: Type, target: Type) {
            if (!(source.flags & TypeFlags.Union)) {
                return isTypeAssignableTo(source, target);
            }
            for (const t of (<UnionType>source).types) {
                if (isTypeAssignableTo(t, target)) {
                    return true;
                }
            }
            return false;
        }

        // Remove those constituent types of declaredType to which no constituent type of assignedType is assignable.
        // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
        // we remove type string.
        function getAssignmentReducedType(declaredType: UnionType, assignedType: Type) {
            if (declaredType !== assignedType) {
                const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
                if (reducedType !== neverType) {
                    return reducedType;
                }
            }
            return declaredType;
        }

        function getTypeFactsOfTypes(types: Type[]): TypeFacts {
            let result: TypeFacts = TypeFacts.None;
            for (const t of types) {
                result |= getTypeFacts(t);
            }
            return result;
        }

        function isFunctionObjectType(type: ObjectType): boolean {
            // We do a quick check for a "bind" property before performing the more expensive subtype
            // check. This gives us a quicker out in the common case where an object type is not a function.
            const resolved = resolveStructuredTypeMembers(type);
            return !!(resolved.callSignatures.length || resolved.constructSignatures.length ||
                resolved.members["bind"] && isTypeSubtypeOf(type, globalFunctionType));
        }

        function getTypeFacts(type: Type): TypeFacts {
            const flags = type.flags;
            if (flags & TypeFlags.String) {
                return strictNullChecks ? TypeFacts.StringStrictFacts : TypeFacts.StringFacts;
            }
            if (flags & TypeFlags.StringLiteral) {
                return strictNullChecks ?
                    type === emptyStringType ? TypeFacts.EmptyStringStrictFacts : TypeFacts.NonEmptyStringStrictFacts :
                    type === emptyStringType ? TypeFacts.EmptyStringFacts : TypeFacts.NonEmptyStringFacts;
            }
            if (flags & (TypeFlags.Number | TypeFlags.Enum)) {
                return strictNullChecks ? TypeFacts.NumberStrictFacts : TypeFacts.NumberFacts;
            }
            if (flags & (TypeFlags.NumberLiteral | TypeFlags.EnumLiteral)) {
                const isZero = type === zeroType || type.flags & TypeFlags.EnumLiteral && (<LiteralType>type).text === "0";
                return strictNullChecks ?
                    isZero ? TypeFacts.ZeroStrictFacts : TypeFacts.NonZeroStrictFacts :
                    isZero ? TypeFacts.ZeroFacts : TypeFacts.NonZeroFacts;
            }
            if (flags & TypeFlags.Boolean) {
                return strictNullChecks ? TypeFacts.BooleanStrictFacts : TypeFacts.BooleanFacts;
            }
            if (flags & TypeFlags.BooleanLike) {
                return strictNullChecks ?
                    type === falseType ? TypeFacts.FalseStrictFacts : TypeFacts.TrueStrictFacts :
                    type === falseType ? TypeFacts.FalseFacts : TypeFacts.TrueFacts;
            }
            if (flags & TypeFlags.ObjectType) {
                return isFunctionObjectType(<ObjectType>type) ?
                    strictNullChecks ? TypeFacts.FunctionStrictFacts : TypeFacts.FunctionFacts :
                    strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
            }
            if (flags & (TypeFlags.Void | TypeFlags.Undefined)) {
                return TypeFacts.UndefinedFacts;
            }
            if (flags & TypeFlags.Null) {
                return TypeFacts.NullFacts;
            }
            if (flags & TypeFlags.ESSymbol) {
                return strictNullChecks ? TypeFacts.SymbolStrictFacts : TypeFacts.SymbolFacts;
            }
            if (flags & TypeFlags.TypeParameter) {
                const constraint = getConstraintOfTypeParameter(<TypeParameter>type);
                return getTypeFacts(constraint || emptyObjectType);
            }
            if (flags & TypeFlags.UnionOrIntersection) {
                return getTypeFactsOfTypes((<UnionOrIntersectionType>type).types);
            }
            return TypeFacts.All;
        }

        function getTypeWithFacts(type: Type, include: TypeFacts) {
            return filterType(type, t => (getTypeFacts(t) & include) !== 0);
        }

        function getTypeWithDefault(type: Type, defaultExpression: Expression) {
            if (defaultExpression) {
                const defaultType = checkExpression(defaultExpression);
                return getUnionType([getTypeWithFacts(type, TypeFacts.NEUndefined), defaultType]);
            }
            return type;
        }

        function getTypeOfDestructuredProperty(type: Type, name: Identifier | LiteralExpression | ComputedPropertyName) {
            const text = getTextOfPropertyName(name);
            return getTypeOfPropertyOfType(type, text) ||
                isNumericLiteralName(text) && getIndexTypeOfType(type, IndexKind.Number) ||
                getIndexTypeOfType(type, IndexKind.String) ||
                unknownType;
        }

        function getTypeOfDestructuredArrayElement(type: Type, index: number) {
            return isTupleLikeType(type) && getTypeOfPropertyOfType(type, "" + index) ||
                checkIteratedTypeOrElementType(type, /*errorNode*/ undefined, /*allowStringInput*/ false) ||
                unknownType;
        }

        function getTypeOfDestructuredSpreadElement(type: Type) {
            return createArrayType(checkIteratedTypeOrElementType(type, /*errorNode*/ undefined, /*allowStringInput*/ false) || unknownType);
        }

        function getAssignedTypeOfBinaryExpression(node: BinaryExpression): Type {
            return node.parent.kind === SyntaxKind.ArrayLiteralExpression || node.parent.kind === SyntaxKind.PropertyAssignment ?
                getTypeWithDefault(getAssignedType(node), node.right) :
                checkExpression(node.right);
        }

        function getAssignedTypeOfArrayLiteralElement(node: ArrayLiteralExpression, element: Expression): Type {
            return getTypeOfDestructuredArrayElement(getAssignedType(node), indexOf(node.elements, element));
        }

        function getAssignedTypeOfSpreadElement(node: SpreadElementExpression): Type {
            return getTypeOfDestructuredSpreadElement(getAssignedType(<ArrayLiteralExpression>node.parent));
        }

        function getAssignedTypeOfPropertyAssignment(node: PropertyAssignment | ShorthandPropertyAssignment): Type {
            return getTypeOfDestructuredProperty(getAssignedType(<ObjectLiteralExpression>node.parent), node.name);
        }

        function getAssignedTypeOfShorthandPropertyAssignment(node: ShorthandPropertyAssignment): Type {
            return getTypeWithDefault(getAssignedTypeOfPropertyAssignment(node), node.objectAssignmentInitializer);
        }

        function getAssignedType(node: Expression): Type {
            const parent = node.parent;
            switch (parent.kind) {
                case SyntaxKind.ForInStatement:
                    return stringType;
                case SyntaxKind.ForOfStatement:
                    return checkRightHandSideOfForOf((<ForOfStatement>parent).expression) || unknownType;
                case SyntaxKind.BinaryExpression:
                    return getAssignedTypeOfBinaryExpression(<BinaryExpression>parent);
                case SyntaxKind.DeleteExpression:
                    return undefinedType;
                case SyntaxKind.ArrayLiteralExpression:
                    return getAssignedTypeOfArrayLiteralElement(<ArrayLiteralExpression>parent, node);
                case SyntaxKind.SpreadElementExpression:
                    return getAssignedTypeOfSpreadElement(<SpreadElementExpression>parent);
                case SyntaxKind.PropertyAssignment:
                    return getAssignedTypeOfPropertyAssignment(<PropertyAssignment>parent);
                case SyntaxKind.ShorthandPropertyAssignment:
                    return getAssignedTypeOfShorthandPropertyAssignment(<ShorthandPropertyAssignment>parent);
            }
            return unknownType;
        }

        function getInitialTypeOfBindingElement(node: BindingElement): Type {
            const pattern = <BindingPattern>node.parent;
            const parentType = getInitialType(<VariableDeclaration | BindingElement>pattern.parent);
            const type = pattern.kind === SyntaxKind.ObjectBindingPattern ?
                getTypeOfDestructuredProperty(parentType, node.propertyName || <Identifier>node.name) :
                !node.dotDotDotToken ?
                    getTypeOfDestructuredArrayElement(parentType, indexOf(pattern.elements, node)) :
                    getTypeOfDestructuredSpreadElement(parentType);
            return getTypeWithDefault(type, node.initializer);
        }

        function getTypeOfInitializer(node: Expression) {
            // Return the cached type if one is available. If the type of the variable was inferred
            // from its initializer, we'll already have cached the type. Otherwise we compute it now
            // without caching such that transient types are reflected.
            const links = getNodeLinks(node);
            return links.resolvedType || checkExpression(node);
        }

        function getInitialTypeOfVariableDeclaration(node: VariableDeclaration) {
            if (node.initializer) {
                return getTypeOfInitializer(node.initializer);
            }
            if (node.parent.parent.kind === SyntaxKind.ForInStatement) {
                return stringType;
            }
            if (node.parent.parent.kind === SyntaxKind.ForOfStatement) {
                return checkRightHandSideOfForOf((<ForOfStatement>node.parent.parent).expression) || unknownType;
            }
            return unknownType;
        }

        function getInitialType(node: VariableDeclaration | BindingElement) {
            return node.kind === SyntaxKind.VariableDeclaration ?
                getInitialTypeOfVariableDeclaration(<VariableDeclaration>node) :
                getInitialTypeOfBindingElement(<BindingElement>node);
        }

        function getInitialOrAssignedType(node: VariableDeclaration | BindingElement | Expression) {
            return node.kind === SyntaxKind.VariableDeclaration || node.kind === SyntaxKind.BindingElement ?
                getInitialType(<VariableDeclaration | BindingElement>node) :
                getAssignedType(<Expression>node);
        }

        function getReferenceCandidate(node: Expression): Expression {
            switch (node.kind) {
                case SyntaxKind.ParenthesizedExpression:
                    return getReferenceCandidate((<ParenthesizedExpression>node).expression);
                case SyntaxKind.BinaryExpression:
                    switch ((<BinaryExpression>node).operatorToken.kind) {
                        case SyntaxKind.EqualsToken:
                            return getReferenceCandidate((<BinaryExpression>node).left);
                        case SyntaxKind.CommaToken:
                            return getReferenceCandidate((<BinaryExpression>node).right);
                    }
            }
            return node;
        }

        function getTypeOfSwitchClause(clause: CaseClause | DefaultClause) {
            if (clause.kind === SyntaxKind.CaseClause) {
                const caseType = checkExpression((<CaseClause>clause).expression);
                return isUnitType(caseType) ? caseType : undefined;
            }
            return neverType;
        }

        function getSwitchClauseTypes(switchStatement: SwitchStatement): Type[] {
            const links = getNodeLinks(switchStatement);
            if (!links.switchTypes) {
                // If all case clauses specify expressions that have unit types, we return an array
                // of those unit types. Otherwise we return an empty array.
                const types = map(switchStatement.caseBlock.clauses, getTypeOfSwitchClause);
                links.switchTypes = !contains(types, undefined) ? types : emptyArray;
            }
            return links.switchTypes;
        }

        function eachTypeContainedIn(source: Type, types: Type[]) {
            return source.flags & TypeFlags.Union ? !forEach((<UnionType>source).types, t => !contains(types, t)) : contains(types, source);
        }

        function isTypeSubsetOf(source: Type, target: Type) {
            return source === target || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, <UnionType>target);
        }

        function isTypeSubsetOfUnion(source: Type, target: UnionType) {
            if (source.flags & TypeFlags.Union) {
                for (const t of (<UnionType>source).types) {
                    if (!containsType(target.types, t)) {
                        return false;
                    }
                }
                return true;
            }
            if (source.flags & TypeFlags.EnumLiteral && target.flags & TypeFlags.Enum && (<EnumLiteralType>source).baseType === target) {
                return true;
            }
            return containsType(target.types, source);
        }

        function filterType(type: Type, f: (t: Type) => boolean): Type {
            if (type.flags & TypeFlags.Union) {
                const types = (<UnionType>type).types;
                const filtered = filter(types, f);
                return filtered === types ? type : getUnionTypeFromSortedList(filtered);
            }
            return f(type) ? type : neverType;
        }

        function isIncomplete(flowType: FlowType) {
            return flowType.flags === 0;
        }

        function getTypeFromFlowType(flowType: FlowType) {
            return flowType.flags === 0 ? (<IncompleteType>flowType).type : <Type>flowType;
        }

        function createFlowType(type: Type, incomplete: boolean): FlowType {
            return incomplete ? { flags: 0, type } : type;
        }

        function getFlowTypeOfReference(reference: Node, declaredType: Type, assumeInitialized: boolean, includeOuterFunctions: boolean) {
            let key: string;
            if (!reference.flowNode || assumeInitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                return declaredType;
            }
            const initialType = assumeInitialized ? declaredType : includeFalsyTypes(declaredType, TypeFlags.Undefined);
            const visitedFlowStart = visitedFlowCount;
            const result = getTypeFromFlowType(getTypeAtFlowNode(reference.flowNode));
            visitedFlowCount = visitedFlowStart;
            if (reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(result, TypeFacts.NEUndefinedOrNull) === neverType) {
                return declaredType;
            }
            return result;

            function getTypeAtFlowNode(flow: FlowNode): FlowType {
                while (true) {
                    if (flow.flags & FlowFlags.Shared) {
                        // We cache results of flow type resolution for shared nodes that were previously visited in
                        // the same getFlowTypeOfReference invocation. A node is considered shared when it is the
                        // antecedent of more than one node.
                        for (let i = visitedFlowStart; i < visitedFlowCount; i++) {
                            if (visitedFlowNodes[i] === flow) {
                                return visitedFlowTypes[i];
                            }
                        }
                    }
                    let type: FlowType;
                    if (flow.flags & FlowFlags.Assignment) {
                        type = getTypeAtFlowAssignment(<FlowAssignment>flow);
                        if (!type) {
                            flow = (<FlowAssignment>flow).antecedent;
                            continue;
                        }
                    }
                    else if (flow.flags & FlowFlags.Condition) {
                        type = getTypeAtFlowCondition(<FlowCondition>flow);
                    }
                    else if (flow.flags & FlowFlags.SwitchClause) {
                        type = getTypeAtSwitchClause(<FlowSwitchClause>flow);
                    }
                    else if (flow.flags & FlowFlags.Label) {
                        if ((<FlowLabel>flow).antecedents.length === 1) {
                            flow = (<FlowLabel>flow).antecedents[0];
                            continue;
                        }
                        type = flow.flags & FlowFlags.BranchLabel ?
                            getTypeAtFlowBranchLabel(<FlowLabel>flow) :
                            getTypeAtFlowLoopLabel(<FlowLabel>flow);
                    }
                    else if (flow.flags & FlowFlags.Start) {
                        // Check if we should continue with the control flow of the containing function.
                        const container = (<FlowStart>flow).container;
                        if (container && includeOuterFunctions) {
                            flow = container.flowNode;
                            continue;
                        }
                        // At the top of the flow we have the initial type.
                        type = initialType;
                    }
                    else {
                        // Unreachable code errors are reported in the binding phase. Here we
                        // simply return the declared type to reduce follow-on errors.
                        type = declaredType;
                    }
                    if (flow.flags & FlowFlags.Shared) {
                        // Record visited node and the associated type in the cache.
                        visitedFlowNodes[visitedFlowCount] = flow;
                        visitedFlowTypes[visitedFlowCount] = type;
                        visitedFlowCount++;
                    }
                    return type;
                }
            }

            function getTypeAtFlowAssignment(flow: FlowAssignment) {
                const node = flow.node;
                // Assignments only narrow the computed type if the declared type is a union type. Thus, we
                // only need to evaluate the assigned type if the declared type is a union type.
                if (isMatchingReference(reference, node)) {
                    return declaredType.flags & TypeFlags.Union ?
                        getAssignmentReducedType(<UnionType>declaredType, getInitialOrAssignedType(node)) :
                        declaredType;
                }
                // We didn't have a direct match. However, if the reference is a dotted name, this
                // may be an assignment to a left hand part of the reference. For example, for a
                // reference 'x.y.z', we may be at an assignment to 'x.y' or 'x'. In that case,
                // return the declared type.
                if (containsMatchingReference(reference, node)) {
                    return declaredType;
                }
                // Assignment doesn't affect reference
                return undefined;
            }

            function getTypeAtFlowCondition(flow: FlowCondition): FlowType {
                const flowType = getTypeAtFlowNode(flow.antecedent);
                let type = getTypeFromFlowType(flowType);
                if (type !== neverType) {
                    // If we have an antecedent type (meaning we're reachable in some way), we first
                    // attempt to narrow the antecedent type. If that produces the never type, and if
                    // the antecedent type is incomplete (i.e. a transient type in a loop), then we
                    // take the type guard as an indication that control *could* reach here once we
                    // have the complete type. We proceed by reverting to the declared type and then
                    // narrow that.
                    const assumeTrue = (flow.flags & FlowFlags.TrueCondition) !== 0;
                    type = narrowType(type, flow.expression, assumeTrue);
                    if (type === neverType && isIncomplete(flowType)) {
                        type = narrowType(declaredType, flow.expression, assumeTrue);
                    }
                }
                return createFlowType(type, isIncomplete(flowType));
            }

            function getTypeAtSwitchClause(flow: FlowSwitchClause): FlowType {
                const flowType = getTypeAtFlowNode(flow.antecedent);
                let type = getTypeFromFlowType(flowType);
                const expr = flow.switchStatement.expression;
                if (isMatchingReference(reference, expr)) {
                    type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                }
                else if (isMatchingReferenceDiscriminant(expr)) {
                    type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                }
                return createFlowType(type, isIncomplete(flowType));
            }

            function getTypeAtFlowBranchLabel(flow: FlowLabel): FlowType {
                const antecedentTypes: Type[] = [];
                let subtypeReduction = false;
                let seenIncomplete = false;
                for (const antecedent of flow.antecedents) {
                    const flowType = getTypeAtFlowNode(antecedent);
                    const type = getTypeFromFlowType(flowType);
                    // If the type at a particular antecedent path is the declared type and the
                    // reference is known to always be assigned (i.e. when declared and initial types
                    // are the same), there is no reason to process more antecedents since the only
                    // possible outcome is subtypes that will be removed in the final union type anyway.
                    if (type === declaredType && declaredType === initialType) {
                        return type;
                    }
                    if (!contains(antecedentTypes, type)) {
                        antecedentTypes.push(type);
                    }
                    // If an antecedent type is not a subset of the declared type, we need to perform
                    // subtype reduction. This happens when a "foreign" type is injected into the control
                    // flow using the instanceof operator or a user defined type predicate.
                    if (!isTypeSubsetOf(type, declaredType)) {
                        subtypeReduction = true;
                    }
                    if (isIncomplete(flowType)) {
                        seenIncomplete = true;
                    }
                }
                return createFlowType(getUnionType(antecedentTypes, subtypeReduction), seenIncomplete);
            }

            function getTypeAtFlowLoopLabel(flow: FlowLabel): FlowType {
                // If we have previously computed the control flow type for the reference at
                // this flow loop junction, return the cached type.
                const id = getFlowNodeId(flow);
                const cache = flowLoopCaches[id] || (flowLoopCaches[id] = createMap<Type>());
                if (!key) {
                    key = getFlowCacheKey(reference);
                }
                if (cache[key]) {
                    return cache[key];
                }
                // If this flow loop junction and reference are already being processed, return
                // the union of the types computed for each branch so far, marked as incomplete.
                // We should never see an empty array here because the first antecedent of a loop
                // junction is always the non-looping control flow path that leads to the top.
                for (let i = flowLoopStart; i < flowLoopCount; i++) {
                    if (flowLoopNodes[i] === flow && flowLoopKeys[i] === key) {
                        return createFlowType(getUnionType(flowLoopTypes[i]), /*incomplete*/ true);
                    }
                }
                // Add the flow loop junction and reference to the in-process stack and analyze
                // each antecedent code path.
                const antecedentTypes: Type[] = [];
                let subtypeReduction = false;
                flowLoopNodes[flowLoopCount] = flow;
                flowLoopKeys[flowLoopCount] = key;
                flowLoopTypes[flowLoopCount] = antecedentTypes;
                for (const antecedent of flow.antecedents) {
                    flowLoopCount++;
                    const type = getTypeFromFlowType(getTypeAtFlowNode(antecedent));
                    flowLoopCount--;
                    // If we see a value appear in the cache it is a sign that control flow  analysis
                    // was restarted and completed by checkExpressionCached. We can simply pick up
                    // the resulting type and bail out.
                    if (cache[key]) {
                        return cache[key];
                    }
                    if (!contains(antecedentTypes, type)) {
                        antecedentTypes.push(type);
                    }
                    // If an antecedent type is not a subset of the declared type, we need to perform
                    // subtype reduction. This happens when a "foreign" type is injected into the control
                    // flow using the instanceof operator or a user defined type predicate.
                    if (!isTypeSubsetOf(type, declaredType)) {
                        subtypeReduction = true;
                    }
                    // If the type at a particular antecedent path is the declared type there is no
                    // reason to process more antecedents since the only possible outcome is subtypes
                    // that will be removed in the final union type anyway.
                    if (type === declaredType) {
                        break;
                    }
                }
                return cache[key] = getUnionType(antecedentTypes, subtypeReduction);
            }

            function isMatchingReferenceDiscriminant(expr: Expression) {
                return expr.kind === SyntaxKind.PropertyAccessExpression &&
                    declaredType.flags & TypeFlags.Union &&
                    isMatchingReference(reference, (<PropertyAccessExpression>expr).expression) &&
                    isDiscriminantProperty(declaredType, (<PropertyAccessExpression>expr).name.text);
            }

            function narrowTypeByDiscriminant(type: Type, propAccess: PropertyAccessExpression, narrowType: (t: Type) => Type): Type {
                const propName = propAccess.name.text;
                const propType = getTypeOfPropertyOfType(type, propName);
                const narrowedPropType = propType && narrowType(propType);
                return propType === narrowedPropType ? type : filterType(type, t => isTypeComparableTo(getTypeOfPropertyOfType(t, propName), narrowedPropType));
            }

            function narrowTypeByTruthiness(type: Type, expr: Expression, assumeTrue: boolean): Type {
                if (isMatchingReference(reference, expr)) {
                    return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                }
                if (isMatchingReferenceDiscriminant(expr)) {
                    return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                }
                if (containsMatchingReferenceDiscriminant(reference, expr)) {
                    return declaredType;
                }
                return type;
            }

            function narrowTypeByBinaryExpression(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                switch (expr.operatorToken.kind) {
                    case SyntaxKind.EqualsToken:
                        return narrowTypeByTruthiness(type, expr.left, assumeTrue);
                    case SyntaxKind.EqualsEqualsToken:
                    case SyntaxKind.ExclamationEqualsToken:
                    case SyntaxKind.EqualsEqualsEqualsToken:
                    case SyntaxKind.ExclamationEqualsEqualsToken:
                        const operator = expr.operatorToken.kind;
                        const left = getReferenceCandidate(expr.left);
                        const right = getReferenceCandidate(expr.right);
                        if (left.kind === SyntaxKind.TypeOfExpression && right.kind === SyntaxKind.StringLiteral) {
                            return narrowTypeByTypeof(type, <TypeOfExpression>left, operator, <LiteralExpression>right, assumeTrue);
                        }
                        if (right.kind === SyntaxKind.TypeOfExpression && left.kind === SyntaxKind.StringLiteral) {
                            return narrowTypeByTypeof(type, <TypeOfExpression>right, operator, <LiteralExpression>left, assumeTrue);
                        }
                        if (isMatchingReference(reference, left)) {
                            return narrowTypeByEquality(type, operator, right, assumeTrue);
                        }
                        if (isMatchingReference(reference, right)) {
                            return narrowTypeByEquality(type, operator, left, assumeTrue);
                        }
                        if (isMatchingReferenceDiscriminant(left)) {
                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                        }
                        if (isMatchingReferenceDiscriminant(right)) {
                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                        }
                        if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                            return declaredType;
                        }
                        break;
                    case SyntaxKind.InstanceOfKeyword:
                        return narrowTypeByInstanceof(type, expr, assumeTrue);
                    case SyntaxKind.CommaToken:
                        return narrowType(type, expr.right, assumeTrue);
                }
                return type;
            }

            function narrowTypeByEquality(type: Type, operator: SyntaxKind, value: Expression, assumeTrue: boolean): Type {
                if (operator === SyntaxKind.ExclamationEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken) {
                    assumeTrue = !assumeTrue;
                }
                const valueType = checkExpression(value);
                if (valueType.flags & TypeFlags.Nullable) {
                    if (!strictNullChecks) {
                        return type;
                    }
                    const doubleEquals = operator === SyntaxKind.EqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsToken;
                    const facts = doubleEquals
                        ? assumeTrue ? TypeFacts.EQUndefinedOrNull : TypeFacts.NEUndefinedOrNull
                        : value.kind === SyntaxKind.NullKeyword
                            ? assumeTrue ? TypeFacts.EQNull : TypeFacts.NENull
                            : assumeTrue ? TypeFacts.EQUndefined : TypeFacts.NEUndefined;
                    return getTypeWithFacts(type, facts);
                }
                if (type.flags & TypeFlags.NotUnionOrUnit) {
                    return type;
                }
                if (assumeTrue) {
                    const narrowedType = filterType(type, t => areTypesComparable(t, valueType));
                    return narrowedType !== neverType ? narrowedType : type;
                }
                return isUnitType(valueType) ? filterType(type, t => t !== valueType) : type;
            }

            function narrowTypeByTypeof(type: Type, typeOfExpr: TypeOfExpression, operator: SyntaxKind, literal: LiteralExpression, assumeTrue: boolean): Type {
                // We have '==', '!=', '====', or !==' operator with 'typeof xxx' and string literal operands
                const target = getReferenceCandidate(typeOfExpr.expression);
                if (!isMatchingReference(reference, target)) {
                    // For a reference of the form 'x.y', a 'typeof x === ...' type guard resets the
                    // narrowed type of 'y' to its declared type.
                    if (containsMatchingReference(reference, target)) {
                        return declaredType;
                    }
                    return type;
                }
                if (operator === SyntaxKind.ExclamationEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken) {
                    assumeTrue = !assumeTrue;
                }
                if (assumeTrue && !(type.flags & TypeFlags.Union)) {
                    // We narrow a non-union type to an exact primitive type if the non-union type
                    // is a supertype of that primitive type. For example, type 'any' can be narrowed
                    // to one of the primitive types.
                    const targetType = typeofTypesByName[literal.text];
                    if (targetType && isTypeSubtypeOf(targetType, type)) {
                        return targetType;
                    }
                }
                const facts = assumeTrue
                    ? typeofEQFacts[literal.text] || TypeFacts.TypeofEQHostObject
                    : typeofNEFacts[literal.text] || TypeFacts.TypeofNEHostObject;
                return getTypeWithFacts(type, facts);
            }

            function narrowTypeBySwitchOnDiscriminant(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number) {
                // We only narrow if all case expressions specify values with unit types
                const switchTypes = getSwitchClauseTypes(switchStatement);
                if (!switchTypes.length) {
                    return type;
                }
                const clauseTypes = switchTypes.slice(clauseStart, clauseEnd);
                const hasDefaultClause = clauseStart === clauseEnd || contains(clauseTypes, neverType);
                const discriminantType = getUnionType(clauseTypes);
                const caseType = discriminantType === neverType ? neverType : filterType(type, t => isTypeComparableTo(discriminantType, t));
                if (!hasDefaultClause) {
                    return caseType;
                }
                const defaultType = filterType(type, t => !(isUnitType(t) && contains(switchTypes, t)));
                return caseType === neverType ? defaultType : getUnionType([caseType, defaultType]);
            }

            function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                const left = getReferenceCandidate(expr.left);
                if (!isMatchingReference(reference, left)) {
                    // For a reference of the form 'x.y', an 'x instanceof T' type guard resets the
                    // narrowed type of 'y' to its declared type.
                    if (containsMatchingReference(reference, left)) {
                        return declaredType;
                    }
                    return type;
                }
                // We never narrow type any in an instanceof guard
                if (isTypeAny(type)) {
                    return type;
                }

                // Check that right operand is a function type with a prototype property
                const rightType = checkExpression(expr.right);
                if (!isTypeSubtypeOf(rightType, globalFunctionType)) {
                    return type;
                }

                let targetType: Type;
                const prototypeProperty = getPropertyOfType(rightType, "prototype");
                if (prototypeProperty) {
                    // Target type is type of the prototype property
                    const prototypePropertyType = getTypeOfSymbol(prototypeProperty);
                    if (!isTypeAny(prototypePropertyType)) {
                        targetType = prototypePropertyType;
                    }
                }

                if (!targetType) {
                    // Target type is type of construct signature
                    let constructSignatures: Signature[];
                    if (rightType.flags & TypeFlags.Interface) {
                        constructSignatures = resolveDeclaredMembers(<InterfaceType>rightType).declaredConstructSignatures;
                    }
                    else if (rightType.flags & TypeFlags.Anonymous) {
                        constructSignatures = getSignaturesOfType(rightType, SignatureKind.Construct);
                    }
                    if (constructSignatures && constructSignatures.length) {
                        targetType = getUnionType(map(constructSignatures, signature => getReturnTypeOfSignature(getErasedSignature(signature))));
                    }
                }

                if (targetType) {
                    return getNarrowedType(type, targetType, assumeTrue);
                }

                return type;
            }

            function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean) {
                if (!assumeTrue) {
                    return filterType(type, t => !isTypeInstanceOf(t, candidate));
                }
                // If the current type is a union type, remove all constituents that couldn't be instances of
                // the candidate type. If one or more constituents remain, return a union of those.
                if (type.flags & TypeFlags.Union) {
                    const assignableType = filterType(type, t => isTypeInstanceOf(t, candidate));
                    if (assignableType !== neverType) {
                        return assignableType;
                    }
                }
                // If the candidate type is a subtype of the target type, narrow to the candidate type.
                // Otherwise, if the target type is assignable to the candidate type, keep the target type.
                // Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
                // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
                // two types.
                const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
                return isTypeSubtypeOf(candidate, targetType) ? candidate :
                    isTypeAssignableTo(type, candidate) ? type :
                    isTypeAssignableTo(candidate, targetType) ? candidate :
                    getIntersectionType([type, candidate]);
            }

            function narrowTypeByTypePredicate(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
                if (type.flags & TypeFlags.Any || !hasMatchingArgument(callExpression, reference)) {
                    return type;
                }
                const signature = getResolvedSignature(callExpression);
                const predicate = signature.typePredicate;
                if (!predicate) {
                    return type;
                }
                if (isIdentifierTypePredicate(predicate)) {
                    const predicateArgument = callExpression.arguments[predicate.parameterIndex];
                    if (predicateArgument) {
                        if (isMatchingReference(reference, predicateArgument)) {
                            return getNarrowedType(type, predicate.type, assumeTrue);
                        }
                        if (containsMatchingReference(reference, predicateArgument)) {
                            return declaredType;
                        }
                    }
                }
                else {
                    const invokedExpression = skipParenthesizedNodes(callExpression.expression);
                    if (invokedExpression.kind === SyntaxKind.ElementAccessExpression || invokedExpression.kind === SyntaxKind.PropertyAccessExpression) {
                        const accessExpression = invokedExpression as ElementAccessExpression | PropertyAccessExpression;
                        const possibleReference = skipParenthesizedNodes(accessExpression.expression);
                        if (isMatchingReference(reference, possibleReference)) {
                            return getNarrowedType(type, predicate.type, assumeTrue);
                        }
                        if (containsMatchingReference(reference, possibleReference)) {
                            return declaredType;
                        }
                    }
                }
                return type;
            }

            // Narrow the given type based on the given expression having the assumed boolean value. The returned type
            // will be a subtype or the same type as the argument.
            function narrowType(type: Type, expr: Expression, assumeTrue: boolean): Type {
                switch (expr.kind) {
                    case SyntaxKind.Identifier:
                    case SyntaxKind.ThisKeyword:
                    case SyntaxKind.PropertyAccessExpression:
                        return narrowTypeByTruthiness(type, expr, assumeTrue);
                    case SyntaxKind.CallExpression:
                        return narrowTypeByTypePredicate(type, <CallExpression>expr, assumeTrue);
                    case SyntaxKind.ParenthesizedExpression:
                        return narrowType(type, (<ParenthesizedExpression>expr).expression, assumeTrue);
                    case SyntaxKind.BinaryExpression:
                        return narrowTypeByBinaryExpression(type, <BinaryExpression>expr, assumeTrue);
                    case SyntaxKind.PrefixUnaryExpression:
                        if ((<PrefixUnaryExpression>expr).operator === SyntaxKind.ExclamationToken) {
                            return narrowType(type, (<PrefixUnaryExpression>expr).operand, !assumeTrue);
                        }
                        break;
                }
                return type;
            }
        }

        function getTypeOfSymbolAtLocation(symbol: Symbol, location: Node) {
            // If we have an identifier or a property access at the given location, if the location is
            // an dotted name expression, and if the location is not an assignment target, obtain the type
            // of the expression (which will reflect control flow analysis). If the expression indeed
            // resolved to the given symbol, return the narrowed type.
            if (location.kind === SyntaxKind.Identifier) {
                if (isRightSideOfQualifiedNameOrPropertyAccess(location)) {
                    location = location.parent;
                }
                if (isExpression(location) && !isAssignmentTarget(location)) {
                    const type = checkExpression(<Expression>location);
                    if (getExportSymbolOfValueSymbolIfExported(getNodeLinks(location).resolvedSymbol) === symbol) {
                        return type;
                    }
                }
            }
            // The location isn't a reference to the given symbol, meaning we're being asked
            // a hypothetical question of what type the symbol would have if there was a reference
            // to it at the given location. Since we have no control flow information for the
            // hypotherical reference (control flow information is created and attached by the
            // binder), we simply return the declared type of the symbol.
            return getTypeOfSymbol(symbol);
        }

        function skipParenthesizedNodes(expression: Expression): Expression {
            while (expression.kind === SyntaxKind.ParenthesizedExpression) {
                expression = (expression as ParenthesizedExpression).expression;
            }
            return expression;
        }

        function getControlFlowContainer(node: Node): Node {
            while (true) {
                node = node.parent;
                if (isFunctionLike(node) || node.kind === SyntaxKind.ModuleBlock || node.kind === SyntaxKind.SourceFile || node.kind === SyntaxKind.PropertyDeclaration) {
                    return node;
                }
            }
        }

        function isDeclarationIncludedInFlow(reference: Node, declaration: Declaration, includeOuterFunctions: boolean) {
            const declarationContainer = getControlFlowContainer(declaration);
            let container = getControlFlowContainer(reference);
            while (container !== declarationContainer &&
                (container.kind === SyntaxKind.FunctionExpression || container.kind === SyntaxKind.ArrowFunction) &&
                (includeOuterFunctions || getImmediatelyInvokedFunctionExpression(<FunctionExpression>container))) {
                container = getControlFlowContainer(container);
            }
            return container === declarationContainer;
        }

        function checkIdentifier(node: Identifier): Type {
            const symbol = getResolvedSymbol(node);

            // As noted in ECMAScript 6 language spec, arrow functions never have an arguments objects.
            // Although in down-level emit of arrow function, we emit it using function expression which means that
            // arguments objects will be bound to the inner object; emitting arrow function natively in ES6, arguments objects
            // will be bound to non-arrow function that contain this arrow function. This results in inconsistent behavior.
            // To avoid that we will give an error to users if they use arguments objects in arrow function so that they
            // can explicitly bound arguments objects
            if (symbol === argumentsSymbol) {
                const container = getContainingFunction(node);
                if (container.kind === SyntaxKind.ArrowFunction) {
                    if (languageVersion < ScriptTarget.ES6) {
                        error(node, Diagnostics.The_arguments_object_cannot_be_referenced_in_an_arrow_function_in_ES3_and_ES5_Consider_using_a_standard_function_expression);
                    }
                }

                if (node.flags & NodeFlags.AwaitContext) {
                    getNodeLinks(container).flags |= NodeCheckFlags.CaptureArguments;
                }
            }

            if (symbol.flags & SymbolFlags.Alias && !isInTypeQuery(node) && !isConstEnumOrConstEnumOnlyModule(resolveAlias(symbol))) {
                markAliasSymbolAsReferenced(symbol);
            }

            const localOrExportSymbol = getExportSymbolOfValueSymbolIfExported(symbol);

            // Due to the emit for class decorators, any reference to the class from inside of the class body
            // must instead be rewritten to point to a temporary variable to avoid issues with the double-bind
            // behavior of class names in ES6.
            if (languageVersion === ScriptTarget.ES6
                && localOrExportSymbol.flags & SymbolFlags.Class
                && localOrExportSymbol.valueDeclaration.kind === SyntaxKind.ClassDeclaration
                && nodeIsDecorated(localOrExportSymbol.valueDeclaration)) {
                let container = getContainingClass(node);
                while (container !== undefined) {
                    if (container === localOrExportSymbol.valueDeclaration && container.name !== node) {
                        getNodeLinks(container).flags |= NodeCheckFlags.ClassWithBodyScopedClassBinding;
                        getNodeLinks(node).flags |= NodeCheckFlags.BodyScopedClassBinding;
                        break;
                    }

                    container = getContainingClass(container);
                }
            }

            checkCollisionWithCapturedSuperVariable(node, node);
            checkCollisionWithCapturedThisVariable(node, node);
            checkNestedBlockScopedBinding(node, symbol);

            const type = getTypeOfSymbol(localOrExportSymbol);
            if (!(localOrExportSymbol.flags & SymbolFlags.Variable) || isAssignmentTarget(node)) {
                return type;
            }
            const declaration = localOrExportSymbol.valueDeclaration;
            const includeOuterFunctions = isReadonlySymbol(localOrExportSymbol);
            const assumeInitialized = !strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || !declaration ||
                getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                !isDeclarationIncludedInFlow(node, declaration, includeOuterFunctions);
            const flowType = getFlowTypeOfReference(node, type, assumeInitialized, includeOuterFunctions);
            if (!assumeInitialized && !(getFalsyFlags(type) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
                error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                // Return the declared type to reduce follow-on errors
                return type;
            }
            return flowType;
        }

        function isInsideFunction(node: Node, threshold: Node): boolean {
            let current = node;
            while (current && current !== threshold) {
                if (isFunctionLike(current)) {
                    return true;
                }
                current = current.parent;
            }

            return false;
        }

        function checkNestedBlockScopedBinding(node: Identifier, symbol: Symbol): void {
            if (languageVersion >= ScriptTarget.ES6 ||
                (symbol.flags & (SymbolFlags.BlockScopedVariable | SymbolFlags.Class)) === 0 ||
                symbol.valueDeclaration.parent.kind === SyntaxKind.CatchClause) {
                return;
            }

            // 1. walk from the use site up to the declaration and check
            // if there is anything function like between declaration and use-site (is binding/class is captured in function).
            // 2. walk from the declaration up to the boundary of lexical environment and check
            // if there is an iteration statement in between declaration and boundary (is binding/class declared inside iteration statement)

            const container = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
            const usedInFunction = isInsideFunction(node.parent, container);
            let current = container;

            let containedInIterationStatement = false;
            while (current && !nodeStartsNewLexicalEnvironment(current)) {
                if (isIterationStatement(current, /*lookInLabeledStatements*/ false)) {
                    containedInIterationStatement = true;
                    break;
                }
                current = current.parent;
            }

            if (containedInIterationStatement) {
                if (usedInFunction) {
                    // mark iteration statement as containing block-scoped binding captured in some function
                    getNodeLinks(current).flags |= NodeCheckFlags.LoopWithCapturedBlockScopedBinding;
                }

                // mark variables that are declared in loop initializer and reassigned inside the body of ForStatement.
                // if body of ForStatement will be converted to function then we'll need a extra machinery to propagate reassigned values back.
                if (container.kind === SyntaxKind.ForStatement &&
                    getAncestor(symbol.valueDeclaration, SyntaxKind.VariableDeclarationList).parent === container &&
                    isAssignedInBodyOfForStatement(node, <ForStatement>container)) {
                    getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.NeedsLoopOutParameter;
                }

                // set 'declared inside loop' bit on the block-scoped binding
                getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.BlockScopedBindingInLoop;
            }

            if (usedInFunction) {
                getNodeLinks(symbol.valueDeclaration).flags |= NodeCheckFlags.CapturedBlockScopedBinding;
            }
        }

        function isAssignedInBodyOfForStatement(node: Identifier, container: ForStatement): boolean {
            let current: Node = node;
            // skip parenthesized nodes
            while (current.parent.kind === SyntaxKind.ParenthesizedExpression) {
                current = current.parent;
            }

            // check if node is used as LHS in some assignment expression
            let isAssigned = false;
            if (isAssignmentTarget(current)) {
                isAssigned = true;
            }
            else if ((current.parent.kind === SyntaxKind.PrefixUnaryExpression || current.parent.kind === SyntaxKind.PostfixUnaryExpression)) {
                const expr = <PrefixUnaryExpression | PostfixUnaryExpression>current.parent;
                isAssigned = expr.operator === SyntaxKind.PlusPlusToken || expr.operator === SyntaxKind.MinusMinusToken;
            }

            if (!isAssigned) {
                return false;
            }

            // at this point we know that node is the target of assignment
            // now check that modification happens inside the statement part of the ForStatement
            while (current !== container) {
                if (current === container.statement) {
                    return true;
                }
                else {
                    current = current.parent;
                }
            }
            return false;
        }

        function captureLexicalThis(node: Node, container: Node): void {
            getNodeLinks(node).flags |= NodeCheckFlags.LexicalThis;
            if (container.kind === SyntaxKind.PropertyDeclaration || container.kind === SyntaxKind.Constructor) {
                const classNode = container.parent;
                getNodeLinks(classNode).flags |= NodeCheckFlags.CaptureThis;
            }
            else {
                getNodeLinks(container).flags |= NodeCheckFlags.CaptureThis;
            }
        }

        function findFirstSuperCall(n: Node): Node {
            if (isSuperCallExpression(n)) {
                return n;
            }
            else if (isFunctionLike(n)) {
                return undefined;
            }
            return forEachChild(n, findFirstSuperCall);
        }

        /**
         * Return a cached result if super-statement is already found.
         * Otherwise, find a super statement in a given constructor function and cache the result in the node-links of the constructor
         *
         * @param constructor constructor-function to look for super statement
         */
        function getSuperCallInConstructor(constructor: ConstructorDeclaration): ExpressionStatement {
            const links = getNodeLinks(constructor);

            // Only trying to find super-call if we haven't yet tried to find one.  Once we try, we will record the result
            if (links.hasSuperCall === undefined) {
                links.superCall = <ExpressionStatement>findFirstSuperCall(constructor.body);
                links.hasSuperCall = links.superCall ? true : false;
            }
            return links.superCall;
        }

        /**
         * Check if the given class-declaration extends null then return true.
         * Otherwise, return false
         * @param classDecl a class declaration to check if it extends null
         */
        function classDeclarationExtendsNull(classDecl: ClassDeclaration): boolean {
            const classSymbol = getSymbolOfNode(classDecl);
            const classInstanceType = <InterfaceType>getDeclaredTypeOfSymbol(classSymbol);
            const baseConstructorType = getBaseConstructorTypeOfClass(classInstanceType);

            return baseConstructorType === nullWideningType;
        }

        function checkThisExpression(node: Node): Type {
            // Stop at the first arrow function so that we can
            // tell whether 'this' needs to be captured.
            let container = getThisContainer(node, /* includeArrowFunctions */ true);
            let needToCaptureLexicalThis = false;

            if (container.kind === SyntaxKind.Constructor) {
                const containingClassDecl = <ClassDeclaration>container.parent;
                const baseTypeNode = getClassExtendsHeritageClauseElement(containingClassDecl);

                // If a containing class does not have extends clause or the class extends null
                // skip checking whether super statement is called before "this" accessing.
                if (baseTypeNode && !classDeclarationExtendsNull(containingClassDecl)) {
                    const superCall = getSuperCallInConstructor(<ConstructorDeclaration>container);

                    // We should give an error in the following cases:
                    //      - No super-call
                    //      - "this" is accessing before super-call.
                    //          i.e super(this)
                    //              this.x; super();
                    // We want to make sure that super-call is done before accessing "this" so that
                    // "this" is not accessed as a parameter of the super-call.
                    if (!superCall || superCall.end > node.pos) {
                        // In ES6, super inside constructor of class-declaration has to precede "this" accessing
                        error(node, Diagnostics.super_must_be_called_before_accessing_this_in_the_constructor_of_a_derived_class);
                    }
                }
            }

            // Now skip arrow functions to get the "real" owner of 'this'.
            if (container.kind === SyntaxKind.ArrowFunction) {
                container = getThisContainer(container, /* includeArrowFunctions */ false);

                // When targeting es6, arrow function lexically bind "this" so we do not need to do the work of binding "this" in emitted code
                needToCaptureLexicalThis = (languageVersion < ScriptTarget.ES6);
            }

            switch (container.kind) {
                case SyntaxKind.ModuleDeclaration:
                    error(node, Diagnostics.this_cannot_be_referenced_in_a_module_or_namespace_body);
                    // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    break;
                case SyntaxKind.EnumDeclaration:
                    error(node, Diagnostics.this_cannot_be_referenced_in_current_location);
                    // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    break;
                case SyntaxKind.Constructor:
                    if (isInConstructorArgumentInitializer(node, container)) {
                        error(node, Diagnostics.this_cannot_be_referenced_in_constructor_arguments);
                        // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    }
                    break;
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    if (container.flags & NodeFlags.Static) {
                        error(node, Diagnostics.this_cannot_be_referenced_in_a_static_property_initializer);
                        // do not return here so in case if lexical this is captured - it will be reflected in flags on NodeLinks
                    }
                    break;
                case SyntaxKind.ComputedPropertyName:
                    error(node, Diagnostics.this_cannot_be_referenced_in_a_computed_property_name);
                    break;
            }

            if (needToCaptureLexicalThis) {
                captureLexicalThis(node, container);
            }
            if (isFunctionLike(container) &&
                (!isInParameterInitializerBeforeContainingFunction(node) || getFunctionLikeThisParameter(container))) {
                // Note: a parameter initializer should refer to class-this unless function-this is explicitly annotated.

                // If this is a function in a JS file, it might be a class method. Check if it's the RHS
                // of a x.prototype.y = function [name]() { .... }
                if (container.kind === SyntaxKind.FunctionExpression &&
                    isInJavaScriptFile(container.parent) &&
                    getSpecialPropertyAssignmentKind(container.parent) === SpecialPropertyAssignmentKind.PrototypeProperty) {
                    // Get the 'x' of 'x.prototype.y = f' (here, 'f' is 'container')
                    const className = (((container.parent as BinaryExpression)   // x.prototype.y = f
                        .left as PropertyAccessExpression)       // x.prototype.y
                        .expression as PropertyAccessExpression) // x.prototype
                        .expression;                             // x
                    const classSymbol = checkExpression(className).symbol;
                    if (classSymbol && classSymbol.members && (classSymbol.flags & SymbolFlags.Function)) {
                        return getInferredClassType(classSymbol);
                    }
                }
                const type = getContextuallyTypedThisType(container);
                if (type) {
                    return type;
                }

                const thisType = getThisTypeOfDeclaration(container);
                if (thisType) {
                    return thisType;
                }
            }
            if (isClassLike(container.parent)) {
                const symbol = getSymbolOfNode(container.parent);
                const type = container.flags & NodeFlags.Static ? getTypeOfSymbol(symbol) : (<InterfaceType>getDeclaredTypeOfSymbol(symbol)).thisType;
                return getFlowTypeOfReference(node, type, /*assumeInitialized*/ true, /*includeOuterFunctions*/ true);
            }

            if (isInJavaScriptFile(node)) {
                const type = getTypeForThisExpressionFromJSDoc(container);
                if (type && type !== unknownType) {
                    return type;
                }
            }

            if (compilerOptions.noImplicitThis) {
                // With noImplicitThis, functions may not reference 'this' if it has type 'any'
                error(node, Diagnostics.this_implicitly_has_type_any_because_it_does_not_have_a_type_annotation);
            }
            return anyType;
        }

        function getTypeForThisExpressionFromJSDoc(node: Node) {
            const typeTag = getJSDocTypeTag(node);
            if (typeTag && typeTag.typeExpression && typeTag.typeExpression.type && typeTag.typeExpression.type.kind === SyntaxKind.JSDocFunctionType) {
                const jsDocFunctionType = <JSDocFunctionType>typeTag.typeExpression.type;
                if (jsDocFunctionType.parameters.length > 0 && jsDocFunctionType.parameters[0].type.kind === SyntaxKind.JSDocThisType) {
                    return getTypeFromTypeNode(jsDocFunctionType.parameters[0].type);
                }
            }
        }

        function isInConstructorArgumentInitializer(node: Node, constructorDecl: Node): boolean {
            for (let n = node; n && n !== constructorDecl; n = n.parent) {
                if (n.kind === SyntaxKind.Parameter) {
                    return true;
                }
            }
            return false;
        }

        function checkSuperExpression(node: Node): Type {
            const isCallExpression = node.parent.kind === SyntaxKind.CallExpression && (<CallExpression>node.parent).expression === node;

            let container = getSuperContainer(node, /*stopOnFunctions*/ true);
            let needToCaptureLexicalThis = false;

            if (!isCallExpression) {
                // adjust the container reference in case if super is used inside arrow functions with arbitrary deep nesting
                while (container && container.kind === SyntaxKind.ArrowFunction) {
                    container = getSuperContainer(container, /*stopOnFunctions*/ true);
                    needToCaptureLexicalThis = languageVersion < ScriptTarget.ES6;
                }
            }

            const canUseSuperExpression = isLegalUsageOfSuperExpression(container);
            let nodeCheckFlag: NodeCheckFlags = 0;

            if (!canUseSuperExpression) {
                // issue more specific error if super is used in computed property name
                // class A { foo() { return "1" }}
                // class B {
                //     [super.foo()]() {}
                // }
                let current = node;
                while (current && current !== container && current.kind !== SyntaxKind.ComputedPropertyName) {
                    current = current.parent;
                }
                if (current && current.kind === SyntaxKind.ComputedPropertyName) {
                    error(node, Diagnostics.super_cannot_be_referenced_in_a_computed_property_name);
                }
                else if (isCallExpression) {
                    error(node, Diagnostics.Super_calls_are_not_permitted_outside_constructors_or_in_nested_functions_inside_constructors);
                }
                else if (!container || !container.parent || !(isClassLike(container.parent) || container.parent.kind === SyntaxKind.ObjectLiteralExpression)) {
                    error(node, Diagnostics.super_can_only_be_referenced_in_members_of_derived_classes_or_object_literal_expressions);
                }
                else {
                    error(node, Diagnostics.super_property_access_is_permitted_only_in_a_constructor_member_function_or_member_accessor_of_a_derived_class);
                }
                return unknownType;
            }

            if ((container.flags & NodeFlags.Static) || isCallExpression) {
                nodeCheckFlag = NodeCheckFlags.SuperStatic;
            }
            else {
                nodeCheckFlag = NodeCheckFlags.SuperInstance;
            }

            getNodeLinks(node).flags |= nodeCheckFlag;

            // Due to how we emit async functions, we need to specialize the emit for an async method that contains a `super` reference.
            // This is due to the fact that we emit the body of an async function inside of a generator function. As generator
            // functions cannot reference `super`, we emit a helper inside of the method body, but outside of the generator. This helper
            // uses an arrow function, which is permitted to reference `super`.
            //
            // There are two primary ways we can access `super` from within an async method. The first is getting the value of a property
            // or indexed access on super, either as part of a right-hand-side expression or call expression. The second is when setting the value
            // of a property or indexed access, either as part of an assignment expression or destructuring assignment.
            //
            // The simplest case is reading a value, in which case we will emit something like the following:
            //
            //  // ts
            //  ...
            //  async asyncMethod() {
            //    let x = await super.asyncMethod();
            //    return x;
            //  }
            //  ...
            //
            //  // js
            //  ...
            //  asyncMethod() {
            //      const _super = name => super[name];
            //      return __awaiter(this, arguments, Promise, function *() {
            //          let x = yield _super("asyncMethod").call(this);
            //          return x;
            //      });
            //  }
            //  ...
            //
            // The more complex case is when we wish to assign a value, especially as part of a destructuring assignment. As both cases
            // are legal in ES6, but also likely less frequent, we emit the same more complex helper for both scenarios:
            //
            //  // ts
            //  ...
            //  async asyncMethod(ar: Promise<any[]>) {
            //      [super.a, super.b] = await ar;
            //  }
            //  ...
            //
            //  // js
            //  ...
            //  asyncMethod(ar) {
            //      const _super = (function (geti, seti) {
            //          const cache = Object.create(null);
            //          return name => cache[name] || (cache[name] = { get value() { return geti(name); }, set value(v) { seti(name, v); } });
            //      })(name => super[name], (name, value) => super[name] = value);
            //      return __awaiter(this, arguments, Promise, function *() {
            //          [_super("a").value, _super("b").value] = yield ar;
            //      });
            //  }
            //  ...
            //
            // This helper creates an object with a "value" property that wraps the `super` property or indexed access for both get and set.
            // This is required for destructuring assignments, as a call expression cannot be used as the target of a destructuring assignment
            // while a property access can.
            if (container.kind === SyntaxKind.MethodDeclaration && container.flags & NodeFlags.Async) {
                if (isSuperPropertyOrElementAccess(node.parent) && isAssignmentTarget(node.parent)) {
                    getNodeLinks(container).flags |= NodeCheckFlags.AsyncMethodWithSuperBinding;
                }
                else {
                    getNodeLinks(container).flags |= NodeCheckFlags.AsyncMethodWithSuper;
                }
            }

            if (needToCaptureLexicalThis) {
                // call expressions are allowed only in constructors so they should always capture correct 'this'
                // super property access expressions can also appear in arrow functions -
                // in this case they should also use correct lexical this
                captureLexicalThis(node.parent, container);
            }

            if (container.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                if (languageVersion < ScriptTarget.ES6) {
                    error(node, Diagnostics.super_is_only_allowed_in_members_of_object_literal_expressions_when_option_target_is_ES2015_or_higher);
                    return unknownType;
                }
                else {
                    // for object literal assume that type of 'super' is 'any'
                    return anyType;
                }
            }

            // at this point the only legal case for parent is ClassLikeDeclaration
            const classLikeDeclaration = <ClassLikeDeclaration>container.parent;
            const classType = <InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(classLikeDeclaration));
            const baseClassType = classType && getBaseTypes(classType)[0];
            if (!baseClassType) {
                if (!getClassExtendsHeritageClauseElement(classLikeDeclaration)) {
                    error(node, Diagnostics.super_can_only_be_referenced_in_a_derived_class);
                }
                return unknownType;
            }

            if (container.kind === SyntaxKind.Constructor && isInConstructorArgumentInitializer(node, container)) {
                // issue custom error message for super property access in constructor arguments (to be aligned with old compiler)
                error(node, Diagnostics.super_cannot_be_referenced_in_constructor_arguments);
                return unknownType;
            }

            return nodeCheckFlag === NodeCheckFlags.SuperStatic
                ? getBaseConstructorTypeOfClass(classType)
                : getTypeWithThisArgument(baseClassType, classType.thisType);

            function isLegalUsageOfSuperExpression(container: Node): boolean {
                if (!container) {
                    return false;
                }

                if (isCallExpression) {
                    // TS 1.0 SPEC (April 2014): 4.8.1
                    // Super calls are only permitted in constructors of derived classes
                    return container.kind === SyntaxKind.Constructor;
                }
                else {
                    // TS 1.0 SPEC (April 2014)
                    // 'super' property access is allowed
                    // - In a constructor, instance member function, instance member accessor, or instance member variable initializer where this references a derived class instance
                    // - In a static member function or static member accessor

                    // topmost container must be something that is directly nested in the class declaration\object literal expression
                    if (isClassLike(container.parent) || container.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                        if (container.flags & NodeFlags.Static) {
                            return container.kind === SyntaxKind.MethodDeclaration ||
                                container.kind === SyntaxKind.MethodSignature ||
                                container.kind === SyntaxKind.GetAccessor ||
                                container.kind === SyntaxKind.SetAccessor;
                        }
                        else {
                            return container.kind === SyntaxKind.MethodDeclaration ||
                                container.kind === SyntaxKind.MethodSignature ||
                                container.kind === SyntaxKind.GetAccessor ||
                                container.kind === SyntaxKind.SetAccessor ||
                                container.kind === SyntaxKind.PropertyDeclaration ||
                                container.kind === SyntaxKind.PropertySignature ||
                                container.kind === SyntaxKind.Constructor;
                        }
                    }
                }

                return false;
            }
        }

        function getContextuallyTypedThisType(func: FunctionLikeDeclaration): Type {
            if (isContextSensitiveFunctionOrObjectLiteralMethod(func) && func.kind !== SyntaxKind.ArrowFunction) {
                const contextualSignature = getContextualSignature(func);
                if (contextualSignature) {
                    return getThisTypeOfSignature(contextualSignature);
                }
            }

            return undefined;
        }

        // Return contextual type of parameter or undefined if no contextual type is available
        function getContextuallyTypedParameterType(parameter: ParameterDeclaration): Type {
            const func = parameter.parent;
            if (isContextSensitiveFunctionOrObjectLiteralMethod(func)) {
                const iife = getImmediatelyInvokedFunctionExpression(func);
                if (iife) {
                    const indexOfParameter = indexOf(func.parameters, parameter);
                    if (iife.arguments && indexOfParameter < iife.arguments.length) {
                        if (parameter.dotDotDotToken) {
                            const restTypes: Type[] = [];
                            for (let i = indexOfParameter; i < iife.arguments.length; i++) {
                                restTypes.push(getTypeOfExpression(iife.arguments[i]));
                            }
                            return createArrayType(getUnionType(restTypes));
                        }
                        const links = getNodeLinks(iife);
                        const cached = links.resolvedSignature;
                        links.resolvedSignature = anySignature;
                        const type = checkExpression(iife.arguments[indexOfParameter]);
                        links.resolvedSignature = cached;
                        return type;
                    }
                }
                const contextualSignature = getContextualSignature(func);
                if (contextualSignature) {
                    const funcHasRestParameters = hasRestParameter(func);
                    const len = func.parameters.length - (funcHasRestParameters ? 1 : 0);
                    const indexOfParameter = indexOf(func.parameters, parameter);
                    if (indexOfParameter < len) {
                        return getTypeAtPosition(contextualSignature, indexOfParameter);
                    }

                    // If last parameter is contextually rest parameter get its type
                    if (funcHasRestParameters &&
                        indexOfParameter === (func.parameters.length - 1) &&
                        isRestParameterIndex(contextualSignature, func.parameters.length - 1)) {
                        return getTypeOfSymbol(lastOrUndefined(contextualSignature.parameters));
                    }
                }
            }
            return undefined;
        }

        // In a variable, parameter or property declaration with a type annotation,
        //   the contextual type of an initializer expression is the type of the variable, parameter or property.
        // Otherwise, in a parameter declaration of a contextually typed function expression,
        //   the contextual type of an initializer expression is the contextual type of the parameter.
        // Otherwise, in a variable or parameter declaration with a binding pattern name,
        //   the contextual type of an initializer expression is the type implied by the binding pattern.
        // Otherwise, in a binding pattern inside a variable or parameter declaration,
        //   the contextual type of an initializer expression is the type annotation of the containing declaration, if present.
        function getContextualTypeForInitializerExpression(node: Expression): Type {
            const declaration = <VariableLikeDeclaration>node.parent;
            if (node === declaration.initializer) {
                if (declaration.type) {
                    return getTypeFromTypeNode(declaration.type);
                }
                if (declaration.kind === SyntaxKind.Parameter) {
                    const type = getContextuallyTypedParameterType(<ParameterDeclaration>declaration);
                    if (type) {
                        return type;
                    }
                }
                if (isBindingPattern(declaration.name)) {
                    return getTypeFromBindingPattern(<BindingPattern>declaration.name, /*includePatternInType*/ true);
                }
                if (isBindingPattern(declaration.parent)) {
                    const parentDeclaration = declaration.parent.parent;
                    const name = declaration.propertyName || declaration.name;
                    if (isVariableLike(parentDeclaration) &&
                        parentDeclaration.type &&
                        !isBindingPattern(name)) {
                        const text = getTextOfPropertyName(name);
                        if (text) {
                            return getTypeOfPropertyOfType(getTypeFromTypeNode(parentDeclaration.type), text);
                        }
                    }
                }
            }
            return undefined;
        }

        function getContextualTypeForReturnExpression(node: Expression): Type {
            const func = getContainingFunction(node);

            if (isAsyncFunctionLike(func)) {
                const contextualReturnType = getContextualReturnType(func);
                if (contextualReturnType) {
                    return getPromisedType(contextualReturnType);
                }

                return undefined;
            }

            if (func && !func.asteriskToken) {
                return getContextualReturnType(func);
            }

            return undefined;
        }

        function getContextualTypeForYieldOperand(node: YieldExpression): Type {
            const func = getContainingFunction(node);
            if (func) {
                const contextualReturnType = getContextualReturnType(func);
                if (contextualReturnType) {
                    return node.asteriskToken
                        ? contextualReturnType
                        : getElementTypeOfIterableIterator(contextualReturnType);
                }
            }

            return undefined;
        }

        function isInParameterInitializerBeforeContainingFunction(node: Node) {
            while (node.parent && !isFunctionLike(node.parent)) {
                if (node.parent.kind === SyntaxKind.Parameter && (<ParameterDeclaration>node.parent).initializer === node) {
                    return true;
                }

                node = node.parent;
            }

            return false;
        }

        function getContextualReturnType(functionDecl: FunctionLikeDeclaration): Type {
            // If the containing function has a return type annotation, is a constructor, or is a get accessor whose
            // corresponding set accessor has a type annotation, return statements in the function are contextually typed
            if (functionDecl.type ||
                functionDecl.kind === SyntaxKind.Constructor ||
                functionDecl.kind === SyntaxKind.GetAccessor && getSetAccessorTypeAnnotationNode(<AccessorDeclaration>getDeclarationOfKind(functionDecl.symbol, SyntaxKind.SetAccessor))) {
                return getReturnTypeOfSignature(getSignatureFromDeclaration(functionDecl));
            }

            // Otherwise, if the containing function is contextually typed by a function type with exactly one call signature
            // and that call signature is non-generic, return statements are contextually typed by the return type of the signature
            const signature = getContextualSignatureForFunctionLikeDeclaration(<FunctionExpression>functionDecl);
            if (signature) {
                return getReturnTypeOfSignature(signature);
            }

            return undefined;
        }

        // In a typed function call, an argument or substitution expression is contextually typed by the type of the corresponding parameter.
        function getContextualTypeForArgument(callTarget: CallLikeExpression, arg: Expression): Type {
            const args = getEffectiveCallArguments(callTarget);
            const argIndex = indexOf(args, arg);
            if (argIndex >= 0) {
                const signature = getResolvedOrAnySignature(callTarget);
                return getTypeAtPosition(signature, argIndex);
            }
            return undefined;
        }

        function getContextualTypeForSubstitutionExpression(template: TemplateExpression, substitutionExpression: Expression) {
            if (template.parent.kind === SyntaxKind.TaggedTemplateExpression) {
                return getContextualTypeForArgument(<TaggedTemplateExpression>template.parent, substitutionExpression);
            }

            return undefined;
        }

        function getContextualTypeForBinaryOperand(node: Expression): Type {
            const binaryExpression = <BinaryExpression>node.parent;
            const operator = binaryExpression.operatorToken.kind;
            if (operator >= SyntaxKind.FirstAssignment && operator <= SyntaxKind.LastAssignment) {
                // In an assignment expression, the right operand is contextually typed by the type of the left operand.
                if (node === binaryExpression.right) {
                    return checkExpression(binaryExpression.left);
                }
            }
            else if (operator === SyntaxKind.BarBarToken) {
                // When an || expression has a contextual type, the operands are contextually typed by that type. When an ||
                // expression has no contextual type, the right operand is contextually typed by the type of the left operand.
                let type = getContextualType(binaryExpression);
                if (!type && node === binaryExpression.right) {
                    type = checkExpression(binaryExpression.left);
                }
                return type;
            }
            else if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.CommaToken) {
                if (node === binaryExpression.right) {
                    return getContextualType(binaryExpression);
                }
            }

            return undefined;
        }

        // Apply a mapping function to a contextual type and return the resulting type. If the contextual type
        // is a union type, the mapping function is applied to each constituent type and a union of the resulting
        // types is returned.
        function applyToContextualType(type: Type, mapper: (t: Type) => Type): Type {
            if (!(type.flags & TypeFlags.Union)) {
                return mapper(type);
            }
            const types = (<UnionType>type).types;
            let mappedType: Type;
            let mappedTypes: Type[];
            for (const current of types) {
                const t = mapper(current);
                if (t) {
                    if (!mappedType) {
                        mappedType = t;
                    }
                    else if (!mappedTypes) {
                        mappedTypes = [mappedType, t];
                    }
                    else {
                        mappedTypes.push(t);
                    }
                }
            }
            return mappedTypes ? getUnionType(mappedTypes) : mappedType;
        }

        function getTypeOfPropertyOfContextualType(type: Type, name: string) {
            return applyToContextualType(type, t => {
                const prop = t.flags & TypeFlags.StructuredType ? getPropertyOfType(t, name) : undefined;
                return prop ? getTypeOfSymbol(prop) : undefined;
            });
        }

        function getIndexTypeOfContextualType(type: Type, kind: IndexKind) {
            return applyToContextualType(type, t => getIndexTypeOfStructuredType(t, kind));
        }

        // Return true if the given contextual type is a tuple-like type
        function contextualTypeIsTupleLikeType(type: Type): boolean {
            return !!(type.flags & TypeFlags.Union ? forEach((<UnionType>type).types, isTupleLikeType) : isTupleLikeType(type));
        }

        // In an object literal contextually typed by a type T, the contextual type of a property assignment is the type of
        // the matching property in T, if one exists. Otherwise, it is the type of the numeric index signature in T, if one
        // exists. Otherwise, it is the type of the string index signature in T, if one exists.
        function getContextualTypeForObjectLiteralMethod(node: MethodDeclaration): Type {
            Debug.assert(isObjectLiteralMethod(node));
            if (isInsideWithStatementBody(node)) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }

            return getContextualTypeForObjectLiteralElement(node);
        }

        function getContextualTypeForObjectLiteralElement(element: ObjectLiteralElement) {
            const objectLiteral = <ObjectLiteralExpression>element.parent;
            const type = getApparentTypeOfContextualType(objectLiteral);
            if (type) {
                if (!hasDynamicName(element)) {
                    // For a (non-symbol) computed property, there is no reason to look up the name
                    // in the type. It will just be "__computed", which does not appear in any
                    // SymbolTable.
                    const symbolName = getSymbolOfNode(element).name;
                    const propertyType = getTypeOfPropertyOfContextualType(type, symbolName);
                    if (propertyType) {
                        return propertyType;
                    }
                }

                return isNumericName(element.name) && getIndexTypeOfContextualType(type, IndexKind.Number) ||
                    getIndexTypeOfContextualType(type, IndexKind.String);
            }

            return undefined;
        }

        // In an array literal contextually typed by a type T, the contextual type of an element expression at index N is
        // the type of the property with the numeric name N in T, if one exists. Otherwise, if T has a numeric index signature,
        // it is the type of the numeric index signature in T. Otherwise, in ES6 and higher, the contextual type is the iterated
        // type of T.
        function getContextualTypeForElementExpression(node: Expression): Type {
            const arrayLiteral = <ArrayLiteralExpression>node.parent;
            const type = getApparentTypeOfContextualType(arrayLiteral);
            if (type) {
                const index = indexOf(arrayLiteral.elements, node);
                return getTypeOfPropertyOfContextualType(type, "" + index)
                    || getIndexTypeOfContextualType(type, IndexKind.Number)
                    || (languageVersion >= ScriptTarget.ES6 ? getElementTypeOfIterable(type, /*errorNode*/ undefined) : undefined);
            }
            return undefined;
        }

        // In a contextually typed conditional expression, the true/false expressions are contextually typed by the same type.
        function getContextualTypeForConditionalOperand(node: Expression): Type {
            const conditional = <ConditionalExpression>node.parent;
            return node === conditional.whenTrue || node === conditional.whenFalse ? getContextualType(conditional) : undefined;
        }

        function getContextualTypeForJsxAttribute(attribute: JsxAttribute | JsxSpreadAttribute) {
            const kind = attribute.kind;
            const jsxElement = attribute.parent as JsxOpeningLikeElement;
            const attrsType = getJsxElementAttributesType(jsxElement);

            if (attribute.kind === SyntaxKind.JsxAttribute) {
                if (!attrsType || isTypeAny(attrsType)) {
                    return undefined;
                }
                return getTypeOfPropertyOfType(attrsType, (attribute as JsxAttribute).name.text);
            }
            else if (attribute.kind === SyntaxKind.JsxSpreadAttribute) {
                return attrsType;
            }

            Debug.fail(`Expected JsxAttribute or JsxSpreadAttribute, got ts.SyntaxKind[${kind}]`);
        }

        // Return the contextual type for a given expression node. During overload resolution, a contextual type may temporarily
        // be "pushed" onto a node using the contextualType property.
        function getApparentTypeOfContextualType(node: Expression): Type {
            const type = getContextualType(node);
            return type && getApparentType(type);
        }

        /**
         * Woah! Do you really want to use this function?
         *
         * Unless you're trying to get the *non-apparent* type for a
         * value-literal type or you're authoring relevant portions of this algorithm,
         * you probably meant to use 'getApparentTypeOfContextualType'.
         * Otherwise this may not be very useful.
         *
         * In cases where you *are* working on this function, you should understand
         * when it is appropriate to use 'getContextualType' and 'getApparentTypeOfContextualType'.
         *
         *   - Use 'getContextualType' when you are simply going to propagate the result to the expression.
         *   - Use 'getApparentTypeOfContextualType' when you're going to need the members of the type.
         *
         * @param node the expression whose contextual type will be returned.
         * @returns the contextual type of an expression.
         */
        function getContextualType(node: Expression): Type {
            if (isInsideWithStatementBody(node)) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }
            if (node.contextualType) {
                return node.contextualType;
            }
            const parent = node.parent;
            switch (parent.kind) {
                case SyntaxKind.VariableDeclaration:
                case SyntaxKind.Parameter:
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                case SyntaxKind.BindingElement:
                    return getContextualTypeForInitializerExpression(node);
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.ReturnStatement:
                    return getContextualTypeForReturnExpression(node);
                case SyntaxKind.YieldExpression:
                    return getContextualTypeForYieldOperand(<YieldExpression>parent);
                case SyntaxKind.CallExpression:
                case SyntaxKind.NewExpression:
                    return getContextualTypeForArgument(<CallExpression>parent, node);
                case SyntaxKind.TypeAssertionExpression:
                case SyntaxKind.AsExpression:
                    return getTypeFromTypeNode((<AssertionExpression>parent).type);
                case SyntaxKind.BinaryExpression:
                    return getContextualTypeForBinaryOperand(node);
                case SyntaxKind.PropertyAssignment:
                    return getContextualTypeForObjectLiteralElement(<ObjectLiteralElement>parent);
                case SyntaxKind.ArrayLiteralExpression:
                    return getContextualTypeForElementExpression(node);
                case SyntaxKind.ConditionalExpression:
                    return getContextualTypeForConditionalOperand(node);
                case SyntaxKind.TemplateSpan:
                    Debug.assert(parent.parent.kind === SyntaxKind.TemplateExpression);
                    return getContextualTypeForSubstitutionExpression(<TemplateExpression>parent.parent, node);
                case SyntaxKind.ParenthesizedExpression:
                    return getContextualType(<ParenthesizedExpression>parent);
                case SyntaxKind.JsxExpression:
                    return getContextualType(<JsxExpression>parent);
                case SyntaxKind.JsxAttribute:
                case SyntaxKind.JsxSpreadAttribute:
                    return getContextualTypeForJsxAttribute(<JsxAttribute | JsxSpreadAttribute>parent);
            }
            return undefined;
        }

        function isLiteralTypeLocation(node: Node): boolean {
            const parent = node.parent;
            switch (parent.kind) {
                case SyntaxKind.BinaryExpression:
                    switch ((<BinaryExpression>parent).operatorToken.kind) {
                        case SyntaxKind.EqualsEqualsEqualsToken:
                        case SyntaxKind.ExclamationEqualsEqualsToken:
                        case SyntaxKind.EqualsEqualsToken:
                        case SyntaxKind.ExclamationEqualsToken:
                            return true;
                    }
                    break;
                case SyntaxKind.ConditionalExpression:
                    return (node === (<ConditionalExpression>parent).whenTrue ||
                        node === (<ConditionalExpression>parent).whenFalse) &&
                        isLiteralTypeLocation(parent);
                case SyntaxKind.ParenthesizedExpression:
                    return isLiteralTypeLocation(parent);
                case SyntaxKind.CaseClause:
                case SyntaxKind.LiteralType:
                    return true;
            }
            return false;
        }

        // If the given type is an object or union type, if that type has a single signature, and if
        // that signature is non-generic, return the signature. Otherwise return undefined.
        function getNonGenericSignature(type: Type): Signature {
            const signatures = getSignaturesOfStructuredType(type, SignatureKind.Call);
            if (signatures.length === 1) {
                const signature = signatures[0];
                if (!signature.typeParameters) {
                    return signature;
                }
            }
        }

        function isFunctionExpressionOrArrowFunction(node: Node): node is FunctionExpression {
            return node.kind === SyntaxKind.FunctionExpression || node.kind === SyntaxKind.ArrowFunction;
        }

        function getContextualSignatureForFunctionLikeDeclaration(node: FunctionLikeDeclaration): Signature {
            // Only function expressions, arrow functions, and object literal methods are contextually typed.
            return isFunctionExpressionOrArrowFunction(node) || isObjectLiteralMethod(node)
                ? getContextualSignature(<FunctionExpression>node)
                : undefined;
        }

        function getContextualTypeForFunctionLikeDeclaration(node: FunctionExpression | MethodDeclaration) {
            return isObjectLiteralMethod(node) ?
                getContextualTypeForObjectLiteralMethod(node) :
                getApparentTypeOfContextualType(node);
        }

        // Return the contextual signature for a given expression node. A contextual type provides a
        // contextual signature if it has a single call signature and if that call signature is non-generic.
        // If the contextual type is a union type, get the signature from each type possible and if they are
        // all identical ignoring their return type, the result is same signature but with return type as
        // union type of return types from these signatures
        function getContextualSignature(node: FunctionExpression | MethodDeclaration): Signature {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));
            const type = getContextualTypeForFunctionLikeDeclaration(node);
            if (!type) {
                return undefined;
            }
            if (!(type.flags & TypeFlags.Union)) {
                return getNonGenericSignature(type);
            }
            let signatureList: Signature[];
            const types = (<UnionType>type).types;
            for (const current of types) {
                const signature = getNonGenericSignature(current);
                if (signature) {
                    if (!signatureList) {
                        // This signature will contribute to contextual union signature
                        signatureList = [signature];
                    }
                    else if (!compareSignaturesIdentical(signatureList[0], signature, /*partialMatch*/ false, /*ignoreThisTypes*/ true, /*ignoreReturnTypes*/ true, compareTypesIdentical)) {
                        // Signatures aren't identical, do not use
                        return undefined;
                    }
                    else {
                        // Use this signature for contextual union signature
                        signatureList.push(signature);
                    }
                }
            }

            // Result is union of signatures collected (return type is union of return types of this signature set)
            let result: Signature;
            if (signatureList) {
                result = cloneSignature(signatureList[0]);
                // Clear resolved return type we possibly got from cloneSignature
                result.resolvedReturnType = undefined;
                result.unionSignatures = signatureList;
            }
            return result;
        }

        /**
         * Detect if the mapper implies an inference context. Specifically, there are 4 possible values
         * for a mapper. Let's go through each one of them:
         *
         *    1. undefined - this means we are not doing inferential typing, but we may do contextual typing,
         *       which could cause us to assign a parameter a type
         *    2. identityMapper - means we want to avoid assigning a parameter a type, whether or not we are in
         *       inferential typing (context is undefined for the identityMapper)
         *    3. a mapper created by createInferenceMapper - we are doing inferential typing, we want to assign
         *       types to parameters and fix type parameters (context is defined)
         *    4. an instantiation mapper created by createTypeMapper or createTypeEraser - this should never be
         *       passed as the contextual mapper when checking an expression (context is undefined for these)
         *
         * isInferentialContext is detecting if we are in case 3
         */
        function isInferentialContext(mapper: TypeMapper) {
            return mapper && mapper.context;
        }

        function checkSpreadElementExpression(node: SpreadElementExpression, contextualMapper?: TypeMapper): Type {
            // It is usually not safe to call checkExpressionCached if we can be contextually typing.
            // You can tell that we are contextually typing because of the contextualMapper parameter.
            // While it is true that a spread element can have a contextual type, it does not do anything
            // with this type. It is neither affected by it, nor does it propagate it to its operand.
            // So the fact that contextualMapper is passed is not important, because the operand of a spread
            // element is not contextually typed.
            const arrayOrIterableType = checkExpressionCached(node.expression, contextualMapper);
            return checkIteratedTypeOrElementType(arrayOrIterableType, node.expression, /*allowStringInput*/ false);
        }

        function hasDefaultValue(node: BindingElement | Expression): boolean {
            return (node.kind === SyntaxKind.BindingElement && !!(<BindingElement>node).initializer) ||
                (node.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>node).operatorToken.kind === SyntaxKind.EqualsToken);
        }

        function checkArrayLiteral(node: ArrayLiteralExpression, contextualMapper?: TypeMapper): Type {
            const elements = node.elements;
            let hasSpreadElement = false;
            const elementTypes: Type[] = [];
            const inDestructuringPattern = isAssignmentTarget(node);
            for (const e of elements) {
                if (inDestructuringPattern && e.kind === SyntaxKind.SpreadElementExpression) {
                    // Given the following situation:
                    //    var c: {};
                    //    [...c] = ["", 0];
                    //
                    // c is represented in the tree as a spread element in an array literal.
                    // But c really functions as a rest element, and its purpose is to provide
                    // a contextual type for the right hand side of the assignment. Therefore,
                    // instead of calling checkExpression on "...c", which will give an error
                    // if c is not iterable/array-like, we need to act as if we are trying to
                    // get the contextual element type from it. So we do something similar to
                    // getContextualTypeForElementExpression, which will crucially not error
                    // if there is no index type / iterated type.
                    const restArrayType = checkExpression((<SpreadElementExpression>e).expression, contextualMapper);
                    const restElementType = getIndexTypeOfType(restArrayType, IndexKind.Number) ||
                        (languageVersion >= ScriptTarget.ES6 ? getElementTypeOfIterable(restArrayType, /*errorNode*/ undefined) : undefined);
                    if (restElementType) {
                        elementTypes.push(restElementType);
                    }
                }
                else {
                    const type = checkExpression(e, contextualMapper);
                    elementTypes.push(type);
                }
                hasSpreadElement = hasSpreadElement || e.kind === SyntaxKind.SpreadElementExpression;
            }
            if (!hasSpreadElement) {
                // If array literal is actually a destructuring pattern, mark it as an implied type. We do this such
                // that we get the same behavior for "var [x, y] = []" and "[x, y] = []".
                if (inDestructuringPattern && elementTypes.length) {
                    const type = createNewTupleType(elementTypes);
                    type.pattern = node;
                    return type;
                }
                const contextualType = getApparentTypeOfContextualType(node);
                if (contextualType && contextualTypeIsTupleLikeType(contextualType)) {
                    const pattern = contextualType.pattern;
                    // If array literal is contextually typed by a binding pattern or an assignment pattern, pad the resulting
                    // tuple type with the corresponding binding or assignment element types to make the lengths equal.
                    if (pattern && (pattern.kind === SyntaxKind.ArrayBindingPattern || pattern.kind === SyntaxKind.ArrayLiteralExpression)) {
                        const patternElements = (<BindingPattern | ArrayLiteralExpression>pattern).elements;
                        for (let i = elementTypes.length; i < patternElements.length; i++) {
                            const patternElement = patternElements[i];
                            if (hasDefaultValue(patternElement)) {
                                elementTypes.push((<TupleType>contextualType).elementTypes[i]);
                            }
                            else {
                                if (patternElement.kind !== SyntaxKind.OmittedExpression) {
                                    error(patternElement, Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
                                }
                                elementTypes.push(unknownType);
                            }
                        }
                    }
                    if (elementTypes.length) {
                        return createTupleType(elementTypes);
                    }
                }
            }
            return createArrayType(elementTypes.length ?
                getUnionType(elementTypes, /*subtypeReduction*/ true) :
                strictNullChecks ? neverType : undefinedWideningType);
        }

        function isNumericName(name: DeclarationName): boolean {
            return name.kind === SyntaxKind.ComputedPropertyName ? isNumericComputedName(<ComputedPropertyName>name) : isNumericLiteralName((<Identifier>name).text);
        }

        function isNumericComputedName(name: ComputedPropertyName): boolean {
            // It seems odd to consider an expression of type Any to result in a numeric name,
            // but this behavior is consistent with checkIndexedAccess
            return isTypeAnyOrAllConstituentTypesHaveKind(checkComputedPropertyName(name), TypeFlags.NumberLike);
        }

        function isTypeAnyOrAllConstituentTypesHaveKind(type: Type, kind: TypeFlags): boolean {
            return isTypeAny(type) || isTypeOfKind(type, kind);
        }

        function isNumericLiteralName(name: string) {
            // The intent of numeric names is that
            //     - they are names with text in a numeric form, and that
            //     - setting properties/indexing with them is always equivalent to doing so with the numeric literal 'numLit',
            //         acquired by applying the abstract 'ToNumber' operation on the name's text.
            //
            // The subtlety is in the latter portion, as we cannot reliably say that anything that looks like a numeric literal is a numeric name.
            // In fact, it is the case that the text of the name must be equal to 'ToString(numLit)' for this to hold.
            //
            // Consider the property name '"0xF00D"'. When one indexes with '0xF00D', they are actually indexing with the value of 'ToString(0xF00D)'
            // according to the ECMAScript specification, so it is actually as if the user indexed with the string '"61453"'.
            // Thus, the text of all numeric literals equivalent to '61543' such as '0xF00D', '0xf00D', '0170015', etc. are not valid numeric names
            // because their 'ToString' representation is not equal to their original text.
            // This is motivated by ECMA-262 sections 9.3.1, 9.8.1, 11.1.5, and 11.2.1.
            //
            // Here, we test whether 'ToString(ToNumber(name))' is exactly equal to 'name'.
            // The '+' prefix operator is equivalent here to applying the abstract ToNumber operation.
            // Applying the 'toString()' method on a number gives us the abstract ToString operation on a number.
            //
            // Note that this accepts the values 'Infinity', '-Infinity', and 'NaN', and that this is intentional.
            // This is desired behavior, because when indexing with them as numeric entities, you are indexing
            // with the strings '"Infinity"', '"-Infinity"', and '"NaN"' respectively.
            return (+name).toString() === name;
        }

        function checkComputedPropertyName(node: ComputedPropertyName): Type {
            const links = getNodeLinks(node.expression);
            if (!links.resolvedType) {
                links.resolvedType = checkExpression(node.expression);

                // This will allow types number, string, symbol or any. It will also allow enums, the unknown
                // type, and any union of these types (like string | number).
                if (!isTypeAnyOrAllConstituentTypesHaveKind(links.resolvedType, TypeFlags.NumberLike | TypeFlags.StringLike | TypeFlags.ESSymbol)) {
                    error(node, Diagnostics.A_computed_property_name_must_be_of_type_string_number_symbol_or_any);
                }
                else {
                    checkThatExpressionIsProperSymbolReference(node.expression, links.resolvedType, /*reportError*/ true);
                }
            }

            return links.resolvedType;
        }

        function getObjectLiteralIndexInfo(node: ObjectLiteralExpression, properties: Symbol[], kind: IndexKind): IndexInfo {
            const propTypes: Type[] = [];
            for (let i = 0; i < properties.length; i++) {
                if (kind === IndexKind.String || isNumericName(node.properties[i].name)) {
                    propTypes.push(getTypeOfSymbol(properties[i]));
                }
            }
            const unionType = propTypes.length ? getUnionType(propTypes, /*subtypeReduction*/ true) : undefinedType;
            return createIndexInfo(unionType, /*isReadonly*/ false);
        }

        function checkObjectLiteral(node: ObjectLiteralExpression, contextualMapper?: TypeMapper): Type {
            const inDestructuringPattern = isAssignmentTarget(node);
            // Grammar checking
            checkGrammarObjectLiteralExpression(node, inDestructuringPattern);

            const propertiesTable = createMap<Symbol>();
            const propertiesArray: Symbol[] = [];
            const contextualType = getApparentTypeOfContextualType(node);
            const contextualTypeHasPattern = contextualType && contextualType.pattern &&
                (contextualType.pattern.kind === SyntaxKind.ObjectBindingPattern || contextualType.pattern.kind === SyntaxKind.ObjectLiteralExpression);
            let typeFlags: TypeFlags = 0;
            let patternWithComputedProperties = false;
            let hasComputedStringProperty = false;
            let hasComputedNumberProperty = false;

            for (const memberDecl of node.properties) {
                let member = memberDecl.symbol;
                if (memberDecl.kind === SyntaxKind.PropertyAssignment ||
                    memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment ||
                    isObjectLiteralMethod(memberDecl)) {
                    let type: Type;
                    if (memberDecl.kind === SyntaxKind.PropertyAssignment) {
                        type = checkPropertyAssignment(<PropertyAssignment>memberDecl, contextualMapper);
                    }
                    else if (memberDecl.kind === SyntaxKind.MethodDeclaration) {
                        type = checkObjectLiteralMethod(<MethodDeclaration>memberDecl, contextualMapper);
                    }
                    else {
                        Debug.assert(memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment);
                        type = checkExpression((<ShorthandPropertyAssignment>memberDecl).name, contextualMapper);
                    }
                    typeFlags |= type.flags;
                    const prop = <TransientSymbol>createSymbol(SymbolFlags.Property | SymbolFlags.Transient | member.flags, member.name);
                    if (inDestructuringPattern) {
                        // If object literal is an assignment pattern and if the assignment pattern specifies a default value
                        // for the property, make the property optional.
                        const isOptional =
                            (memberDecl.kind === SyntaxKind.PropertyAssignment && hasDefaultValue((<PropertyAssignment>memberDecl).initializer)) ||
                            (memberDecl.kind === SyntaxKind.ShorthandPropertyAssignment && (<ShorthandPropertyAssignment>memberDecl).objectAssignmentInitializer);
                        if (isOptional) {
                            prop.flags |= SymbolFlags.Optional;
                        }
                        if (hasDynamicName(memberDecl)) {
                            patternWithComputedProperties = true;
                        }
                    }
                    else if (contextualTypeHasPattern && !(contextualType.flags & TypeFlags.ObjectLiteralPatternWithComputedProperties)) {
                        // If object literal is contextually typed by the implied type of a binding pattern, and if the
                        // binding pattern specifies a default value for the property, make the property optional.
                        const impliedProp = getPropertyOfType(contextualType, member.name);
                        if (impliedProp) {
                            prop.flags |= impliedProp.flags & SymbolFlags.Optional;
                        }
                        else if (!compilerOptions.suppressExcessPropertyErrors) {
                            error(memberDecl.name, Diagnostics.Object_literal_may_only_specify_known_properties_and_0_does_not_exist_in_type_1,
                                symbolToString(member), typeToString(contextualType));
                        }
                    }
                    prop.declarations = member.declarations;
                    prop.parent = member.parent;
                    if (member.valueDeclaration) {
                        prop.valueDeclaration = member.valueDeclaration;
                    }

                    prop.type = type;
                    prop.target = member;
                    member = prop;
                }
                else {
                    // TypeScript 1.0 spec (April 2014)
                    // A get accessor declaration is processed in the same manner as
                    // an ordinary function declaration(section 6.1) with no parameters.
                    // A set accessor declaration is processed in the same manner
                    // as an ordinary function declaration with a single parameter and a Void return type.
                    Debug.assert(memberDecl.kind === SyntaxKind.GetAccessor || memberDecl.kind === SyntaxKind.SetAccessor);
                    checkAccessorDeclaration(<AccessorDeclaration>memberDecl);
                }

                if (hasDynamicName(memberDecl)) {
                    if (isNumericName(memberDecl.name)) {
                        hasComputedNumberProperty = true;
                    }
                    else {
                        hasComputedStringProperty = true;
                    }
                }
                else {
                    propertiesTable[member.name] = member;
                }
                propertiesArray.push(member);
            }

            // If object literal is contextually typed by the implied type of a binding pattern, augment the result
            // type with those properties for which the binding pattern specifies a default value.
            if (contextualTypeHasPattern) {
                for (const prop of getPropertiesOfType(contextualType)) {
                    if (!propertiesTable[prop.name]) {
                        if (!(prop.flags & SymbolFlags.Optional)) {
                            error(prop.valueDeclaration || (<TransientSymbol>prop).bindingElement,
                                Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
                        }
                        propertiesTable[prop.name] = prop;
                        propertiesArray.push(prop);
                    }
                }
            }

            const stringIndexInfo = hasComputedStringProperty ? getObjectLiteralIndexInfo(node, propertiesArray, IndexKind.String) : undefined;
            const numberIndexInfo = hasComputedNumberProperty ? getObjectLiteralIndexInfo(node, propertiesArray, IndexKind.Number) : undefined;
            const result = createAnonymousType(node.symbol, propertiesTable, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
            const freshObjectLiteralFlag = compilerOptions.suppressExcessPropertyErrors ? 0 : TypeFlags.FreshObjectLiteral;
            result.flags |= TypeFlags.ObjectLiteral | TypeFlags.ContainsObjectLiteral | freshObjectLiteralFlag | (typeFlags & TypeFlags.PropagatingFlags) | (patternWithComputedProperties ? TypeFlags.ObjectLiteralPatternWithComputedProperties : 0);
            if (inDestructuringPattern) {
                result.pattern = node;
            }
            return result;
        }

        function checkJsxSelfClosingElement(node: JsxSelfClosingElement) {
            checkJsxOpeningLikeElement(node);
            return jsxElementType || anyType;
        }

        function checkJsxElement(node: JsxElement) {
            // Check attributes
            checkJsxOpeningLikeElement(node.openingElement);

            // Perform resolution on the closing tag so that rename/go to definition/etc work
            if (isJsxIntrinsicIdentifier(node.closingElement.tagName)) {
                getIntrinsicTagSymbol(node.closingElement);
            }
            else {
                checkExpression(node.closingElement.tagName);
            }

            // Check children
            for (const child of node.children) {
                switch (child.kind) {
                    case SyntaxKind.JsxExpression:
                        checkJsxExpression(<JsxExpression>child);
                        break;
                    case SyntaxKind.JsxElement:
                        checkJsxElement(<JsxElement>child);
                        break;
                    case SyntaxKind.JsxSelfClosingElement:
                        checkJsxSelfClosingElement(<JsxSelfClosingElement>child);
                        break;
                }
            }

            return jsxElementType || anyType;
        }

        /**
         * Returns true iff the JSX element name would be a valid JS identifier, ignoring restrictions about keywords not being identifiers
         */
        function isUnhyphenatedJsxName(name: string) {
            // - is the only character supported in JSX attribute names that isn't valid in JavaScript identifiers
            return name.indexOf("-") < 0;
        }

        /**
         * Returns true iff React would emit this tag name as a string rather than an identifier or qualified name
         */
        function isJsxIntrinsicIdentifier(tagName: JsxTagNameExpression) {
            // TODO (yuisu): comment
            if (tagName.kind === SyntaxKind.PropertyAccessExpression || tagName.kind === SyntaxKind.ThisKeyword) {
                return false;
            }
            else {
                return isIntrinsicJsxName((<Identifier>tagName).text);
            }
        }

        function checkJsxAttribute(node: JsxAttribute, elementAttributesType: Type, nameTable: Map<boolean>) {
            let correspondingPropType: Type = undefined;

            // Look up the corresponding property for this attribute
            if (elementAttributesType === emptyObjectType && isUnhyphenatedJsxName(node.name.text)) {
                // If there is no 'props' property, you may not have non-"data-" attributes
                error(node.parent, Diagnostics.JSX_element_class_does_not_support_attributes_because_it_does_not_have_a_0_property, getJsxElementPropertiesName());
            }
            else if (elementAttributesType && !isTypeAny(elementAttributesType)) {
                const correspondingPropSymbol = getPropertyOfType(elementAttributesType, node.name.text);
                correspondingPropType = correspondingPropSymbol && getTypeOfSymbol(correspondingPropSymbol);
                if (isUnhyphenatedJsxName(node.name.text)) {
                    // Maybe there's a string indexer?
                    const indexerType = getIndexTypeOfType(elementAttributesType, IndexKind.String);
                    if (indexerType) {
                        correspondingPropType = indexerType;
                    }
                    else {
                        // If there's no corresponding property with this name, error
                        if (!correspondingPropType) {
                            error(node.name, Diagnostics.Property_0_does_not_exist_on_type_1, node.name.text, typeToString(elementAttributesType));
                            return unknownType;
                        }
                    }
                }
            }

            let exprType: Type;
            if (node.initializer) {
                exprType = checkExpression(node.initializer);
            }
            else {
                // <Elem attr /> is sugar for <Elem attr={true} />
                exprType = booleanType;
            }

            if (correspondingPropType) {
                checkTypeAssignableTo(exprType, correspondingPropType, node);
            }

            nameTable[node.name.text] = true;
            return exprType;
        }

        function checkJsxSpreadAttribute(node: JsxSpreadAttribute, elementAttributesType: Type, nameTable: Map<boolean>) {
            const type = checkExpression(node.expression);
            const props = getPropertiesOfType(type);
            for (const prop of props) {
                // Is there a corresponding property in the element attributes type? Skip checking of properties
                // that have already been assigned to, as these are not actually pushed into the resulting type
                if (!nameTable[prop.name]) {
                    const targetPropSym = getPropertyOfType(elementAttributesType, prop.name);
                    if (targetPropSym) {
                        const msg = chainDiagnosticMessages(undefined, Diagnostics.Property_0_of_JSX_spread_attribute_is_not_assignable_to_target_property, prop.name);
                        checkTypeAssignableTo(getTypeOfSymbol(prop), getTypeOfSymbol(targetPropSym), node, undefined, msg);
                    }

                    nameTable[prop.name] = true;
                }
            }
            return type;
        }

        function getJsxType(name: string) {
            if (jsxTypes[name] === undefined) {
                return jsxTypes[name] = getExportedTypeFromNamespace(JsxNames.JSX, name) || unknownType;
            }
            return jsxTypes[name];
        }

        /**
          * Looks up an intrinsic tag name and returns a symbol that either points to an intrinsic
          * property (in which case nodeLinks.jsxFlags will be IntrinsicNamedElement) or an intrinsic
          * string index signature (in which case nodeLinks.jsxFlags will be IntrinsicIndexedElement).
          * May also return unknownSymbol if both of these lookups fail.
          */
        function getIntrinsicTagSymbol(node: JsxOpeningLikeElement | JsxClosingElement): Symbol {
            const links = getNodeLinks(node);
            if (!links.resolvedSymbol) {
                const intrinsicElementsType = getJsxType(JsxNames.IntrinsicElements);
                if (intrinsicElementsType !== unknownType) {
                    // Property case
                    const intrinsicProp = getPropertyOfType(intrinsicElementsType, (<Identifier>node.tagName).text);
                    if (intrinsicProp) {
                        links.jsxFlags |= JsxFlags.IntrinsicNamedElement;
                        return links.resolvedSymbol = intrinsicProp;
                    }

                    // Intrinsic string indexer case
                    const indexSignatureType = getIndexTypeOfType(intrinsicElementsType, IndexKind.String);
                    if (indexSignatureType) {
                        links.jsxFlags |= JsxFlags.IntrinsicIndexedElement;
                        return links.resolvedSymbol = intrinsicElementsType.symbol;
                    }

                    // Wasn't found
                    error(node, Diagnostics.Property_0_does_not_exist_on_type_1, (<Identifier>node.tagName).text, "JSX." + JsxNames.IntrinsicElements);
                    return links.resolvedSymbol = unknownSymbol;
                }
                else {
                    if (compilerOptions.noImplicitAny) {
                        error(node, Diagnostics.JSX_element_implicitly_has_type_any_because_no_interface_JSX_0_exists, JsxNames.IntrinsicElements);
                    }
                    return links.resolvedSymbol = unknownSymbol;
                }
            }
            return links.resolvedSymbol;
        }

        /**
         * Given a JSX element that is a class element, finds the Element Instance Type. If the
         * element is not a class element, or the class element type cannot be determined, returns 'undefined'.
         * For example, in the element <MyClass>, the element instance type is `MyClass` (not `typeof MyClass`).
         */
        function getJsxElementInstanceType(node: JsxOpeningLikeElement, valueType: Type) {
            Debug.assert(!(valueType.flags & TypeFlags.Union));
            if (isTypeAny(valueType)) {
                // Short-circuit if the class tag is using an element type 'any'
                return anyType;
            }

            // Resolve the signatures, preferring constructor
            let signatures = getSignaturesOfType(valueType, SignatureKind.Construct);
            if (signatures.length === 0) {
                // No construct signatures, try call signatures
                signatures = getSignaturesOfType(valueType, SignatureKind.Call);
                if (signatures.length === 0) {
                    // We found no signatures at all, which is an error
                    error(node.tagName, Diagnostics.JSX_element_type_0_does_not_have_any_construct_or_call_signatures, getTextOfNode(node.tagName));
                    return unknownType;
                }
            }

            return getUnionType(signatures.map(getReturnTypeOfSignature), /*subtypeReduction*/ true);
        }

        /// e.g. "props" for React.d.ts,
        /// or 'undefined' if ElementAttributesProperty doesn't exist (which means all
        ///     non-intrinsic elements' attributes type is 'any'),
        /// or '' if it has 0 properties (which means every
        ///     non-intrinsic elements' attributes type is the element instance type)
        function getJsxElementPropertiesName() {
            // JSX
            const jsxNamespace = getGlobalSymbol(JsxNames.JSX, SymbolFlags.Namespace, /*diagnosticMessage*/undefined);
            // JSX.ElementAttributesProperty [symbol]
            const attribsPropTypeSym = jsxNamespace && getSymbol(jsxNamespace.exports, JsxNames.ElementAttributesPropertyNameContainer, SymbolFlags.Type);
            // JSX.ElementAttributesProperty [type]
            const attribPropType = attribsPropTypeSym && getDeclaredTypeOfSymbol(attribsPropTypeSym);
            // The properties of JSX.ElementAttributesProperty
            const attribProperties = attribPropType && getPropertiesOfType(attribPropType);

            if (attribProperties) {
                // Element Attributes has zero properties, so the element attributes type will be the class instance type
                if (attribProperties.length === 0) {
                    return "";
                }
                // Element Attributes has one property, so the element attributes type will be the type of the corresponding
                // property of the class instance type
                else if (attribProperties.length === 1) {
                    return attribProperties[0].name;
                }
                // More than one property on ElementAttributesProperty is an error
                else {
                    error(attribsPropTypeSym.declarations[0], Diagnostics.The_global_type_JSX_0_may_not_have_more_than_one_property, JsxNames.ElementAttributesPropertyNameContainer);
                    return undefined;
                }
            }
            else {
                // No interface exists, so the element attributes type will be an implicit any
                return undefined;
            }
        }

        /**
         * Given React element instance type and the class type, resolve the Jsx type
         * Pass elemType to handle individual type in the union typed element type.
         */
        function getResolvedJsxType(node: JsxOpeningLikeElement, elemType?: Type, elemClassType?: Type): Type {
            if (!elemType) {
                elemType = checkExpression(node.tagName);
            }
            if (elemType.flags & TypeFlags.Union) {
                const types = (<UnionOrIntersectionType>elemType).types;
                return getUnionType(types.map(type => {
                    return getResolvedJsxType(node, type, elemClassType);
                }), /*subtypeReduction*/ true);
            }

            // If the elemType is a string type, we have to return anyType to prevent an error downstream as we will try to find construct or call signature of the type
            if (elemType.flags & TypeFlags.String) {
                return anyType;
            }
            else if (elemType.flags & TypeFlags.StringLiteral) {
                // If the elemType is a stringLiteral type, we can then provide a check to make sure that the string literal type is one of the Jsx intrinsic element type
                const intrinsicElementsType = getJsxType(JsxNames.IntrinsicElements);
                if (intrinsicElementsType !== unknownType) {
                    const stringLiteralTypeName = (<LiteralType>elemType).text;
                    const intrinsicProp = getPropertyOfType(intrinsicElementsType, stringLiteralTypeName);
                    if (intrinsicProp) {
                        return getTypeOfSymbol(intrinsicProp);
                    }
                    const indexSignatureType = getIndexTypeOfType(intrinsicElementsType, IndexKind.String);
                    if (indexSignatureType) {
                        return indexSignatureType;
                    }
                    error(node, Diagnostics.Property_0_does_not_exist_on_type_1, stringLiteralTypeName, "JSX." + JsxNames.IntrinsicElements);
                }
                // If we need to report an error, we already done so here. So just return any to prevent any more error downstream
                return anyType;
            }

            // Get the element instance type (the result of newing or invoking this tag)
            const elemInstanceType = getJsxElementInstanceType(node, elemType);

            if (!elemClassType || !isTypeAssignableTo(elemInstanceType, elemClassType)) {
                // Is this is a stateless function component? See if its single signature's return type is
                // assignable to the JSX Element Type
                if (jsxElementType) {
                    const callSignatures = elemType && getSignaturesOfType(elemType, SignatureKind.Call);
                    const callSignature = callSignatures && callSignatures.length > 0 && callSignatures[0];
                    const callReturnType = callSignature && getReturnTypeOfSignature(callSignature);
                    let paramType = callReturnType && (callSignature.parameters.length === 0 ? emptyObjectType : getTypeOfSymbol(callSignature.parameters[0]));
                    if (callReturnType && isTypeAssignableTo(callReturnType, jsxElementType)) {
                        // Intersect in JSX.IntrinsicAttributes if it exists
                        const intrinsicAttributes = getJsxType(JsxNames.IntrinsicAttributes);
                        if (intrinsicAttributes !== unknownType) {
                            paramType = intersectTypes(intrinsicAttributes, paramType);
                        }
                        return paramType;
                    }
                }
            }

            // Issue an error if this return type isn't assignable to JSX.ElementClass
            if (elemClassType) {
                checkTypeRelatedTo(elemInstanceType, elemClassType, assignableRelation, node, Diagnostics.JSX_element_type_0_is_not_a_constructor_function_for_JSX_elements);
            }

            if (isTypeAny(elemInstanceType)) {
                return elemInstanceType;
            }

            const propsName = getJsxElementPropertiesName();
            if (propsName === undefined) {
                // There is no type ElementAttributesProperty, return 'any'
                return anyType;
            }
            else if (propsName === "") {
                // If there is no e.g. 'props' member in ElementAttributesProperty, use the element class type instead
                return elemInstanceType;
            }
            else {
                const attributesType = getTypeOfPropertyOfType(elemInstanceType, propsName);

                if (!attributesType) {
                    // There is no property named 'props' on this instance type
                    return emptyObjectType;
                }
                else if (isTypeAny(attributesType) || (attributesType === unknownType)) {
                    // Props is of type 'any' or unknown
                    return attributesType;
                }
                else if (attributesType.flags & TypeFlags.Union) {
                    // Props cannot be a union type
                    error(node.tagName, Diagnostics.JSX_element_attributes_type_0_may_not_be_a_union_type, typeToString(attributesType));
                    return anyType;
                }
                else {
                    // Normal case -- add in IntrinsicClassElements<T> and IntrinsicElements
                    let apparentAttributesType = attributesType;
                    const intrinsicClassAttribs = getJsxType(JsxNames.IntrinsicClassAttributes);
                    if (intrinsicClassAttribs !== unknownType) {
                        const typeParams = getLocalTypeParametersOfClassOrInterfaceOrTypeAlias(intrinsicClassAttribs.symbol);
                        if (typeParams) {
                            if (typeParams.length === 1) {
                                apparentAttributesType = intersectTypes(createTypeReference(<GenericType>intrinsicClassAttribs, [elemInstanceType]), apparentAttributesType);
                            }
                        }
                        else {
                            apparentAttributesType = intersectTypes(attributesType, intrinsicClassAttribs);
                        }
                    }

                    const intrinsicAttribs = getJsxType(JsxNames.IntrinsicAttributes);
                    if (intrinsicAttribs !== unknownType) {
                        apparentAttributesType = intersectTypes(intrinsicAttribs, apparentAttributesType);
                    }

                    return apparentAttributesType;
                }
            }
        }

        /**
         * Given an opening/self-closing element, get the 'element attributes type', i.e. the type that tells
         * us which attributes are valid on a given element.
         */
        function getJsxElementAttributesType(node: JsxOpeningLikeElement): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedJsxType) {
                if (isJsxIntrinsicIdentifier(node.tagName)) {
                    const symbol = getIntrinsicTagSymbol(node);
                    if (links.jsxFlags & JsxFlags.IntrinsicNamedElement) {
                        return links.resolvedJsxType = getTypeOfSymbol(symbol);
                    }
                    else if (links.jsxFlags & JsxFlags.IntrinsicIndexedElement) {
                        return links.resolvedJsxType = getIndexInfoOfSymbol(symbol, IndexKind.String).type;
                    }
                    else {
                        return links.resolvedJsxType = unknownType;
                    }
                }
                else {
                    const elemClassType = getJsxGlobalElementClassType();
                    return links.resolvedJsxType = getResolvedJsxType(node, undefined, elemClassType);
                }
            }
            return links.resolvedJsxType;
        }

        /**
         * Given a JSX attribute, returns the symbol for the corresponds property
         * of the element attributes type. Will return unknownSymbol for attributes
         * that have no matching element attributes type property.
         */
        function getJsxAttributePropertySymbol(attrib: JsxAttribute): Symbol {
            const attributesType = getJsxElementAttributesType(<JsxOpeningElement>attrib.parent);
            const prop = getPropertyOfType(attributesType, attrib.name.text);
            return prop || unknownSymbol;
        }

        function getJsxGlobalElementClassType(): Type {
            if (!jsxElementClassType) {
                jsxElementClassType = getExportedTypeFromNamespace(JsxNames.JSX, JsxNames.ElementClass);
            }
            return jsxElementClassType;
        }

        /// Returns all the properties of the Jsx.IntrinsicElements interface
        function getJsxIntrinsicTagNames(): Symbol[] {
            const intrinsics = getJsxType(JsxNames.IntrinsicElements);
            return intrinsics ? getPropertiesOfType(intrinsics) : emptyArray;
        }

        function checkJsxPreconditions(errorNode: Node) {
            // Preconditions for using JSX
            if ((compilerOptions.jsx || JsxEmit.None) === JsxEmit.None) {
                error(errorNode, Diagnostics.Cannot_use_JSX_unless_the_jsx_flag_is_provided);
            }

            if (jsxElementType === undefined) {
                if (compilerOptions.noImplicitAny) {
                    error(errorNode, Diagnostics.JSX_element_implicitly_has_type_any_because_the_global_type_JSX_Element_does_not_exist);
                }
            }
        }

        function checkJsxOpeningLikeElement(node: JsxOpeningLikeElement) {
            checkGrammarJsxElement(node);
            checkJsxPreconditions(node);

            // The reactNamespace symbol should be marked as 'used' so we don't incorrectly elide its import. And if there
            // is no reactNamespace symbol in scope when targeting React emit, we should issue an error.
            const reactRefErr = compilerOptions.jsx === JsxEmit.React ? Diagnostics.Cannot_find_name_0 : undefined;
            const reactNamespace = compilerOptions.reactNamespace ? compilerOptions.reactNamespace : "React";
            const reactSym = resolveName(node.tagName, reactNamespace, SymbolFlags.Value, reactRefErr, reactNamespace);
            if (reactSym) {
                getSymbolLinks(reactSym).referenced = true;
            }

            const targetAttributesType = getJsxElementAttributesType(node);

            const nameTable = createMap<boolean>();
            // Process this array in right-to-left order so we know which
            // attributes (mostly from spreads) are being overwritten and
            // thus should have their types ignored
            let sawSpreadedAny = false;
            for (let i = node.attributes.length - 1; i >= 0; i--) {
                if (node.attributes[i].kind === SyntaxKind.JsxAttribute) {
                    checkJsxAttribute(<JsxAttribute>(node.attributes[i]), targetAttributesType, nameTable);
                }
                else {
                    Debug.assert(node.attributes[i].kind === SyntaxKind.JsxSpreadAttribute);
                    const spreadType = checkJsxSpreadAttribute(<JsxSpreadAttribute>(node.attributes[i]), targetAttributesType, nameTable);
                    if (isTypeAny(spreadType)) {
                        sawSpreadedAny = true;
                    }
                }
            }

            // Check that all required properties have been provided. If an 'any'
            // was spreaded in, though, assume that it provided all required properties
            if (targetAttributesType && !sawSpreadedAny) {
                const targetProperties = getPropertiesOfType(targetAttributesType);
                for (let i = 0; i < targetProperties.length; i++) {
                    if (!(targetProperties[i].flags & SymbolFlags.Optional) &&
                        !nameTable[targetProperties[i].name]) {

                        error(node, Diagnostics.Property_0_is_missing_in_type_1, targetProperties[i].name, typeToString(targetAttributesType));
                    }
                }
            }
        }

        function checkJsxExpression(node: JsxExpression) {
            if (node.expression) {
                return checkExpression(node.expression);
            }
            else {
                return unknownType;
            }
        }

        // If a symbol is a synthesized symbol with no value declaration, we assume it is a property. Example of this are the synthesized
        // '.prototype' property as well as synthesized tuple index properties.
        function getDeclarationKindFromSymbol(s: Symbol) {
            return s.valueDeclaration ? s.valueDeclaration.kind : SyntaxKind.PropertyDeclaration;
        }

        function getDeclarationFlagsFromSymbol(s: Symbol): NodeFlags {
            return s.valueDeclaration ? getCombinedNodeFlags(s.valueDeclaration) : s.flags & SymbolFlags.Prototype ? NodeFlags.Public | NodeFlags.Static : 0;
        }

        /**
         * Check whether the requested property access is valid.
         * Returns true if node is a valid property access, and false otherwise.
         * @param node The node to be checked.
         * @param left The left hand side of the property access (e.g.: the super in `super.foo`).
         * @param type The type of left.
         * @param prop The symbol for the right hand side of the property access.
         */
        function checkClassPropertyAccess(node: PropertyAccessExpression | QualifiedName | VariableLikeDeclaration, left: Expression | QualifiedName, type: Type, prop: Symbol): boolean {
            const flags = getDeclarationFlagsFromSymbol(prop);
            const declaringClass = <InterfaceType>getDeclaredTypeOfSymbol(getParentOfSymbol(prop));
            const errorNode = node.kind === SyntaxKind.PropertyAccessExpression || node.kind === SyntaxKind.VariableDeclaration ?
                (<PropertyAccessExpression | VariableDeclaration>node).name :
                (<QualifiedName>node).right;
            if (left.kind === SyntaxKind.SuperKeyword) {
                // TS 1.0 spec (April 2014): 4.8.2
                // - In a constructor, instance member function, instance member accessor, or
                //   instance member variable initializer where this references a derived class instance,
                //   a super property access is permitted and must specify a public instance member function of the base class.
                // - In a static member function or static member accessor
                //   where this references the constructor function object of a derived class,
                //   a super property access is permitted and must specify a public static member function of the base class.
                if (languageVersion < ScriptTarget.ES6 && getDeclarationKindFromSymbol(prop) !== SyntaxKind.MethodDeclaration) {
                    // `prop` refers to a *property* declared in the super class
                    // rather than a *method*, so it does not satisfy the above criteria.

                    error(errorNode, Diagnostics.Only_public_and_protected_methods_of_the_base_class_are_accessible_via_the_super_keyword);
                    return false;
                }

                if (flags & NodeFlags.Abstract) {
                    // A method cannot be accessed in a super property access if the method is abstract.
                    // This error could mask a private property access error. But, a member
                    // cannot simultaneously be private and abstract, so this will trigger an
                    // additional error elsewhere.

                    error(errorNode, Diagnostics.Abstract_method_0_in_class_1_cannot_be_accessed_via_super_expression, symbolToString(prop), typeToString(declaringClass));
                    return false;
                }
            }

            // Public properties are otherwise accessible.
            if (!(flags & (NodeFlags.Private | NodeFlags.Protected))) {
                return true;
            }

            // Property is known to be private or protected at this point

            // Private property is accessible if the property is within the declaring class
            if (flags & NodeFlags.Private) {
                const declaringClassDeclaration = <ClassLikeDeclaration>getClassLikeDeclarationOfSymbol(getParentOfSymbol(prop));
                if (!isNodeWithinClass(node, declaringClassDeclaration)) {
                    error(errorNode, Diagnostics.Property_0_is_private_and_only_accessible_within_class_1, symbolToString(prop), typeToString(declaringClass));
                    return false;
                }
                return true;
            }

            // Property is known to be protected at this point

            // All protected properties of a supertype are accessible in a super access
            if (left.kind === SyntaxKind.SuperKeyword) {
                return true;
            }

            // Get the enclosing class that has the declaring class as its base type
            const enclosingClass = forEachEnclosingClass(node, enclosingDeclaration => {
                const enclosingClass = <InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(enclosingDeclaration));
                return hasBaseType(enclosingClass, declaringClass) ? enclosingClass : undefined;
            });

            // A protected property is accessible if the property is within the declaring class or classes derived from it
            if (!enclosingClass) {
                error(errorNode, Diagnostics.Property_0_is_protected_and_only_accessible_within_class_1_and_its_subclasses, symbolToString(prop), typeToString(declaringClass));
                return false;
            }
            // No further restrictions for static properties
            if (flags & NodeFlags.Static) {
                return true;
            }
            // An instance property must be accessed through an instance of the enclosing class
            if (type.flags & TypeFlags.ThisType) {
                // get the original type -- represented as the type constraint of the 'this' type
                type = getConstraintOfTypeParameter(<TypeParameter>type);
            }

            // TODO: why is the first part of this check here?
            if (!(getTargetType(type).flags & (TypeFlags.Class | TypeFlags.Interface) && hasBaseType(<InterfaceType>type, enclosingClass))) {
                error(errorNode, Diagnostics.Property_0_is_protected_and_only_accessible_through_an_instance_of_class_1, symbolToString(prop), typeToString(enclosingClass));
                return false;
            }
            return true;
        }

        function checkNonNullExpression(node: Expression | QualifiedName) {
            const type = checkExpression(node);
            if (strictNullChecks) {
                const kind = getFalsyFlags(type) & TypeFlags.Nullable;
                if (kind) {
                    error(node, kind & TypeFlags.Undefined ? kind & TypeFlags.Null ?
                        Diagnostics.Object_is_possibly_null_or_undefined :
                        Diagnostics.Object_is_possibly_undefined :
                        Diagnostics.Object_is_possibly_null);
                }
                return getNonNullableType(type);
            }
            return type;
        }

        function checkPropertyAccessExpression(node: PropertyAccessExpression) {
            return checkPropertyAccessExpressionOrQualifiedName(node, node.expression, node.name);
        }

        function checkQualifiedName(node: QualifiedName) {
            return checkPropertyAccessExpressionOrQualifiedName(node, node.left, node.right);
        }

        function checkPropertyAccessExpressionOrQualifiedName(node: PropertyAccessExpression | QualifiedName, left: Expression | QualifiedName, right: Identifier) {
            const type = checkNonNullExpression(left);
            if (isTypeAny(type)) {
                return type;
            }

            const apparentType = getApparentType(getWidenedType(type));
            if (apparentType === unknownType || (type.flags & TypeFlags.TypeParameter && isTypeAny(apparentType))) {
                // handle cases when type is Type parameter with invalid or any constraint
                return apparentType;
            }
            const prop = getPropertyOfType(apparentType, right.text);
            if (!prop) {
                if (right.text && !checkAndReportErrorForExtendingInterface(node)) {
                    error(right, Diagnostics.Property_0_does_not_exist_on_type_1, declarationNameToString(right), typeToString(type.flags & TypeFlags.ThisType ? apparentType : type));
                }
                return unknownType;
            }

            if (noUnusedIdentifiers &&
                (prop.flags & SymbolFlags.ClassMember) &&
                prop.valueDeclaration && (prop.valueDeclaration.flags & NodeFlags.Private)) {
                if (prop.flags & SymbolFlags.Instantiated) {
                    getSymbolLinks(prop).target.isReferenced = true;

                }
                else {
                    prop.isReferenced = true;
                }
            }

            getNodeLinks(node).resolvedSymbol = prop;

            if (prop.parent && prop.parent.flags & SymbolFlags.Class) {
                checkClassPropertyAccess(node, left, apparentType, prop);
            }

            let propType = getTypeOfSymbol(prop);
            if (prop.flags & SymbolFlags.EnumMember && isLiteralContextForType(<Expression>node, propType)) {
                propType = getDeclaredTypeOfSymbol(prop);
            }

            // Only compute control flow type if this is a property access expression that isn't an
            // assignment target, and the referenced property was declared as a variable, property,
            // accessor, or optional method.
            if (node.kind !== SyntaxKind.PropertyAccessExpression || isAssignmentTarget(node) ||
                !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) &&
                !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
                return propType;
            }
            return getFlowTypeOfReference(node, propType, /*assumeInitialized*/ true, /*includeOuterFunctions*/ false);
        }

        function isValidPropertyAccess(node: PropertyAccessExpression | QualifiedName, propertyName: string): boolean {
            const left = node.kind === SyntaxKind.PropertyAccessExpression
                ? (<PropertyAccessExpression>node).expression
                : (<QualifiedName>node).left;

            const type = checkExpression(left);
            if (type !== unknownType && !isTypeAny(type)) {
                const prop = getPropertyOfType(getWidenedType(type), propertyName);
                if (prop && prop.parent && prop.parent.flags & SymbolFlags.Class) {
                    return checkClassPropertyAccess(node, left, type, prop);
                }
            }
            return true;
        }

        /**
         * Return the symbol of the for-in variable declared or referenced by the given for-in statement.
         */
        function getForInVariableSymbol(node: ForInStatement): Symbol {
            const initializer = node.initializer;
            if (initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variable = (<VariableDeclarationList>initializer).declarations[0];
                if (variable && !isBindingPattern(variable.name)) {
                    return getSymbolOfNode(variable);
                }
            }
            else if (initializer.kind === SyntaxKind.Identifier) {
                return getResolvedSymbol(<Identifier>initializer);
            }
            return undefined;
        }

        /**
         * Return true if the given type is considered to have numeric property names.
         */
        function hasNumericPropertyNames(type: Type) {
            return getIndexTypeOfType(type, IndexKind.Number) && !getIndexTypeOfType(type, IndexKind.String);
        }

        /**
         * Return true if given node is an expression consisting of an identifier (possibly parenthesized)
         * that references a for-in variable for an object with numeric property names.
         */
        function isForInVariableForNumericPropertyNames(expr: Expression) {
            const e = skipParenthesizedNodes(expr);
            if (e.kind === SyntaxKind.Identifier) {
                const symbol = getResolvedSymbol(<Identifier>e);
                if (symbol.flags & SymbolFlags.Variable) {
                    let child: Node = expr;
                    let node = expr.parent;
                    while (node) {
                        if (node.kind === SyntaxKind.ForInStatement &&
                            child === (<ForInStatement>node).statement &&
                            getForInVariableSymbol(<ForInStatement>node) === symbol &&
                            hasNumericPropertyNames(checkExpression((<ForInStatement>node).expression))) {
                            return true;
                        }
                        child = node;
                        node = node.parent;
                    }
                }
            }
            return false;
        }

        function checkIndexedAccess(node: ElementAccessExpression): Type {
            // Grammar checking
            if (!node.argumentExpression) {
                const sourceFile = getSourceFileOfNode(node);
                if (node.parent.kind === SyntaxKind.NewExpression && (<NewExpression>node.parent).expression === node) {
                    const start = skipTrivia(sourceFile.text, node.expression.end);
                    const end = node.end;
                    grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.new_T_cannot_be_used_to_create_an_array_Use_new_Array_T_instead);
                }
                else {
                    const start = node.end - "]".length;
                    const end = node.end;
                    grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.Expression_expected);
                }
            }

            // Obtain base constraint such that we can bail out if the constraint is an unknown type
            const objectType = getApparentType(checkNonNullExpression(node.expression));
            const indexType = node.argumentExpression ? checkExpression(node.argumentExpression) : unknownType;

            if (objectType === unknownType) {
                return unknownType;
            }

            const isConstEnum = isConstEnumObjectType(objectType);
            if (isConstEnum &&
                (!node.argumentExpression || node.argumentExpression.kind !== SyntaxKind.StringLiteral)) {
                error(node.argumentExpression, Diagnostics.A_const_enum_member_can_only_be_accessed_using_a_string_literal);
                return unknownType;
            }

            // TypeScript 1.0 spec (April 2014): 4.10 Property Access
            // - If IndexExpr is a string literal or a numeric literal and ObjExpr's apparent type has a property with the name
            //    given by that literal(converted to its string representation in the case of a numeric literal), the property access is of the type of that property.
            // - Otherwise, if ObjExpr's apparent type has a numeric index signature and IndexExpr is of type Any, the Number primitive type, or an enum type,
            //    the property access is of the type of that index signature.
            // - Otherwise, if ObjExpr's apparent type has a string index signature and IndexExpr is of type Any, the String or Number primitive type, or an enum type,
            //    the property access is of the type of that index signature.
            // - Otherwise, if IndexExpr is of type Any, the String or Number primitive type, or an enum type, the property access is of type Any.

            // See if we can index as a property.
            if (node.argumentExpression) {
                const name = getPropertyNameForIndexedAccess(node.argumentExpression, indexType);
                if (name !== undefined) {
                    const prop = getPropertyOfType(objectType, name);
                    if (prop) {
                        getNodeLinks(node).resolvedSymbol = prop;
                        return getTypeOfSymbol(prop);
                    }
                    else if (isConstEnum) {
                        error(node.argumentExpression, Diagnostics.Property_0_does_not_exist_on_const_enum_1, name, symbolToString(objectType.symbol));
                        return unknownType;
                    }
                }
            }

            // Check for compatible indexer types.
            if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbol)) {

                // Try to use a number indexer.
                if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.NumberLike) || isForInVariableForNumericPropertyNames(node.argumentExpression)) {
                    const numberIndexInfo = getIndexInfoOfType(objectType, IndexKind.Number);
                    if (numberIndexInfo) {
                        getNodeLinks(node).resolvedIndexInfo = numberIndexInfo;
                        return numberIndexInfo.type;
                    }
                }

                // Try to use string indexing.
                const stringIndexInfo = getIndexInfoOfType(objectType, IndexKind.String);
                if (stringIndexInfo) {
                    getNodeLinks(node).resolvedIndexInfo = stringIndexInfo;
                    return stringIndexInfo.type;
                }

                // Fall back to any.
                if (compilerOptions.noImplicitAny && !compilerOptions.suppressImplicitAnyIndexErrors && !isTypeAny(objectType)) {
                    error(node, getIndexTypeOfType(objectType, IndexKind.Number) ?
                        Diagnostics.Element_implicitly_has_an_any_type_because_index_expression_is_not_of_type_number :
                        Diagnostics.Index_signature_of_object_type_implicitly_has_an_any_type);
                }

                return anyType;
            }

            // REVIEW: Users should know the type that was actually used.
            error(node, Diagnostics.An_index_expression_argument_must_be_of_type_string_number_symbol_or_any);

            return unknownType;
        }

        /**
         * If indexArgumentExpression is a string literal or number literal, returns its text.
         * If indexArgumentExpression is a constant value, returns its string value.
         * If indexArgumentExpression is a well known symbol, returns the property name corresponding
         *    to this symbol, as long as it is a proper symbol reference.
         * Otherwise, returns undefined.
         */
        function getPropertyNameForIndexedAccess(indexArgumentExpression: Expression, indexArgumentType: Type): string {
            if (indexArgumentExpression.kind === SyntaxKind.StringLiteral || indexArgumentExpression.kind === SyntaxKind.NumericLiteral) {
                return (<LiteralExpression>indexArgumentExpression).text;
            }
            if (indexArgumentExpression.kind === SyntaxKind.ElementAccessExpression || indexArgumentExpression.kind === SyntaxKind.PropertyAccessExpression) {
                const value = getConstantValue(<ElementAccessExpression | PropertyAccessExpression>indexArgumentExpression);
                if (value !== undefined) {
                    return value.toString();
                }
            }
            if (checkThatExpressionIsProperSymbolReference(indexArgumentExpression, indexArgumentType, /*reportError*/ false)) {
                const rightHandSideName = (<Identifier>(<PropertyAccessExpression>indexArgumentExpression).name).text;
                return getPropertyNameForKnownSymbolName(rightHandSideName);
            }

            return undefined;
        }

        /**
         * A proper symbol reference requires the following:
         *   1. The property access denotes a property that exists
         *   2. The expression is of the form Symbol.<identifier>
         *   3. The property access is of the primitive type symbol.
         *   4. Symbol in this context resolves to the global Symbol object
         */
        function checkThatExpressionIsProperSymbolReference(expression: Expression, expressionType: Type, reportError: boolean): boolean {
            if (expressionType === unknownType) {
                // There is already an error, so no need to report one.
                return false;
            }

            if (!isWellKnownSymbolSyntactically(expression)) {
                return false;
            }

            // Make sure the property type is the primitive symbol type
            if ((expressionType.flags & TypeFlags.ESSymbol) === 0) {
                if (reportError) {
                    error(expression, Diagnostics.A_computed_property_name_of_the_form_0_must_be_of_type_symbol, getTextOfNode(expression));
                }
                return false;
            }

            // The name is Symbol.<someName>, so make sure Symbol actually resolves to the
            // global Symbol object
            const leftHandSide = <Identifier>(<PropertyAccessExpression>expression).expression;
            const leftHandSideSymbol = getResolvedSymbol(leftHandSide);
            if (!leftHandSideSymbol) {
                return false;
            }

            const globalESSymbol = getGlobalESSymbolConstructorSymbol();
            if (!globalESSymbol) {
                // Already errored when we tried to look up the symbol
                return false;
            }

            if (leftHandSideSymbol !== globalESSymbol) {
                if (reportError) {
                    error(leftHandSide, Diagnostics.Symbol_reference_does_not_refer_to_the_global_Symbol_constructor_object);
                }
                return false;
            }

            return true;
        }

        function resolveUntypedCall(node: CallLikeExpression): Signature {
            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                checkExpression((<TaggedTemplateExpression>node).template);
            }
            else if (node.kind !== SyntaxKind.Decorator) {
                forEach((<CallExpression>node).arguments, argument => {
                    checkExpression(argument);
                });
            }
            return anySignature;
        }

        function resolveErrorCall(node: CallLikeExpression): Signature {
            resolveUntypedCall(node);
            return unknownSignature;
        }

        // Re-order candidate signatures into the result array. Assumes the result array to be empty.
        // The candidate list orders groups in reverse, but within a group signatures are kept in declaration order
        // A nit here is that we reorder only signatures that belong to the same symbol,
        // so order how inherited signatures are processed is still preserved.
        // interface A { (x: string): void }
        // interface B extends A { (x: 'foo'): string }
        // const b: B;
        // b('foo') // <- here overloads should be processed as [(x:'foo'): string, (x: string): void]
        function reorderCandidates(signatures: Signature[], result: Signature[]): void {
            let lastParent: Node;
            let lastSymbol: Symbol;
            let cutoffIndex = 0;
            let index: number;
            let specializedIndex = -1;
            let spliceIndex: number;
            Debug.assert(!result.length);
            for (const signature of signatures) {
                const symbol = signature.declaration && getSymbolOfNode(signature.declaration);
                const parent = signature.declaration && signature.declaration.parent;
                if (!lastSymbol || symbol === lastSymbol) {
                    if (lastParent && parent === lastParent) {
                        index++;
                    }
                    else {
                        lastParent = parent;
                        index = cutoffIndex;
                    }
                }
                else {
                    // current declaration belongs to a different symbol
                    // set cutoffIndex so re-orderings in the future won't change result set from 0 to cutoffIndex
                    index = cutoffIndex = result.length;
                    lastParent = parent;
                }
                lastSymbol = symbol;

                // specialized signatures always need to be placed before non-specialized signatures regardless
                // of the cutoff position; see GH#1133
                if (signature.hasLiteralTypes) {
                    specializedIndex++;
                    spliceIndex = specializedIndex;
                    // The cutoff index always needs to be greater than or equal to the specialized signature index
                    // in order to prevent non-specialized signatures from being added before a specialized
                    // signature.
                    cutoffIndex++;
                }
                else {
                    spliceIndex = index;
                }

                result.splice(spliceIndex, 0, signature);
            }
        }

        function getSpreadArgumentIndex(args: Expression[]): number {
            for (let i = 0; i < args.length; i++) {
                const arg = args[i];
                if (arg && arg.kind === SyntaxKind.SpreadElementExpression) {
                    return i;
                }
            }
            return -1;
        }

        function hasCorrectArity(node: CallLikeExpression, args: Expression[], signature: Signature, signatureHelpTrailingComma = false) {
            let argCount: number;            // Apparent number of arguments we will have in this call
            let typeArguments: NodeArray<TypeNode>;  // Type arguments (undefined if none)
            let callIsIncomplete: boolean;           // In incomplete call we want to be lenient when we have too few arguments
            let isDecorator: boolean;
            let spreadArgIndex = -1;

            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                const tagExpression = <TaggedTemplateExpression>node;

                // Even if the call is incomplete, we'll have a missing expression as our last argument,
                // so we can say the count is just the arg list length
                argCount = args.length;
                typeArguments = undefined;

                if (tagExpression.template.kind === SyntaxKind.TemplateExpression) {
                    // If a tagged template expression lacks a tail literal, the call is incomplete.
                    // Specifically, a template only can end in a TemplateTail or a Missing literal.
                    const templateExpression = <TemplateExpression>tagExpression.template;
                    const lastSpan = lastOrUndefined(templateExpression.templateSpans);
                    Debug.assert(lastSpan !== undefined); // we should always have at least one span.
                    callIsIncomplete = nodeIsMissing(lastSpan.literal) || !!lastSpan.literal.isUnterminated;
                }
                else {
                    // If the template didn't end in a backtick, or its beginning occurred right prior to EOF,
                    // then this might actually turn out to be a TemplateHead in the future;
                    // so we consider the call to be incomplete.
                    const templateLiteral = <LiteralExpression>tagExpression.template;
                    Debug.assert(templateLiteral.kind === SyntaxKind.NoSubstitutionTemplateLiteral);
                    callIsIncomplete = !!templateLiteral.isUnterminated;
                }
            }
            else if (node.kind === SyntaxKind.Decorator) {
                isDecorator = true;
                typeArguments = undefined;
                argCount = getEffectiveArgumentCount(node, /*args*/ undefined, signature);
            }
            else {
                const callExpression = <CallExpression>node;
                if (!callExpression.arguments) {
                    // This only happens when we have something of the form: 'new C'
                    Debug.assert(callExpression.kind === SyntaxKind.NewExpression);

                    return signature.minArgumentCount === 0;
                }

                argCount = signatureHelpTrailingComma ? args.length + 1 : args.length;

                // If we are missing the close paren, the call is incomplete.
                callIsIncomplete = (<CallExpression>callExpression).arguments.end === callExpression.end;

                typeArguments = callExpression.typeArguments;
                spreadArgIndex = getSpreadArgumentIndex(args);
            }

            // If the user supplied type arguments, but the number of type arguments does not match
            // the declared number of type parameters, the call has an incorrect arity.
            const hasRightNumberOfTypeArgs = !typeArguments ||
                (signature.typeParameters && typeArguments.length === signature.typeParameters.length);
            if (!hasRightNumberOfTypeArgs) {
                return false;
            }

            // If spread arguments are present, check that they correspond to a rest parameter. If so, no
            // further checking is necessary.
            if (spreadArgIndex >= 0) {
                return isRestParameterIndex(signature, spreadArgIndex);
            }

            // Too many arguments implies incorrect arity.
            if (!signature.hasRestParameter && argCount > signature.parameters.length) {
                return false;
            }

            // If the call is incomplete, we should skip the lower bound check.
            const hasEnoughArguments = argCount >= signature.minArgumentCount;
            return callIsIncomplete || hasEnoughArguments;
        }

        // If type has a single call signature and no other members, return that signature. Otherwise, return undefined.
        function getSingleCallSignature(type: Type): Signature {
            if (type.flags & TypeFlags.ObjectType) {
                const resolved = resolveStructuredTypeMembers(<ObjectType>type);
                if (resolved.callSignatures.length === 1 && resolved.constructSignatures.length === 0 &&
                    resolved.properties.length === 0 && !resolved.stringIndexInfo && !resolved.numberIndexInfo) {
                    return resolved.callSignatures[0];
                }
            }
            return undefined;
        }

        // Instantiate a generic signature in the context of a non-generic signature (section 3.8.5 in TypeScript spec)
        function instantiateSignatureInContextOf(signature: Signature, contextualSignature: Signature, contextualMapper: TypeMapper): Signature {
            const context = createInferenceContext(signature.typeParameters, /*inferUnionTypes*/ true);
            forEachMatchingParameterType(contextualSignature, signature, (source, target) => {
                // Type parameters from outer context referenced by source type are fixed by instantiation of the source type
                inferTypes(context, instantiateType(source, contextualMapper), target);
            });
            return getSignatureInstantiation(signature, getInferredTypes(context));
        }

        function inferTypeArguments(node: CallLikeExpression, signature: Signature, args: Expression[], excludeArgument: boolean[], context: InferenceContext): void {
            const typeParameters = signature.typeParameters;
            const inferenceMapper = getInferenceMapper(context);

            // Clear out all the inference results from the last time inferTypeArguments was called on this context
            for (let i = 0; i < typeParameters.length; i++) {
                // As an optimization, we don't have to clear (and later recompute) inferred types
                // for type parameters that have already been fixed on the previous call to inferTypeArguments.
                // It would be just as correct to reset all of them. But then we'd be repeating the same work
                // for the type parameters that were fixed, namely the work done by getInferredType.
                if (!context.inferences[i].isFixed) {
                    context.inferredTypes[i] = undefined;
                }
            }

            // On this call to inferTypeArguments, we may get more inferences for certain type parameters that were not
            // fixed last time. This means that a type parameter that failed inference last time may succeed this time,
            // or vice versa. Therefore, the failedTypeParameterIndex is useless if it points to an unfixed type parameter,
            // because it may change. So here we reset it. However, getInferredType will not revisit any type parameters
            // that were previously fixed. So if a fixed type parameter failed previously, it will fail again because
            // it will contain the exact same set of inferences. So if we reset the index from a fixed type parameter,
            // we will lose information that we won't recover this time around.
            if (context.failedTypeParameterIndex !== undefined && !context.inferences[context.failedTypeParameterIndex].isFixed) {
                context.failedTypeParameterIndex = undefined;
            }

            const thisType = getThisTypeOfSignature(signature);
            if (thisType) {
                const thisArgumentNode = getThisArgumentOfCall(node);
                const thisArgumentType = thisArgumentNode ? checkExpression(thisArgumentNode) : voidType;
                inferTypes(context, thisArgumentType, thisType);
            }

            // We perform two passes over the arguments. In the first pass we infer from all arguments, but use
            // wildcards for all context sensitive function expressions.
            const argCount = getEffectiveArgumentCount(node, args, signature);
            for (let i = 0; i < argCount; i++) {
                const arg = getEffectiveArgument(node, args, i);
                // If the effective argument is 'undefined', then it is an argument that is present but is synthetic.
                if (arg === undefined || arg.kind !== SyntaxKind.OmittedExpression) {
                    const paramType = getTypeAtPosition(signature, i);
                    let argType = getEffectiveArgumentType(node, i, arg);

                    // If the effective argument type is 'undefined', there is no synthetic type
                    // for the argument. In that case, we should check the argument.
                    if (argType === undefined) {
                        // For context sensitive arguments we pass the identityMapper, which is a signal to treat all
                        // context sensitive function expressions as wildcards
                        const mapper = excludeArgument && excludeArgument[i] !== undefined ? identityMapper : inferenceMapper;
                        argType = checkExpressionWithContextualType(arg, paramType, mapper);
                    }

                    inferTypes(context, argType, paramType);
                }
            }

            // In the second pass we visit only context sensitive arguments, and only those that aren't excluded, this
            // time treating function expressions normally (which may cause previously inferred type arguments to be fixed
            // as we construct types for contextually typed parameters)
            // Decorators will not have `excludeArgument`, as their arguments cannot be contextually typed.
            // Tagged template expressions will always have `undefined` for `excludeArgument[0]`.
            if (excludeArgument) {
                for (let i = 0; i < argCount; i++) {
                    // No need to check for omitted args and template expressions, their exclusion value is always undefined
                    if (excludeArgument[i] === false) {
                        const arg = args[i];
                        const paramType = getTypeAtPosition(signature, i);
                        inferTypes(context, checkExpressionWithContextualType(arg, paramType, inferenceMapper), paramType);
                    }
                }
            }

            getInferredTypes(context);
        }

        function checkTypeArguments(signature: Signature, typeArgumentNodes: TypeNode[], typeArgumentTypes: Type[], reportErrors: boolean, headMessage?: DiagnosticMessage): boolean {
            const typeParameters = signature.typeParameters;
            let typeArgumentsAreAssignable = true;
            let mapper: TypeMapper;
            for (let i = 0; i < typeParameters.length; i++) {
                if (typeArgumentsAreAssignable /* so far */) {
                    const constraint = getConstraintOfTypeParameter(typeParameters[i]);
                    if (constraint) {
                        let errorInfo: DiagnosticMessageChain;
                        let typeArgumentHeadMessage = Diagnostics.Type_0_does_not_satisfy_the_constraint_1;
                        if (reportErrors && headMessage) {
                            errorInfo = chainDiagnosticMessages(errorInfo, typeArgumentHeadMessage);
                            typeArgumentHeadMessage = headMessage;
                        }
                        if (!mapper) {
                            mapper = createTypeMapper(typeParameters, typeArgumentTypes);
                        }
                        const typeArgument = typeArgumentTypes[i];
                        typeArgumentsAreAssignable = checkTypeAssignableTo(
                            typeArgument,
                            getTypeWithThisArgument(instantiateType(constraint, mapper), typeArgument),
                            reportErrors ? typeArgumentNodes[i] : undefined,
                            typeArgumentHeadMessage,
                            errorInfo);
                    }
                }
            }
            return typeArgumentsAreAssignable;
        }

        function checkApplicableSignature(node: CallLikeExpression, args: Expression[], signature: Signature, relation: Map<RelationComparisonResult>, excludeArgument: boolean[], reportErrors: boolean) {
            const thisType = getThisTypeOfSignature(signature);
            if (thisType && thisType !== voidType && node.kind !== SyntaxKind.NewExpression) {
                // If the called expression is not of the form `x.f` or `x["f"]`, then sourceType = voidType
                // If the signature's 'this' type is voidType, then the check is skipped -- anything is compatible.
                // If the expression is a new expression, then the check is skipped.
                const thisArgumentNode = getThisArgumentOfCall(node);
                const thisArgumentType = thisArgumentNode ? checkExpression(thisArgumentNode) : voidType;
                const errorNode = reportErrors ? (thisArgumentNode || node) : undefined;
                const headMessage = Diagnostics.The_this_context_of_type_0_is_not_assignable_to_method_s_this_of_type_1;
                if (!checkTypeRelatedTo(thisArgumentType, getThisTypeOfSignature(signature), relation, errorNode, headMessage)) {
                    return false;
                }
            }
            const headMessage = Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1;
            const argCount = getEffectiveArgumentCount(node, args, signature);
            for (let i = 0; i < argCount; i++) {
                const arg = getEffectiveArgument(node, args, i);
                // If the effective argument is 'undefined', then it is an argument that is present but is synthetic.
                if (arg === undefined || arg.kind !== SyntaxKind.OmittedExpression) {
                    // Check spread elements against rest type (from arity check we know spread argument corresponds to a rest parameter)
                    const paramType = getTypeAtPosition(signature, i);
                    let argType = getEffectiveArgumentType(node, i, arg);

                    // If the effective argument type is 'undefined', there is no synthetic type
                    // for the argument. In that case, we should check the argument.
                    if (argType === undefined) {
                        argType = checkExpressionWithContextualType(arg, paramType, excludeArgument && excludeArgument[i] ? identityMapper : undefined);
                    }

                    // Use argument expression as error location when reporting errors
                    const errorNode = reportErrors ? getEffectiveArgumentErrorNode(node, i, arg) : undefined;
                    if (!checkTypeRelatedTo(argType, paramType, relation, errorNode, headMessage)) {
                        return false;
                    }
                }
            }

            return true;
        }

        /**
         * Returns the this argument in calls like x.f(...) and x[f](...). Undefined otherwise.
         */
        function getThisArgumentOfCall(node: CallLikeExpression): LeftHandSideExpression {
            if (node.kind === SyntaxKind.CallExpression) {
                const callee = (<CallExpression>node).expression;
                if (callee.kind === SyntaxKind.PropertyAccessExpression) {
                    return (callee as PropertyAccessExpression).expression;
                }
                else if (callee.kind === SyntaxKind.ElementAccessExpression) {
                    return (callee as ElementAccessExpression).expression;
                }
            }
        }

        /**
         * Returns the effective arguments for an expression that works like a function invocation.
         *
         * If 'node' is a CallExpression or a NewExpression, then its argument list is returned.
         * If 'node' is a TaggedTemplateExpression, a new argument list is constructed from the substitution
         *    expressions, where the first element of the list is `undefined`.
         * If 'node' is a Decorator, the argument list will be `undefined`, and its arguments and types
         *    will be supplied from calls to `getEffectiveArgumentCount` and `getEffectiveArgumentType`.
         */
        function getEffectiveCallArguments(node: CallLikeExpression): Expression[] {
            let args: Expression[];
            if (node.kind === SyntaxKind.TaggedTemplateExpression) {
                const template = (<TaggedTemplateExpression>node).template;
                args = [undefined];
                if (template.kind === SyntaxKind.TemplateExpression) {
                    forEach((<TemplateExpression>template).templateSpans, span => {
                        args.push(span.expression);
                    });
                }
            }
            else if (node.kind === SyntaxKind.Decorator) {
                // For a decorator, we return undefined as we will determine
                // the number and types of arguments for a decorator using
                // `getEffectiveArgumentCount` and `getEffectiveArgumentType` below.
                return undefined;
            }
            else {
                args = (<CallExpression>node).arguments || emptyArray;
            }

            return args;
        }


        /**
          * Returns the effective argument count for a node that works like a function invocation.
          * If 'node' is a Decorator, the number of arguments is derived from the decoration
          *    target and the signature:
          *    If 'node.target' is a class declaration or class expression, the effective argument
          *       count is 1.
          *    If 'node.target' is a parameter declaration, the effective argument count is 3.
          *    If 'node.target' is a property declaration, the effective argument count is 2.
          *    If 'node.target' is a method or accessor declaration, the effective argument count
          *       is 3, although it can be 2 if the signature only accepts two arguments, allowing
          *       us to match a property decorator.
          * Otherwise, the argument count is the length of the 'args' array.
          */
        function getEffectiveArgumentCount(node: CallLikeExpression, args: Expression[], signature: Signature) {
            if (node.kind === SyntaxKind.Decorator) {
                switch (node.parent.kind) {
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.ClassExpression:
                        // A class decorator will have one argument (see `ClassDecorator` in core.d.ts)
                        return 1;

                    case SyntaxKind.PropertyDeclaration:
                        // A property declaration decorator will have two arguments (see
                        // `PropertyDecorator` in core.d.ts)
                        return 2;

                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        // A method or accessor declaration decorator will have two or three arguments (see
                        // `PropertyDecorator` and `MethodDecorator` in core.d.ts)

                        // If we are emitting decorators for ES3, we will only pass two arguments.
                        if (languageVersion === ScriptTarget.ES3) {
                            return 2;
                        }

                        // If the method decorator signature only accepts a target and a key, we will only
                        // type check those arguments.
                        return signature.parameters.length >= 3 ? 3 : 2;

                    case SyntaxKind.Parameter:
                        // A parameter declaration decorator will have three arguments (see
                        // `ParameterDecorator` in core.d.ts)

                        return 3;
                }
            }
            else {
                return args.length;
            }
        }

        /**
          * Returns the effective type of the first argument to a decorator.
          * If 'node' is a class declaration or class expression, the effective argument type
          *    is the type of the static side of the class.
          * If 'node' is a parameter declaration, the effective argument type is either the type
          *    of the static or instance side of the class for the parameter's parent method,
          *    depending on whether the method is declared static.
          *    For a constructor, the type is always the type of the static side of the class.
          * If 'node' is a property, method, or accessor declaration, the effective argument
          *    type is the type of the static or instance side of the parent class for class
          *    element, depending on whether the element is declared static.
          */
        function getEffectiveDecoratorFirstArgumentType(node: Node): Type {
            // The first argument to a decorator is its `target`.
            if (node.kind === SyntaxKind.ClassDeclaration) {
                // For a class decorator, the `target` is the type of the class (e.g. the
                // "static" or "constructor" side of the class)
                const classSymbol = getSymbolOfNode(node);
                return getTypeOfSymbol(classSymbol);
            }

            if (node.kind === SyntaxKind.Parameter) {
                // For a parameter decorator, the `target` is the parent type of the
                // parameter's containing method.
                node = node.parent;
                if (node.kind === SyntaxKind.Constructor) {
                    const classSymbol = getSymbolOfNode(node);
                    return getTypeOfSymbol(classSymbol);
                }
            }

            if (node.kind === SyntaxKind.PropertyDeclaration ||
                node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.GetAccessor ||
                node.kind === SyntaxKind.SetAccessor) {
                // For a property or method decorator, the `target` is the
                // "static"-side type of the parent of the member if the member is
                // declared "static"; otherwise, it is the "instance"-side type of the
                // parent of the member.
                return getParentTypeOfClassElement(<ClassElement>node);
            }

            Debug.fail("Unsupported decorator target.");
            return unknownType;
        }

        /**
          * Returns the effective type for the second argument to a decorator.
          * If 'node' is a parameter, its effective argument type is one of the following:
          *    If 'node.parent' is a constructor, the effective argument type is 'any', as we
          *       will emit `undefined`.
          *    If 'node.parent' is a member with an identifier, numeric, or string literal name,
          *       the effective argument type will be a string literal type for the member name.
          *    If 'node.parent' is a computed property name, the effective argument type will
          *       either be a symbol type or the string type.
          * If 'node' is a member with an identifier, numeric, or string literal name, the
          *    effective argument type will be a string literal type for the member name.
          * If 'node' is a computed property name, the effective argument type will either
          *    be a symbol type or the string type.
          * A class decorator does not have a second argument type.
          */
        function getEffectiveDecoratorSecondArgumentType(node: Node) {
            // The second argument to a decorator is its `propertyKey`
            if (node.kind === SyntaxKind.ClassDeclaration) {
                Debug.fail("Class decorators should not have a second synthetic argument.");
                return unknownType;
            }

            if (node.kind === SyntaxKind.Parameter) {
                node = node.parent;
                if (node.kind === SyntaxKind.Constructor) {
                    // For a constructor parameter decorator, the `propertyKey` will be `undefined`.
                    return anyType;
                }

                // For a non-constructor parameter decorator, the `propertyKey` will be either
                // a string or a symbol, based on the name of the parameter's containing method.
            }

            if (node.kind === SyntaxKind.PropertyDeclaration ||
                node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.GetAccessor ||
                node.kind === SyntaxKind.SetAccessor) {
                // The `propertyKey` for a property or method decorator will be a
                // string literal type if the member name is an identifier, number, or string;
                // otherwise, if the member name is a computed property name it will
                // be either string or symbol.
                const element = <ClassElement>node;
                switch (element.name.kind) {
                    case SyntaxKind.Identifier:
                    case SyntaxKind.NumericLiteral:
                    case SyntaxKind.StringLiteral:
                        return getLiteralTypeForText(TypeFlags.StringLiteral, (<Identifier | LiteralExpression>element.name).text);

                    case SyntaxKind.ComputedPropertyName:
                        const nameType = checkComputedPropertyName(<ComputedPropertyName>element.name);
                        if (isTypeOfKind(nameType, TypeFlags.ESSymbol)) {
                            return nameType;
                        }
                        else {
                            return stringType;
                        }

                    default:
                        Debug.fail("Unsupported property name.");
                        return unknownType;
                }
            }

            Debug.fail("Unsupported decorator target.");
            return unknownType;
        }

        /**
          * Returns the effective argument type for the third argument to a decorator.
          * If 'node' is a parameter, the effective argument type is the number type.
          * If 'node' is a method or accessor, the effective argument type is a
          *    `TypedPropertyDescriptor<T>` instantiated with the type of the member.
          * Class and property decorators do not have a third effective argument.
          */
        function getEffectiveDecoratorThirdArgumentType(node: Node) {
            // The third argument to a decorator is either its `descriptor` for a method decorator
            // or its `parameterIndex` for a parameter decorator
            if (node.kind === SyntaxKind.ClassDeclaration) {
                Debug.fail("Class decorators should not have a third synthetic argument.");
                return unknownType;
            }

            if (node.kind === SyntaxKind.Parameter) {
                // The `parameterIndex` for a parameter decorator is always a number
                return numberType;
            }

            if (node.kind === SyntaxKind.PropertyDeclaration) {
                Debug.fail("Property decorators should not have a third synthetic argument.");
                return unknownType;
            }

            if (node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.GetAccessor ||
                node.kind === SyntaxKind.SetAccessor) {
                // The `descriptor` for a method decorator will be a `TypedPropertyDescriptor<T>`
                // for the type of the member.
                const propertyType = getTypeOfNode(node);
                return createTypedPropertyDescriptorType(propertyType);
            }

            Debug.fail("Unsupported decorator target.");
            return unknownType;
        }

        /**
          * Returns the effective argument type for the provided argument to a decorator.
          */
        function getEffectiveDecoratorArgumentType(node: Decorator, argIndex: number): Type {
            if (argIndex === 0) {
                return getEffectiveDecoratorFirstArgumentType(node.parent);
            }
            else if (argIndex === 1) {
                return getEffectiveDecoratorSecondArgumentType(node.parent);
            }
            else if (argIndex === 2) {
                return getEffectiveDecoratorThirdArgumentType(node.parent);
            }

            Debug.fail("Decorators should not have a fourth synthetic argument.");
            return unknownType;
        }

        /**
          * Gets the effective argument type for an argument in a call expression.
          */
        function getEffectiveArgumentType(node: CallLikeExpression, argIndex: number, arg: Expression): Type {
            // Decorators provide special arguments, a tagged template expression provides
            // a special first argument, and string literals get string literal types
            // unless we're reporting errors
            if (node.kind === SyntaxKind.Decorator) {
                return getEffectiveDecoratorArgumentType(<Decorator>node, argIndex);
            }
            else if (argIndex === 0 && node.kind === SyntaxKind.TaggedTemplateExpression) {
                return getGlobalTemplateStringsArrayType();
            }

            // This is not a synthetic argument, so we return 'undefined'
            // to signal that the caller needs to check the argument.
            return undefined;
        }

        /**
          * Gets the effective argument expression for an argument in a call expression.
          */
        function getEffectiveArgument(node: CallLikeExpression, args: Expression[], argIndex: number) {
            // For a decorator or the first argument of a tagged template expression we return undefined.
            if (node.kind === SyntaxKind.Decorator ||
                (argIndex === 0 && node.kind === SyntaxKind.TaggedTemplateExpression)) {
                return undefined;
            }

            return args[argIndex];
        }

        /**
          * Gets the error node to use when reporting errors for an effective argument.
          */
        function getEffectiveArgumentErrorNode(node: CallLikeExpression, argIndex: number, arg: Expression) {
            if (node.kind === SyntaxKind.Decorator) {
                // For a decorator, we use the expression of the decorator for error reporting.
                return (<Decorator>node).expression;
            }
            else if (argIndex === 0 && node.kind === SyntaxKind.TaggedTemplateExpression) {
                // For a the first argument of a tagged template expression, we use the template of the tag for error reporting.
                return (<TaggedTemplateExpression>node).template;
            }
            else {
                return arg;
            }
        }

        function resolveCall(node: CallLikeExpression, signatures: Signature[], candidatesOutArray: Signature[], headMessage?: DiagnosticMessage): Signature {
            const isTaggedTemplate = node.kind === SyntaxKind.TaggedTemplateExpression;
            const isDecorator = node.kind === SyntaxKind.Decorator;

            let typeArguments: TypeNode[];

            if (!isTaggedTemplate && !isDecorator) {
                typeArguments = (<CallExpression>node).typeArguments;

                // We already perform checking on the type arguments on the class declaration itself.
                if ((<CallExpression>node).expression.kind !== SyntaxKind.SuperKeyword) {
                    forEach(typeArguments, checkSourceElement);
                }
            }

            const candidates = candidatesOutArray || [];
            // reorderCandidates fills up the candidates array directly
            reorderCandidates(signatures, candidates);
            if (!candidates.length) {
                reportError(Diagnostics.Supplied_parameters_do_not_match_any_signature_of_call_target);
                return resolveErrorCall(node);
            }

            const args = getEffectiveCallArguments(node);

            // The following applies to any value of 'excludeArgument[i]':
            //    - true:      the argument at 'i' is susceptible to a one-time permanent contextual typing.
            //    - undefined: the argument at 'i' is *not* susceptible to permanent contextual typing.
            //    - false:     the argument at 'i' *was* and *has been* permanently contextually typed.
            //
            // The idea is that we will perform type argument inference & assignability checking once
            // without using the susceptible parameters that are functions, and once more for each of those
            // parameters, contextually typing each as we go along.
            //
            // For a tagged template, then the first argument be 'undefined' if necessary
            // because it represents a TemplateStringsArray.
            //
            // For a decorator, no arguments are susceptible to contextual typing due to the fact
            // decorators are applied to a declaration by the emitter, and not to an expression.
            let excludeArgument: boolean[];
            if (!isDecorator) {
                // We do not need to call `getEffectiveArgumentCount` here as it only
                // applies when calculating the number of arguments for a decorator.
                for (let i = isTaggedTemplate ? 1 : 0; i < args.length; i++) {
                    if (isContextSensitive(args[i])) {
                        if (!excludeArgument) {
                            excludeArgument = new Array(args.length);
                        }
                        excludeArgument[i] = true;
                    }
                }
            }

            // The following variables are captured and modified by calls to chooseOverload.
            // If overload resolution or type argument inference fails, we want to report the
            // best error possible. The best error is one which says that an argument was not
            // assignable to a parameter. This implies that everything else about the overload
            // was fine. So if there is any overload that is only incorrect because of an
            // argument, we will report an error on that one.
            //
            //     function foo(s: string) {}
            //     function foo(n: number) {} // Report argument error on this overload
            //     function foo() {}
            //     foo(true);
            //
            // If none of the overloads even made it that far, there are two possibilities.
            // There was a problem with type arguments for some overload, in which case
            // report an error on that. Or none of the overloads even had correct arity,
            // in which case give an arity error.
            //
            //     function foo<T>(x: T, y: T) {} // Report type argument inference error
            //     function foo() {}
            //     foo(0, true);
            //
            let candidateForArgumentError: Signature;
            let candidateForTypeArgumentError: Signature;
            let resultOfFailedInference: InferenceContext;
            let result: Signature;

            // If we are in signature help, a trailing comma indicates that we intend to provide another argument,
            // so we will only accept overloads with arity at least 1 higher than the current number of provided arguments.
            const signatureHelpTrailingComma =
                candidatesOutArray && node.kind === SyntaxKind.CallExpression && (<CallExpression>node).arguments.hasTrailingComma;

            // Section 4.12.1:
            // if the candidate list contains one or more signatures for which the type of each argument
            // expression is a subtype of each corresponding parameter type, the return type of the first
            // of those signatures becomes the return type of the function call.
            // Otherwise, the return type of the first signature in the candidate list becomes the return
            // type of the function call.
            //
            // Whether the call is an error is determined by assignability of the arguments. The subtype pass
            // is just important for choosing the best signature. So in the case where there is only one
            // signature, the subtype pass is useless. So skipping it is an optimization.
            if (candidates.length > 1) {
                result = chooseOverload(candidates, subtypeRelation, signatureHelpTrailingComma);
            }
            if (!result) {
                // Reinitialize these pointers for round two
                candidateForArgumentError = undefined;
                candidateForTypeArgumentError = undefined;
                resultOfFailedInference = undefined;
                result = chooseOverload(candidates, assignableRelation, signatureHelpTrailingComma);
            }
            if (result) {
                return result;
            }

            // No signatures were applicable. Now report errors based on the last applicable signature with
            // no arguments excluded from assignability checks.
            // If candidate is undefined, it means that no candidates had a suitable arity. In that case,
            // skip the checkApplicableSignature check.
            if (candidateForArgumentError) {
                // excludeArgument is undefined, in this case also equivalent to [undefined, undefined, ...]
                // The importance of excludeArgument is to prevent us from typing function expression parameters
                // in arguments too early. If possible, we'd like to only type them once we know the correct
                // overload. However, this matters for the case where the call is correct. When the call is
                // an error, we don't need to exclude any arguments, although it would cause no harm to do so.
                checkApplicableSignature(node, args, candidateForArgumentError, assignableRelation, /*excludeArgument*/ undefined, /*reportErrors*/ true);
            }
            else if (candidateForTypeArgumentError) {
                if (!isTaggedTemplate && !isDecorator && typeArguments) {
                    const typeArguments = (<CallExpression>node).typeArguments;
                    checkTypeArguments(candidateForTypeArgumentError, typeArguments, map(typeArguments, getTypeFromTypeNode), /*reportErrors*/ true, headMessage);
                }
                else {
                    Debug.assert(resultOfFailedInference.failedTypeParameterIndex >= 0);
                    const failedTypeParameter = candidateForTypeArgumentError.typeParameters[resultOfFailedInference.failedTypeParameterIndex];
                    const inferenceCandidates = getInferenceCandidates(resultOfFailedInference, resultOfFailedInference.failedTypeParameterIndex);

                    let diagnosticChainHead = chainDiagnosticMessages(/*details*/ undefined, // details will be provided by call to reportNoCommonSupertypeError
                        Diagnostics.The_type_argument_for_type_parameter_0_cannot_be_inferred_from_the_usage_Consider_specifying_the_type_arguments_explicitly,
                        typeToString(failedTypeParameter));

                    if (headMessage) {
                        diagnosticChainHead = chainDiagnosticMessages(diagnosticChainHead, headMessage);
                    }

                    reportNoCommonSupertypeError(inferenceCandidates, (<CallExpression>node).expression || (<TaggedTemplateExpression>node).tag, diagnosticChainHead);
                }
            }
            else {
                reportError(Diagnostics.Supplied_parameters_do_not_match_any_signature_of_call_target);
            }

            // No signature was applicable. We have already reported the errors for the invalid signature.
            // If this is a type resolution session, e.g. Language Service, try to get better information that anySignature.
            // Pick the first candidate that matches the arity. This way we can get a contextual type for cases like:
            //  declare function f(a: { xa: number; xb: number; });
            //  f({ |
            if (!produceDiagnostics) {
                for (let candidate of candidates) {
                    if (hasCorrectArity(node, args, candidate)) {
                        if (candidate.typeParameters && typeArguments) {
                            candidate = getSignatureInstantiation(candidate, map(typeArguments, getTypeFromTypeNode));
                        }
                        return candidate;
                    }
                }
            }

            return resolveErrorCall(node);

            function reportError(message: DiagnosticMessage, arg0?: string, arg1?: string, arg2?: string): void {
                let errorInfo: DiagnosticMessageChain;
                errorInfo = chainDiagnosticMessages(errorInfo, message, arg0, arg1, arg2);
                if (headMessage) {
                    errorInfo = chainDiagnosticMessages(errorInfo, headMessage);
                }

                diagnostics.add(createDiagnosticForNodeFromMessageChain(node, errorInfo));
            }

            function chooseOverload(candidates: Signature[], relation: Map<RelationComparisonResult>, signatureHelpTrailingComma = false) {
                for (const originalCandidate of candidates) {
                    if (!hasCorrectArity(node, args, originalCandidate, signatureHelpTrailingComma)) {
                        continue;
                    }

                    let candidate: Signature;
                    let typeArgumentsAreValid: boolean;
                    const inferenceContext = originalCandidate.typeParameters
                        ? createInferenceContext(originalCandidate.typeParameters, /*inferUnionTypes*/ false)
                        : undefined;

                    while (true) {
                        candidate = originalCandidate;
                        if (candidate.typeParameters) {
                            let typeArgumentTypes: Type[];
                            if (typeArguments) {
                                typeArgumentTypes = map(typeArguments, getTypeFromTypeNode);
                                typeArgumentsAreValid = checkTypeArguments(candidate, typeArguments, typeArgumentTypes, /*reportErrors*/ false);
                            }
                            else {
                                inferTypeArguments(node, candidate, args, excludeArgument, inferenceContext);
                                typeArgumentsAreValid = inferenceContext.failedTypeParameterIndex === undefined;
                                typeArgumentTypes = inferenceContext.inferredTypes;
                            }
                            if (!typeArgumentsAreValid) {
                                break;
                            }
                            candidate = getSignatureInstantiation(candidate, typeArgumentTypes);
                        }
                        if (!checkApplicableSignature(node, args, candidate, relation, excludeArgument, /*reportErrors*/ false)) {
                            break;
                        }
                        const index = excludeArgument ? indexOf(excludeArgument, true) : -1;
                        if (index < 0) {
                            return candidate;
                        }
                        excludeArgument[index] = false;
                    }

                    // A post-mortem of this iteration of the loop. The signature was not applicable,
                    // so we want to track it as a candidate for reporting an error. If the candidate
                    // had no type parameters, or had no issues related to type arguments, we can
                    // report an error based on the arguments. If there was an issue with type
                    // arguments, then we can only report an error based on the type arguments.
                    if (originalCandidate.typeParameters) {
                        const instantiatedCandidate = candidate;
                        if (typeArgumentsAreValid) {
                            candidateForArgumentError = instantiatedCandidate;
                        }
                        else {
                            candidateForTypeArgumentError = originalCandidate;
                            if (!typeArguments) {
                                resultOfFailedInference = inferenceContext;
                            }
                        }
                    }
                    else {
                        Debug.assert(originalCandidate === candidate);
                        candidateForArgumentError = originalCandidate;
                    }
                }

                return undefined;
            }

        }

        function resolveCallExpression(node: CallExpression, candidatesOutArray: Signature[]): Signature {
            if (node.expression.kind === SyntaxKind.SuperKeyword) {
                const superType = checkSuperExpression(node.expression);
                if (superType !== unknownType) {
                    // In super call, the candidate signatures are the matching arity signatures of the base constructor function instantiated
                    // with the type arguments specified in the extends clause.
                    const baseTypeNode = getClassExtendsHeritageClauseElement(getContainingClass(node));
                    if (baseTypeNode) {
                        const baseConstructors = getInstantiatedConstructorsForTypeArguments(superType, baseTypeNode.typeArguments);
                        return resolveCall(node, baseConstructors, candidatesOutArray);
                    }
                }
                return resolveUntypedCall(node);
            }

            const funcType = checkNonNullExpression(node.expression);
            const apparentType = getApparentType(funcType);

            if (apparentType === unknownType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            // Technically, this signatures list may be incomplete. We are taking the apparent type,
            // but we are not including call signatures that may have been added to the Object or
            // Function interface, since they have none by default. This is a bit of a leap of faith
            // that the user will not add any.
            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);

            const constructSignatures = getSignaturesOfType(apparentType, SignatureKind.Construct);
            // TS 1.0 spec: 4.12
            // If FuncExpr is of type Any, or of an object type that has no call or construct signatures
            // but is a subtype of the Function interface, the call is an untyped function call. In an
            // untyped function call no TypeArgs are permitted, Args can be any argument list, no contextual
            // types are provided for the argument expressions, and the result is always of type Any.
            // We exclude union types because we may have a union of function types that happen to have
            // no common signatures.
            if (isTypeAny(funcType) ||
                (isTypeAny(apparentType) && funcType.flags & TypeFlags.TypeParameter) ||
                (!callSignatures.length && !constructSignatures.length && !(funcType.flags & TypeFlags.Union) && isTypeAssignableTo(funcType, globalFunctionType))) {
                // The unknownType indicates that an error already occurred (and was reported).  No
                // need to report another error in this case.
                if (funcType !== unknownType && node.typeArguments) {
                    error(node, Diagnostics.Untyped_function_calls_may_not_accept_type_arguments);
                }
                return resolveUntypedCall(node);
            }
            // If FuncExpr's apparent type(section 3.8.1) is a function type, the call is a typed function call.
            // TypeScript employs overload resolution in typed function calls in order to support functions
            // with multiple call signatures.
            if (!callSignatures.length) {
                if (constructSignatures.length) {
                    error(node, Diagnostics.Value_of_type_0_is_not_callable_Did_you_mean_to_include_new, typeToString(funcType));
                }
                else {
                    error(node, Diagnostics.Cannot_invoke_an_expression_whose_type_lacks_a_call_signature);
                }
                return resolveErrorCall(node);
            }
            return resolveCall(node, callSignatures, candidatesOutArray);
        }

        function resolveNewExpression(node: NewExpression, candidatesOutArray: Signature[]): Signature {
            if (node.arguments && languageVersion < ScriptTarget.ES5) {
                const spreadIndex = getSpreadArgumentIndex(node.arguments);
                if (spreadIndex >= 0) {
                    error(node.arguments[spreadIndex], Diagnostics.Spread_operator_in_new_expressions_is_only_available_when_targeting_ECMAScript_5_and_higher);
                }
            }

            let expressionType = checkNonNullExpression(node.expression);

            // If expressionType's apparent type(section 3.8.1) is an object type with one or
            // more construct signatures, the expression is processed in the same manner as a
            // function call, but using the construct signatures as the initial set of candidate
            // signatures for overload resolution. The result type of the function call becomes
            // the result type of the operation.
            expressionType = getApparentType(expressionType);
            if (expressionType === unknownType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            // If the expression is a class of abstract type, then it cannot be instantiated.
            // Note, only class declarations can be declared abstract.
            // In the case of a merged class-module or class-interface declaration,
            // only the class declaration node will have the Abstract flag set.
            const valueDecl = expressionType.symbol && getClassLikeDeclarationOfSymbol(expressionType.symbol);
            if (valueDecl && valueDecl.flags & NodeFlags.Abstract) {
                error(node, Diagnostics.Cannot_create_an_instance_of_the_abstract_class_0, declarationNameToString(valueDecl.name));
                return resolveErrorCall(node);
            }

            // TS 1.0 spec: 4.11
            // If expressionType is of type Any, Args can be any argument
            // list and the result of the operation is of type Any.
            if (isTypeAny(expressionType)) {
                if (node.typeArguments) {
                    error(node, Diagnostics.Untyped_function_calls_may_not_accept_type_arguments);
                }
                return resolveUntypedCall(node);
            }

            // Technically, this signatures list may be incomplete. We are taking the apparent type,
            // but we are not including construct signatures that may have been added to the Object or
            // Function interface, since they have none by default. This is a bit of a leap of faith
            // that the user will not add any.
            const constructSignatures = getSignaturesOfType(expressionType, SignatureKind.Construct);
            if (constructSignatures.length) {
                if (!isConstructorAccessible(node, constructSignatures[0])) {
                    return resolveErrorCall(node);
                }
                return resolveCall(node, constructSignatures, candidatesOutArray);
            }

            // If expressionType's apparent type is an object type with no construct signatures but
            // one or more call signatures, the expression is processed as a function call. A compile-time
            // error occurs if the result of the function call is not Void. The type of the result of the
            // operation is Any. It is an error to have a Void this type.
            const callSignatures = getSignaturesOfType(expressionType, SignatureKind.Call);
            if (callSignatures.length) {
                const signature = resolveCall(node, callSignatures, candidatesOutArray);
                if (getReturnTypeOfSignature(signature) !== voidType) {
                    error(node, Diagnostics.Only_a_void_function_can_be_called_with_the_new_keyword);
                }
                if (getThisTypeOfSignature(signature) === voidType) {
                    error(node, Diagnostics.A_function_that_is_called_with_the_new_keyword_cannot_have_a_this_type_that_is_void);
                }
                return signature;
            }

            error(node, Diagnostics.Cannot_use_new_with_an_expression_whose_type_lacks_a_call_or_construct_signature);
            return resolveErrorCall(node);
        }

        function isConstructorAccessible(node: NewExpression, signature: Signature) {
            if (!signature || !signature.declaration) {
                return true;
            }

            const declaration = signature.declaration;
            const flags = declaration.flags;

            // Public constructor is accessible.
            if (!(flags & (NodeFlags.Private | NodeFlags.Protected))) {
                return true;
            }

            const declaringClassDeclaration = <ClassLikeDeclaration>getClassLikeDeclarationOfSymbol(declaration.parent.symbol);
            const declaringClass = <InterfaceType>getDeclaredTypeOfSymbol(declaration.parent.symbol);

            // A private or protected constructor can only be instantiated within its own class (or a subclass, for protected)
            if (!isNodeWithinClass(node, declaringClassDeclaration)) {
                const containingClass = getContainingClass(node);
                if (containingClass) {
                    const containingType = getTypeOfNode(containingClass);
                    const baseTypes = getBaseTypes(<InterfaceType>containingType);
                    if (baseTypes.length) {
                        const baseType = baseTypes[0];
                        if (flags & NodeFlags.Protected &&
                            baseType.symbol === declaration.parent.symbol) {
                            return true;
                        }
                    }
                }
                if (flags & NodeFlags.Private) {
                    error(node, Diagnostics.Constructor_of_class_0_is_private_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
                }
                if (flags & NodeFlags.Protected) {
                    error(node, Diagnostics.Constructor_of_class_0_is_protected_and_only_accessible_within_the_class_declaration, typeToString(declaringClass));
                }
                return false;
            }

            return true;
        }

        function resolveTaggedTemplateExpression(node: TaggedTemplateExpression, candidatesOutArray: Signature[]): Signature {
            const tagType = checkExpression(node.tag);
            const apparentType = getApparentType(tagType);

            if (apparentType === unknownType) {
                // Another error has already been reported
                return resolveErrorCall(node);
            }

            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);

            if (isTypeAny(tagType) || (!callSignatures.length && !(tagType.flags & TypeFlags.Union) && isTypeAssignableTo(tagType, globalFunctionType))) {
                return resolveUntypedCall(node);
            }

            if (!callSignatures.length) {
                error(node, Diagnostics.Cannot_invoke_an_expression_whose_type_lacks_a_call_signature);
                return resolveErrorCall(node);
            }

            return resolveCall(node, callSignatures, candidatesOutArray);
        }

        /**
          * Gets the localized diagnostic head message to use for errors when resolving a decorator as a call expression.
          */
        function getDiagnosticHeadMessageForDecoratorResolution(node: Decorator) {
            switch (node.parent.kind) {
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.ClassExpression:
                    return Diagnostics.Unable_to_resolve_signature_of_class_decorator_when_called_as_an_expression;

                case SyntaxKind.Parameter:
                    return Diagnostics.Unable_to_resolve_signature_of_parameter_decorator_when_called_as_an_expression;

                case SyntaxKind.PropertyDeclaration:
                    return Diagnostics.Unable_to_resolve_signature_of_property_decorator_when_called_as_an_expression;

                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    return Diagnostics.Unable_to_resolve_signature_of_method_decorator_when_called_as_an_expression;
            }
        }

        /**
          * Resolves a decorator as if it were a call expression.
          */
        function resolveDecorator(node: Decorator, candidatesOutArray: Signature[]): Signature {
            const funcType = checkExpression(node.expression);
            const apparentType = getApparentType(funcType);
            if (apparentType === unknownType) {
                return resolveErrorCall(node);
            }

            const callSignatures = getSignaturesOfType(apparentType, SignatureKind.Call);
            if (funcType === anyType || (!callSignatures.length && !(funcType.flags & TypeFlags.Union) && isTypeAssignableTo(funcType, globalFunctionType))) {
                return resolveUntypedCall(node);
            }

            const headMessage = getDiagnosticHeadMessageForDecoratorResolution(node);
            if (!callSignatures.length) {
                let errorInfo: DiagnosticMessageChain;
                errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Cannot_invoke_an_expression_whose_type_lacks_a_call_signature);
                errorInfo = chainDiagnosticMessages(errorInfo, headMessage);
                diagnostics.add(createDiagnosticForNodeFromMessageChain(node, errorInfo));
                return resolveErrorCall(node);
            }

            return resolveCall(node, callSignatures, candidatesOutArray, headMessage);
        }

        function resolveSignature(node: CallLikeExpression, candidatesOutArray?: Signature[]): Signature {
            switch (node.kind) {
                case SyntaxKind.CallExpression:
                    return resolveCallExpression(<CallExpression>node, candidatesOutArray);
                case SyntaxKind.NewExpression:
                    return resolveNewExpression(<NewExpression>node, candidatesOutArray);
                case SyntaxKind.TaggedTemplateExpression:
                    return resolveTaggedTemplateExpression(<TaggedTemplateExpression>node, candidatesOutArray);
                case SyntaxKind.Decorator:
                    return resolveDecorator(<Decorator>node, candidatesOutArray);
            }
            Debug.fail("Branch in 'resolveSignature' should be unreachable.");
        }

        // candidatesOutArray is passed by signature help in the language service, and collectCandidates
        // must fill it up with the appropriate candidate signatures
        function getResolvedSignature(node: CallLikeExpression, candidatesOutArray?: Signature[]): Signature {
            const links = getNodeLinks(node);
            // If getResolvedSignature has already been called, we will have cached the resolvedSignature.
            // However, it is possible that either candidatesOutArray was not passed in the first time,
            // or that a different candidatesOutArray was passed in. Therefore, we need to redo the work
            // to correctly fill the candidatesOutArray.
            const cached = links.resolvedSignature;
            if (cached && cached !== anySignature && !candidatesOutArray) {
                return cached;
            }
            links.resolvedSignature = anySignature;
            const result = resolveSignature(node, candidatesOutArray);
            // If signature resolution originated in control flow type analysis (for example to compute the
            // assigned type in a flow assignment) we don't cache the result as it may be based on temporary
            // types from the control flow analysis.
            links.resolvedSignature = flowLoopStart === flowLoopCount ? result : cached;
            return result;
        }

        function getResolvedOrAnySignature(node: CallLikeExpression) {
            // If we're already in the process of resolving the given signature, don't resolve again as
            // that could cause infinite recursion. Instead, return anySignature.
            return getNodeLinks(node).resolvedSignature === anySignature ? anySignature : getResolvedSignature(node);
        }

        function getInferredClassType(symbol: Symbol) {
            const links = getSymbolLinks(symbol);
            if (!links.inferredClassType) {
                links.inferredClassType = createAnonymousType(symbol, symbol.members, emptyArray, emptyArray, /*stringIndexType*/ undefined, /*numberIndexType*/ undefined);
            }
            return links.inferredClassType;
        }

        /**
         * Syntactically and semantically checks a call or new expression.
         * @param node The call/new expression to be checked.
         * @returns On success, the expression's signature's return type. On failure, anyType.
         */
        function checkCallExpression(node: CallExpression): Type {
            // Grammar checking; stop grammar-checking if checkGrammarTypeArguments return true
            checkGrammarTypeArguments(node, node.typeArguments) || checkGrammarArguments(node, node.arguments);

            const signature = getResolvedSignature(node);

            if (node.expression.kind === SyntaxKind.SuperKeyword) {
                return voidType;
            }

            if (node.kind === SyntaxKind.NewExpression) {
                const declaration = signature.declaration;

                if (declaration &&
                    declaration.kind !== SyntaxKind.Constructor &&
                    declaration.kind !== SyntaxKind.ConstructSignature &&
                    declaration.kind !== SyntaxKind.ConstructorType &&
                    !isJSDocConstructSignature(declaration)) {

                    // When resolved signature is a call signature (and not a construct signature) the result type is any, unless
                    // the declaring function had members created through 'x.prototype.y = expr' or 'this.y = expr' psuedodeclarations
                    // in a JS file
                    // Note:JS inferred classes might come from a variable declaration instead of a function declaration.
                    // In this case, using getResolvedSymbol directly is required to avoid losing the members from the declaration.
                    const funcSymbol = node.expression.kind === SyntaxKind.Identifier ?
                        getResolvedSymbol(node.expression as Identifier) :
                        checkExpression(node.expression).symbol;
                    if (funcSymbol && funcSymbol.members && (funcSymbol.flags & SymbolFlags.Function || isDeclarationOfFunctionExpression(funcSymbol))) {
                        return getInferredClassType(funcSymbol);
                    }
                    else if (compilerOptions.noImplicitAny) {
                        error(node, Diagnostics.new_expression_whose_target_lacks_a_construct_signature_implicitly_has_an_any_type);
                    }
                    return anyType;
                }
            }

            // In JavaScript files, calls to any identifier 'require' are treated as external module imports
            if (isInJavaScriptFile(node) && isRequireCall(node, /*checkArgumentIsStringLiteral*/true)) {
                return resolveExternalModuleTypeByLiteral(<StringLiteral>node.arguments[0]);
            }

            return getReturnTypeOfSignature(signature);
        }

        function checkTaggedTemplateExpression(node: TaggedTemplateExpression): Type {
            return getReturnTypeOfSignature(getResolvedSignature(node));
        }

        function checkAssertion(node: AssertionExpression) {
            const exprType = getRegularTypeOfObjectLiteral(checkExpression(node.expression));

            checkSourceElement(node.type);
            const targetType = getTypeFromTypeNode(node.type);

            if (produceDiagnostics && targetType !== unknownType) {
                const widenedType = getWidenedType(exprType);
                if (!isTypeComparableTo(targetType, widenedType)) {
                    checkTypeComparableTo(exprType, targetType, node, Diagnostics.Type_0_cannot_be_converted_to_type_1);
                }
            }
            return targetType;
        }

        function checkNonNullAssertion(node: NonNullExpression) {
            return getNonNullableType(checkExpression(node.expression));
        }

        function getTypeOfParameter(symbol: Symbol) {
            const type = getTypeOfSymbol(symbol);
            if (strictNullChecks) {
                const declaration = symbol.valueDeclaration;
                if (declaration && (<VariableLikeDeclaration>declaration).initializer) {
                    return includeFalsyTypes(type, TypeFlags.Undefined);
                }
            }
            return type;
        }

        function getTypeAtPosition(signature: Signature, pos: number): Type {
            return signature.hasRestParameter ?
                pos < signature.parameters.length - 1 ? getTypeOfParameter(signature.parameters[pos]) : getRestTypeOfSignature(signature) :
                pos < signature.parameters.length ? getTypeOfParameter(signature.parameters[pos]) : anyType;
        }

        function assignContextualParameterTypes(signature: Signature, context: Signature, mapper: TypeMapper) {
            const len = signature.parameters.length - (signature.hasRestParameter ? 1 : 0);
            for (let i = 0; i < len; i++) {
                const parameter = signature.parameters[i];
                const contextualParameterType = getTypeAtPosition(context, i);
                assignTypeToParameterAndFixTypeParameters(parameter, contextualParameterType, mapper);
            }
            if (signature.hasRestParameter && isRestParameterIndex(context, signature.parameters.length - 1)) {
                const parameter = lastOrUndefined(signature.parameters);
                const contextualParameterType = getTypeOfSymbol(lastOrUndefined(context.parameters));
                assignTypeToParameterAndFixTypeParameters(parameter, contextualParameterType, mapper);
            }
        }

        // When contextual typing assigns a type to a parameter that contains a binding pattern, we also need to push
        // the destructured type into the contained binding elements.
        function assignBindingElementTypes(node: VariableLikeDeclaration) {
            if (isBindingPattern(node.name)) {
                for (const element of (<BindingPattern>node.name).elements) {
                    if (element.kind !== SyntaxKind.OmittedExpression) {
                        if (element.name.kind === SyntaxKind.Identifier) {
                            getSymbolLinks(getSymbolOfNode(element)).type = getTypeForBindingElement(element);
                        }
                        assignBindingElementTypes(element);
                    }
                }
            }
        }

        function assignTypeToParameterAndFixTypeParameters(parameter: Symbol, contextualType: Type, mapper: TypeMapper) {
            const links = getSymbolLinks(parameter);
            if (!links.type) {
                links.type = instantiateType(contextualType, mapper);
                // if inference didn't come up with anything but {}, fall back to the binding pattern if present.
                if (links.type === emptyObjectType &&
                    (parameter.valueDeclaration.name.kind === SyntaxKind.ObjectBindingPattern ||
                        parameter.valueDeclaration.name.kind === SyntaxKind.ArrayBindingPattern)) {
                    links.type = getTypeFromBindingPattern(<BindingPattern>parameter.valueDeclaration.name);
                }
                assignBindingElementTypes(<ParameterDeclaration>parameter.valueDeclaration);
            }
            else if (isInferentialContext(mapper)) {
                // Even if the parameter already has a type, it might be because it was given a type while
                // processing the function as an argument to a prior signature during overload resolution.
                // If this was the case, it may have caused some type parameters to be fixed. So here,
                // we need to ensure that type parameters at the same positions get fixed again. This is
                // done by calling instantiateType to attach the mapper to the contextualType, and then
                // calling inferTypes to force a walk of contextualType so that all the correct fixing
                // happens. The choice to pass in links.type may seem kind of arbitrary, but it serves
                // to make sure that all the correct positions in contextualType are reached by the walk.
                // Here is an example:
                //
                //      interface Base {
                //          baseProp;
                //      }
                //      interface Derived extends Base {
                //          toBase(): Base;
                //      }
                //
                //      var derived: Derived;
                //
                //      declare function foo<T>(x: T, func: (p: T) => T): T;
                //      declare function foo<T>(x: T, func: (p: T) => T): T;
                //
                //      var result = foo(derived, d => d.toBase());
                //
                // We are typing d while checking the second overload. But we've already given d
                // a type (Derived) from the first overload. However, we still want to fix the
                // T in the second overload so that we do not infer Base as a candidate for T
                // (inferring Base would make type argument inference inconsistent between the two
                // overloads).
                inferTypes(mapper.context, links.type, instantiateType(contextualType, mapper));
            }
        }

        function getReturnTypeFromJSDocComment(func: SignatureDeclaration | FunctionDeclaration): Type {
            const returnTag = getJSDocReturnTag(func);
            if (returnTag && returnTag.typeExpression) {
                return getTypeFromTypeNode(returnTag.typeExpression.type);
            }

            return undefined;
        }

        function createPromiseType(promisedType: Type): Type {
            // creates a `Promise<T>` type where `T` is the promisedType argument
            const globalPromiseType = getGlobalPromiseType();
            if (globalPromiseType !== emptyGenericType) {
                // if the promised type is itself a promise, get the underlying type; otherwise, fallback to the promised type
                promisedType = getAwaitedType(promisedType);
                return createTypeReference(<GenericType>globalPromiseType, [promisedType]);
            }

            return emptyObjectType;
        }

        function createPromiseReturnType(func: FunctionLikeDeclaration, promisedType: Type) {
            const promiseType = createPromiseType(promisedType);
            if (promiseType === emptyObjectType) {
                error(func, Diagnostics.An_async_function_or_method_must_have_a_valid_awaitable_return_type);
                return unknownType;
            }

            return promiseType;
        }

        function getReturnTypeFromBody(func: FunctionLikeDeclaration, contextualMapper?: TypeMapper): Type {
            const contextualSignature = getContextualSignatureForFunctionLikeDeclaration(func);
            if (!func.body) {
                return unknownType;
            }

            const isAsync = isAsyncFunctionLike(func);
            let type: Type;
            if (func.body.kind !== SyntaxKind.Block) {
                type = checkExpressionCached(<Expression>func.body, contextualMapper);
                if (isAsync) {
                    // From within an async function you can return either a non-promise value or a promise. Any
                    // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
                    // return type of the body should be unwrapped to its awaited type, which we will wrap in
                    // the native Promise<T> type later in this function.
                    type = checkAwaitedType(type, func, Diagnostics.Return_expression_in_async_function_does_not_have_a_valid_callable_then_member);
                }
            }
            else {
                let types: Type[];
                const funcIsGenerator = !!func.asteriskToken;
                if (funcIsGenerator) {
                    types = checkAndAggregateYieldOperandTypes(func, contextualMapper);
                    if (types.length === 0) {
                        const iterableIteratorAny = createIterableIteratorType(anyType);
                        if (compilerOptions.noImplicitAny) {
                            error(func.asteriskToken,
                                Diagnostics.Generator_implicitly_has_type_0_because_it_does_not_yield_any_values_Consider_supplying_a_return_type, typeToString(iterableIteratorAny));
                        }
                        return iterableIteratorAny;
                    }
                }
                else {
                    types = checkAndAggregateReturnExpressionTypes(func, contextualMapper);
                    if (!types) {
                        // For an async function, the return type will not be never, but rather a Promise for never.
                        return isAsync ? createPromiseReturnType(func, neverType) : neverType;
                    }
                    if (types.length === 0) {
                        // For an async function, the return type will not be void, but rather a Promise for void.
                        return isAsync ? createPromiseReturnType(func, voidType) : voidType;
                    }
                }
                // When yield/return statements are contextually typed we allow the return type to be a union type.
                // Otherwise we require the yield/return expressions to have a best common supertype.
                type = contextualSignature ? getUnionType(types, /*subtypeReduction*/ true) : getCommonSupertype(types);
                if (!type) {
                    if (funcIsGenerator) {
                        error(func, Diagnostics.No_best_common_type_exists_among_yield_expressions);
                        return createIterableIteratorType(unknownType);
                    }
                    else {
                        error(func, Diagnostics.No_best_common_type_exists_among_return_expressions);
                        // Defer to unioning the return types so we get a) downstream errors earlier and b) better Salsa experience
                        return isAsync ? createPromiseReturnType(func, getUnionType(types, /*subtypeReduction*/ true)) : getUnionType(types, /*subtypeReduction*/ true);
                    }
                }

                if (funcIsGenerator) {
                    type = createIterableIteratorType(type);
                }
            }
            if (!contextualSignature) {
                reportErrorsFromWidening(func, type);
            }

            const widenedType = getWidenedType(type);
            // From within an async function you can return either a non-promise value or a promise. Any
            // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
            // return type of the body is awaited type of the body, wrapped in a native Promise<T> type.
            return isAsync ? createPromiseReturnType(func, widenedType) : widenedType;
        }

        function checkAndAggregateYieldOperandTypes(func: FunctionLikeDeclaration, contextualMapper: TypeMapper): Type[] {
            const aggregatedTypes: Type[] = [];

            forEachYieldExpression(<Block>func.body, yieldExpression => {
                const expr = yieldExpression.expression;
                if (expr) {
                    let type = checkExpressionCached(expr, contextualMapper);

                    if (yieldExpression.asteriskToken) {
                        // A yield* expression effectively yields everything that its operand yields
                        type = checkElementTypeOfIterable(type, yieldExpression.expression);
                    }

                    if (!contains(aggregatedTypes, type)) {
                        aggregatedTypes.push(type);
                    }
                }
            });

            return aggregatedTypes;
        }

        function isExhaustiveSwitchStatement(node: SwitchStatement): boolean {
            if (!node.possiblyExhaustive) {
                return false;
            }
            const type = checkExpression(node.expression);
            if (!isUnitUnionType(type)) {
                return false;
            }
            const switchTypes = getSwitchClauseTypes(node);
            if (!switchTypes.length) {
                return false;
            }
            return eachTypeContainedIn(type, switchTypes);
        }

        function functionHasImplicitReturn(func: FunctionLikeDeclaration) {
            if (!(func.flags & NodeFlags.HasImplicitReturn)) {
                return false;
            }
            const lastStatement = lastOrUndefined((<Block>func.body).statements);
            if (lastStatement && lastStatement.kind === SyntaxKind.SwitchStatement && isExhaustiveSwitchStatement(<SwitchStatement>lastStatement)) {
                return false;
            }
            return true;
        }

        function checkAndAggregateReturnExpressionTypes(func: FunctionLikeDeclaration, contextualMapper: TypeMapper): Type[] {
            const isAsync = isAsyncFunctionLike(func);
            const aggregatedTypes: Type[] = [];
            let hasReturnWithNoExpression = functionHasImplicitReturn(func);
            let hasReturnOfTypeNever = false;
            forEachReturnStatement(<Block>func.body, returnStatement => {
                const expr = returnStatement.expression;
                if (expr) {
                    let type = checkExpressionCached(expr, contextualMapper);
                    if (isAsync) {
                        // From within an async function you can return either a non-promise value or a promise. Any
                        // Promise/A+ compatible implementation will always assimilate any foreign promise, so the
                        // return type of the body should be unwrapped to its awaited type, which should be wrapped in
                        // the native Promise<T> type by the caller.
                        type = checkAwaitedType(type, func, Diagnostics.Return_expression_in_async_function_does_not_have_a_valid_callable_then_member);
                    }
                    if (type === neverType) {
                        hasReturnOfTypeNever = true;
                    }
                    else if (!contains(aggregatedTypes, type)) {
                        aggregatedTypes.push(type);
                    }
                }
                else {
                    hasReturnWithNoExpression = true;
                }
            });
            if (aggregatedTypes.length === 0 && !hasReturnWithNoExpression && (hasReturnOfTypeNever ||
                func.kind === SyntaxKind.FunctionExpression || func.kind === SyntaxKind.ArrowFunction)) {
                return undefined;
            }
            if (strictNullChecks && aggregatedTypes.length && hasReturnWithNoExpression) {
                if (!contains(aggregatedTypes, undefinedType)) {
                    aggregatedTypes.push(undefinedType);
                }
            }
            return aggregatedTypes;
        }

        /**
         * TypeScript Specification 1.0 (6.3) - July 2014
         *   An explicitly typed function whose return type isn't the Void type,
         *   the Any type, or a union type containing the Void or Any type as a constituent
         *   must have at least one return statement somewhere in its body.
         *   An exception to this rule is if the function implementation consists of a single 'throw' statement.
         *
         * @param returnType - return type of the function, can be undefined if return type is not explicitly specified
         */
        function checkAllCodePathsInNonVoidFunctionReturnOrThrow(func: FunctionLikeDeclaration, returnType: Type): void {
            if (!produceDiagnostics) {
                return;
            }

            // Functions with with an explicitly specified 'void' or 'any' return type don't need any return expressions.
            if (returnType && maybeTypeOfKind(returnType, TypeFlags.Any | TypeFlags.Void)) {
                return;
            }

            // If all we have is a function signature, or an arrow function with an expression body, then there is nothing to check.
            // also if HasImplicitReturn flag is not set this means that all codepaths in function body end with return or throw
            if (nodeIsMissing(func.body) || func.body.kind !== SyntaxKind.Block || !functionHasImplicitReturn(func)) {
                return;
            }

            const hasExplicitReturn = func.flags & NodeFlags.HasExplicitReturn;

            if (returnType === neverType) {
                error(func.type, Diagnostics.A_function_returning_never_cannot_have_a_reachable_end_point);
            }
            else if (returnType && !hasExplicitReturn) {
                // minimal check: function has syntactic return type annotation and no explicit return statements in the body
                // this function does not conform to the specification.
                // NOTE: having returnType !== undefined is a precondition for entering this branch so func.type will always be present
                error(func.type, Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value);
            }
            else if (returnType && strictNullChecks && !isTypeAssignableTo(undefinedType, returnType)) {
                error(func.type, Diagnostics.Function_lacks_ending_return_statement_and_return_type_does_not_include_undefined);
            }
            else if (compilerOptions.noImplicitReturns) {
                if (!returnType) {
                    // If return type annotation is omitted check if function has any explicit return statements.
                    // If it does not have any - its inferred return type is void - don't do any checks.
                    // Otherwise get inferred return type from function body and report error only if it is not void / anytype
                    if (!hasExplicitReturn) {
                        return;
                    }
                    const inferredReturnType = getReturnTypeOfSignature(getSignatureFromDeclaration(func));
                    if (isUnwrappedReturnTypeVoidOrAny(func, inferredReturnType)) {
                        return;
                    }
                }
                error(func.type || func, Diagnostics.Not_all_code_paths_return_a_value);
            }
        }

        function checkFunctionExpressionOrObjectLiteralMethod(node: FunctionExpression | MethodDeclaration, contextualMapper?: TypeMapper): Type {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));

            // Grammar checking
            const hasGrammarError = checkGrammarFunctionLikeDeclaration(node);
            if (!hasGrammarError && node.kind === SyntaxKind.FunctionExpression) {
                checkGrammarForGenerator(node);
            }

            // The identityMapper object is used to indicate that function expressions are wildcards
            if (contextualMapper === identityMapper && isContextSensitive(node)) {
                checkNodeDeferred(node);
                return anyFunctionType;
            }

            const links = getNodeLinks(node);
            const type = getTypeOfSymbol(node.symbol);
            const contextSensitive = isContextSensitive(node);
            const mightFixTypeParameters = contextSensitive && isInferentialContext(contextualMapper);

            // Check if function expression is contextually typed and assign parameter types if so.
            // See the comment in assignTypeToParameterAndFixTypeParameters to understand why we need to
            // check mightFixTypeParameters.
            if (mightFixTypeParameters || !(links.flags & NodeCheckFlags.ContextChecked)) {
                const contextualSignature = getContextualSignature(node);
                // If a type check is started at a function expression that is an argument of a function call, obtaining the
                // contextual type may recursively get back to here during overload resolution of the call. If so, we will have
                // already assigned contextual types.
                const contextChecked = !!(links.flags & NodeCheckFlags.ContextChecked);
                if (mightFixTypeParameters || !contextChecked) {
                    links.flags |= NodeCheckFlags.ContextChecked;
                    if (contextualSignature) {
                        const signature = getSignaturesOfType(type, SignatureKind.Call)[0];
                        if (contextSensitive) {
                            assignContextualParameterTypes(signature, contextualSignature, contextualMapper || identityMapper);
                        }
                        if (mightFixTypeParameters || !node.type && !signature.resolvedReturnType) {
                            const returnType = getReturnTypeFromBody(node, contextualMapper);
                            if (!signature.resolvedReturnType) {
                                signature.resolvedReturnType = returnType;
                            }
                        }
                    }

                    if (!contextChecked) {
                        checkSignatureDeclaration(node);
                        checkNodeDeferred(node);
                    }
                }
            }

            if (produceDiagnostics && node.kind !== SyntaxKind.MethodDeclaration && node.kind !== SyntaxKind.MethodSignature) {
                checkCollisionWithCapturedSuperVariable(node, (<FunctionExpression>node).name);
                checkCollisionWithCapturedThisVariable(node, (<FunctionExpression>node).name);
            }

            return type;
        }

        function checkFunctionExpressionOrObjectLiteralMethodDeferred(node: ArrowFunction | FunctionExpression | MethodDeclaration) {
            Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));

            const isAsync = isAsyncFunctionLike(node);
            const returnOrPromisedType = node.type && (isAsync ? checkAsyncFunctionReturnType(node) : getTypeFromTypeNode(node.type));
            if (!node.asteriskToken) {
                // return is not necessary in the body of generators
                checkAllCodePathsInNonVoidFunctionReturnOrThrow(node, returnOrPromisedType);
            }

            if (node.body) {
                if (!node.type) {
                    // There are some checks that are only performed in getReturnTypeFromBody, that may produce errors
                    // we need. An example is the noImplicitAny errors resulting from widening the return expression
                    // of a function. Because checking of function expression bodies is deferred, there was never an
                    // appropriate time to do this during the main walk of the file (see the comment at the top of
                    // checkFunctionExpressionBodies). So it must be done now.
                    getReturnTypeOfSignature(getSignatureFromDeclaration(node));
                }

                if (node.body.kind === SyntaxKind.Block) {
                    checkSourceElement(node.body);
                }
                else {
                    // From within an async function you can return either a non-promise value or a promise. Any
                    // Promise/A+ compatible implementation will always assimilate any foreign promise, so we
                    // should not be checking assignability of a promise to the return type. Instead, we need to
                    // check assignability of the awaited type of the expression body against the promised type of
                    // its return type annotation.
                    const exprType = checkExpression(<Expression>node.body);
                    if (returnOrPromisedType) {
                        if (isAsync) {
                            const awaitedType = checkAwaitedType(exprType, node.body, Diagnostics.Expression_body_for_async_arrow_function_does_not_have_a_valid_callable_then_member);
                            checkTypeAssignableTo(awaitedType, returnOrPromisedType, node.body);
                        }
                        else {
                            checkTypeAssignableTo(exprType, returnOrPromisedType, node.body);
                        }
                    }
                }
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkArithmeticOperandType(operand: Node, type: Type, diagnostic: DiagnosticMessage): boolean {
            if (!isTypeAnyOrAllConstituentTypesHaveKind(type, TypeFlags.NumberLike)) {
                error(operand, diagnostic);
                return false;
            }
            return true;
        }

        function isReadonlySymbol(symbol: Symbol): boolean {
            // The following symbols are considered read-only:
            // Properties with a 'readonly' modifier
            // Variables declared with 'const'
            // Get accessors without matching set accessors
            // Enum members
            // Unions and intersections of the above (unions and intersections eagerly set isReadonly on creation)
            return symbol.isReadonly ||
                symbol.flags & SymbolFlags.Property && (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Readonly) !== 0 ||
                symbol.flags & SymbolFlags.Variable && (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Const) !== 0 ||
                symbol.flags & SymbolFlags.Accessor && !(symbol.flags & SymbolFlags.SetAccessor) ||
                (symbol.flags & SymbolFlags.EnumMember) !== 0;
        }

        function isReferenceToReadonlyEntity(expr: Expression, symbol: Symbol): boolean {
            if (isReadonlySymbol(symbol)) {
                // Allow assignments to readonly properties within constructors of the same class declaration.
                if (symbol.flags & SymbolFlags.Property &&
                    (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) &&
                    (expr as PropertyAccessExpression | ElementAccessExpression).expression.kind === SyntaxKind.ThisKeyword) {
                    // Look for if this is the constructor for the class that `symbol` is a property of.
                    const func = getContainingFunction(expr);
                    if (!(func && func.kind === SyntaxKind.Constructor))
                        return true;
                    // If func.parent is a class and symbol is a (readonly) property of that class, or
                    // if func is a constructor and symbol is a (readonly) parameter property declared in it,
                    // then symbol is writeable here.
                    return !(func.parent === symbol.valueDeclaration.parent || func === symbol.valueDeclaration.parent);
                }
                return true;
            }
            return false;
        }

        function isReferenceThroughNamespaceImport(expr: Expression): boolean {
            if (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) {
                const node = skipParenthesizedNodes((expr as PropertyAccessExpression | ElementAccessExpression).expression);
                if (node.kind === SyntaxKind.Identifier) {
                    const symbol = getNodeLinks(node).resolvedSymbol;
                    if (symbol.flags & SymbolFlags.Alias) {
                        const declaration = getDeclarationOfAliasSymbol(symbol);
                        return declaration && declaration.kind === SyntaxKind.NamespaceImport;
                    }
                }
            }
            return false;
        }

        function checkReferenceExpression(expr: Expression, invalidReferenceMessage: DiagnosticMessage, constantVariableMessage: DiagnosticMessage): boolean {
            // References are combinations of identifiers, parentheses, and property accesses.
            const node = skipParenthesizedNodes(expr);
            if (node.kind !== SyntaxKind.Identifier && node.kind !== SyntaxKind.PropertyAccessExpression && node.kind !== SyntaxKind.ElementAccessExpression) {
                error(expr, invalidReferenceMessage);
                return false;
            }
            // Because we get the symbol from the resolvedSymbol property, it might be of kind
            // SymbolFlags.ExportValue. In this case it is necessary to get the actual export
            // symbol, which will have the correct flags set on it.
            const links = getNodeLinks(node);
            const symbol = getExportSymbolOfValueSymbolIfExported(links.resolvedSymbol);
            if (symbol) {
                if (symbol !== unknownSymbol && symbol !== argumentsSymbol) {
                    // Only variables (and not functions, classes, namespaces, enum objects, or enum members)
                    // are considered references when referenced using a simple identifier.
                    if (node.kind === SyntaxKind.Identifier && !(symbol.flags & SymbolFlags.Variable)) {
                        error(expr, invalidReferenceMessage);
                        return false;
                    }
                    if (isReferenceToReadonlyEntity(node, symbol) || isReferenceThroughNamespaceImport(node)) {
                        error(expr, constantVariableMessage);
                        return false;
                    }
                }
            }
            else if (node.kind === SyntaxKind.ElementAccessExpression) {
                if (links.resolvedIndexInfo && links.resolvedIndexInfo.isReadonly) {
                    error(expr, constantVariableMessage);
                    return false;
                }
            }
            return true;
        }

        function checkDeleteExpression(node: DeleteExpression): Type {
            checkExpression(node.expression);
            return booleanType;
        }

        function checkTypeOfExpression(node: TypeOfExpression): Type {
            checkExpression(node.expression);
            return stringType;
        }

        function checkVoidExpression(node: VoidExpression): Type {
            checkExpression(node.expression);
            return undefinedWideningType;
        }

        function checkAwaitExpression(node: AwaitExpression): Type {
            // Grammar checking
            if (produceDiagnostics) {
                if (!(node.flags & NodeFlags.AwaitContext)) {
                    grammarErrorOnFirstToken(node, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
                }

                if (isInParameterInitializerBeforeContainingFunction(node)) {
                    error(node, Diagnostics.await_expressions_cannot_be_used_in_a_parameter_initializer);
                }
            }

            const operandType = checkExpression(node.expression);
            return checkAwaitedType(operandType, node);
        }

        function checkPrefixUnaryExpression(node: PrefixUnaryExpression): Type {
            const operandType = checkExpression(node.operand);
            if (node.operator === SyntaxKind.MinusToken && node.operand.kind === SyntaxKind.NumericLiteral && isLiteralContextForType(node, numberType)) {
                return getLiteralTypeForText(TypeFlags.NumberLiteral, "" + -(<LiteralExpression>node.operand).text);
            }
            switch (node.operator) {
                case SyntaxKind.PlusToken:
                case SyntaxKind.MinusToken:
                case SyntaxKind.TildeToken:
                    if (maybeTypeOfKind(operandType, TypeFlags.ESSymbol)) {
                        error(node.operand, Diagnostics.The_0_operator_cannot_be_applied_to_type_symbol, tokenToString(node.operator));
                    }
                    return numberType;
                case SyntaxKind.ExclamationToken:
                    const facts = getTypeFacts(operandType) & (TypeFacts.Truthy | TypeFacts.Falsy);
                    return facts === TypeFacts.Truthy ? falseType :
                        facts === TypeFacts.Falsy ? trueType :
                        booleanType;
                case SyntaxKind.PlusPlusToken:
                case SyntaxKind.MinusMinusToken:
                    const ok = checkArithmeticOperandType(node.operand, getNonNullableType(operandType),
                        Diagnostics.An_arithmetic_operand_must_be_of_type_any_number_or_an_enum_type);
                    if (ok) {
                        // run check only if former checks succeeded to avoid reporting cascading errors
                        checkReferenceExpression(node.operand,
                            Diagnostics.The_operand_of_an_increment_or_decrement_operator_must_be_a_variable_property_or_indexer,
                            Diagnostics.The_operand_of_an_increment_or_decrement_operator_cannot_be_a_constant_or_a_read_only_property);
                    }
                    return numberType;
            }
            return unknownType;
        }

        function checkPostfixUnaryExpression(node: PostfixUnaryExpression): Type {
            const operandType = checkExpression(node.operand);
            const ok = checkArithmeticOperandType(node.operand, getNonNullableType(operandType),
                Diagnostics.An_arithmetic_operand_must_be_of_type_any_number_or_an_enum_type);
            if (ok) {
                // run check only if former checks succeeded to avoid reporting cascading errors
                checkReferenceExpression(node.operand,
                    Diagnostics.The_operand_of_an_increment_or_decrement_operator_must_be_a_variable_property_or_indexer,
                    Diagnostics.The_operand_of_an_increment_or_decrement_operator_cannot_be_a_constant_or_a_read_only_property);
            }
            return numberType;
        }

        // Return true if type might be of the given kind. A union or intersection type might be of a given
        // kind if at least one constituent type is of the given kind.
        function maybeTypeOfKind(type: Type, kind: TypeFlags): boolean {
            if (type.flags & kind) {
                return true;
            }
            if (type.flags & TypeFlags.UnionOrIntersection) {
                const types = (<UnionOrIntersectionType>type).types;
                for (const t of types) {
                    if (maybeTypeOfKind(t, kind)) {
                        return true;
                    }
                }
            }
            return false;
        }

        // Return true if type is of the given kind. A union type is of a given kind if all constituent types
        // are of the given kind. An intersection type is of a given kind if at least one constituent type is
        // of the given kind.
        function isTypeOfKind(type: Type, kind: TypeFlags): boolean {
            if (type.flags & kind) {
                return true;
            }
            if (type.flags & TypeFlags.Union) {
                const types = (<UnionOrIntersectionType>type).types;
                for (const t of types) {
                    if (!isTypeOfKind(t, kind)) {
                        return false;
                    }
                }
                return true;
            }
            if (type.flags & TypeFlags.Intersection) {
                const types = (<UnionOrIntersectionType>type).types;
                for (const t of types) {
                    if (isTypeOfKind(t, kind)) {
                        return true;
                    }
                }
            }
            return false;
        }

        function isConstEnumObjectType(type: Type): boolean {
            return type.flags & (TypeFlags.ObjectType | TypeFlags.Anonymous) && type.symbol && isConstEnumSymbol(type.symbol);
        }

        function isConstEnumSymbol(symbol: Symbol): boolean {
            return (symbol.flags & SymbolFlags.ConstEnum) !== 0;
        }

        function checkInstanceOfExpression(left: Expression, right: Expression, leftType: Type, rightType: Type): Type {
            // TypeScript 1.0 spec (April 2014): 4.15.4
            // The instanceof operator requires the left operand to be of type Any, an object type, or a type parameter type,
            // and the right operand to be of type Any or a subtype of the 'Function' interface type.
            // The result is always of the Boolean primitive type.
            // NOTE: do not raise error if leftType is unknown as related error was already reported
            if (isTypeOfKind(leftType, TypeFlags.Primitive)) {
                error(left, Diagnostics.The_left_hand_side_of_an_instanceof_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
            }
            // NOTE: do not raise error if right is unknown as related error was already reported
            if (!(isTypeAny(rightType) || isTypeSubtypeOf(rightType, globalFunctionType))) {
                error(right, Diagnostics.The_right_hand_side_of_an_instanceof_expression_must_be_of_type_any_or_of_a_type_assignable_to_the_Function_interface_type);
            }
            return booleanType;
        }

        function checkInExpression(left: Expression, right: Expression, leftType: Type, rightType: Type): Type {
            // TypeScript 1.0 spec (April 2014): 4.15.5
            // The in operator requires the left operand to be of type Any, the String primitive type, or the Number primitive type,
            // and the right operand to be of type Any, an object type, or a type parameter type.
            // The result is always of the Boolean primitive type.
            if (!isTypeAnyOrAllConstituentTypesHaveKind(leftType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbol)) {
                error(left, Diagnostics.The_left_hand_side_of_an_in_expression_must_be_of_type_any_string_number_or_symbol);
            }
            if (!isTypeAnyOrAllConstituentTypesHaveKind(rightType, TypeFlags.ObjectType | TypeFlags.TypeParameter)) {
                error(right, Diagnostics.The_right_hand_side_of_an_in_expression_must_be_of_type_any_an_object_type_or_a_type_parameter);
            }
            return booleanType;
        }

        function checkObjectLiteralAssignment(node: ObjectLiteralExpression, sourceType: Type, contextualMapper?: TypeMapper): Type {
            const properties = node.properties;
            for (const p of properties) {
                checkObjectLiteralDestructuringPropertyAssignment(sourceType, p, contextualMapper);
            }
            return sourceType;
        }

        function checkObjectLiteralDestructuringPropertyAssignment(objectLiteralType: Type, property: ObjectLiteralElement, contextualMapper?: TypeMapper) {
            if (property.kind === SyntaxKind.PropertyAssignment || property.kind === SyntaxKind.ShorthandPropertyAssignment) {
                const name = <PropertyName>(<PropertyAssignment>property).name;
                if (name.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(<ComputedPropertyName>name);
                }
                if (isComputedNonLiteralName(name)) {
                    return undefined;
                }

                const text = getTextOfPropertyName(name);
                const type = isTypeAny(objectLiteralType)
                    ? objectLiteralType
                    : getTypeOfPropertyOfType(objectLiteralType, text) ||
                    isNumericLiteralName(text) && getIndexTypeOfType(objectLiteralType, IndexKind.Number) ||
                    getIndexTypeOfType(objectLiteralType, IndexKind.String);
                if (type) {
                    if (property.kind === SyntaxKind.ShorthandPropertyAssignment) {
                        return checkDestructuringAssignment(<ShorthandPropertyAssignment>property, type);
                    }
                    else {
                        // non-shorthand property assignments should always have initializers
                        return checkDestructuringAssignment((<PropertyAssignment>property).initializer, type);
                    }
                }
                else {
                    error(name, Diagnostics.Type_0_has_no_property_1_and_no_string_index_signature, typeToString(objectLiteralType), declarationNameToString(name));
                }
            }
            else {
                error(property, Diagnostics.Property_assignment_expected);
            }
        }

        function checkArrayLiteralAssignment(node: ArrayLiteralExpression, sourceType: Type, contextualMapper?: TypeMapper): Type {
            // This elementType will be used if the specific property corresponding to this index is not
            // present (aka the tuple element property). This call also checks that the parentType is in
            // fact an iterable or array (depending on target language).
            const elementType = checkIteratedTypeOrElementType(sourceType, node, /*allowStringInput*/ false) || unknownType;
            const elements = node.elements;
            for (let i = 0; i < elements.length; i++) {
                checkArrayLiteralDestructuringElementAssignment(node, sourceType, i, elementType, contextualMapper);
            }
            return sourceType;
        }

        function checkArrayLiteralDestructuringElementAssignment(node: ArrayLiteralExpression, sourceType: Type,
            elementIndex: number, elementType: Type, contextualMapper?: TypeMapper) {
            const elements = node.elements;
            const element = elements[elementIndex];
            if (element.kind !== SyntaxKind.OmittedExpression) {
                if (element.kind !== SyntaxKind.SpreadElementExpression) {
                    const propName = "" + elementIndex;
                    const type = isTypeAny(sourceType)
                        ? sourceType
                        : isTupleLikeType(sourceType)
                            ? getTypeOfPropertyOfType(sourceType, propName)
                            : elementType;
                    if (type) {
                        return checkDestructuringAssignment(element, type, contextualMapper);
                    }
                    else {
                        // We still need to check element expression here because we may need to set appropriate flag on the expression
                        // such as NodeCheckFlags.LexicalThis on "this"expression.
                        checkExpression(element);
                        if (isTupleType(sourceType)) {
                            error(element, Diagnostics.Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2, typeToString(sourceType), (<TupleType>sourceType).elementTypes.length, elements.length);
                        }
                        else {
                            error(element, Diagnostics.Type_0_has_no_property_1, typeToString(sourceType), propName);
                        }
                    }
                }
                else {
                    if (elementIndex < elements.length - 1) {
                        error(element, Diagnostics.A_rest_element_must_be_last_in_an_array_destructuring_pattern);
                    }
                    else {
                        const restExpression = (<SpreadElementExpression>element).expression;
                        if (restExpression.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>restExpression).operatorToken.kind === SyntaxKind.EqualsToken) {
                            error((<BinaryExpression>restExpression).operatorToken, Diagnostics.A_rest_element_cannot_have_an_initializer);
                        }
                        else {
                            return checkDestructuringAssignment(restExpression, createArrayType(elementType), contextualMapper);
                        }
                    }
                }
            }
            return undefined;
        }

        function checkDestructuringAssignment(exprOrAssignment: Expression | ShorthandPropertyAssignment, sourceType: Type, contextualMapper?: TypeMapper): Type {
            let target: Expression;
            if (exprOrAssignment.kind === SyntaxKind.ShorthandPropertyAssignment) {
                const prop = <ShorthandPropertyAssignment>exprOrAssignment;
                if (prop.objectAssignmentInitializer) {
                    // In strict null checking mode, if a default value of a non-undefined type is specified, remove
                    // undefined from the final type.
                    if (strictNullChecks &&
                        !(getFalsyFlags(checkExpression(prop.objectAssignmentInitializer)) & TypeFlags.Undefined)) {
                        sourceType = getTypeWithFacts(sourceType, TypeFacts.NEUndefined);
                    }
                    checkBinaryLikeExpression(prop.name, prop.equalsToken, prop.objectAssignmentInitializer, contextualMapper);
                }
                target = (<ShorthandPropertyAssignment>exprOrAssignment).name;
            }
            else {
                target = <Expression>exprOrAssignment;
            }

            if (target.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>target).operatorToken.kind === SyntaxKind.EqualsToken) {
                checkBinaryExpression(<BinaryExpression>target, contextualMapper);
                target = (<BinaryExpression>target).left;
            }
            if (target.kind === SyntaxKind.ObjectLiteralExpression) {
                return checkObjectLiteralAssignment(<ObjectLiteralExpression>target, sourceType, contextualMapper);
            }
            if (target.kind === SyntaxKind.ArrayLiteralExpression) {
                return checkArrayLiteralAssignment(<ArrayLiteralExpression>target, sourceType, contextualMapper);
            }
            return checkReferenceAssignment(target, sourceType, contextualMapper);
        }

        function checkReferenceAssignment(target: Expression, sourceType: Type, contextualMapper?: TypeMapper): Type {
            const targetType = checkExpression(target, contextualMapper);
            if (checkReferenceExpression(target, Diagnostics.Invalid_left_hand_side_of_assignment_expression, Diagnostics.Left_hand_side_of_assignment_expression_cannot_be_a_constant_or_a_read_only_property)) {
                checkTypeAssignableTo(sourceType, targetType, target, /*headMessage*/ undefined);
            }
            return sourceType;
        }

        function isTypeEqualityComparableTo(source: Type, target: Type) {
            return (target.flags & TypeFlags.Nullable) !== 0 || isTypeComparableTo(source, target);
        }

        function getBestChoiceType(type1: Type, type2: Type): Type {
            const firstAssignableToSecond = isTypeAssignableTo(type1, type2);
            const secondAssignableToFirst = isTypeAssignableTo(type2, type1);
            return secondAssignableToFirst && !firstAssignableToSecond ? type1 :
                firstAssignableToSecond && !secondAssignableToFirst ? type2 :
                getUnionType([type1, type2], /*subtypeReduction*/ true);
        }

        function checkBinaryExpression(node: BinaryExpression, contextualMapper?: TypeMapper) {
            return checkBinaryLikeExpression(node.left, node.operatorToken, node.right, contextualMapper, node);
        }

        function checkBinaryLikeExpression(left: Expression, operatorToken: Node, right: Expression, contextualMapper?: TypeMapper, errorNode?: Node) {
            const operator = operatorToken.kind;
            if (operator === SyntaxKind.EqualsToken && (left.kind === SyntaxKind.ObjectLiteralExpression || left.kind === SyntaxKind.ArrayLiteralExpression)) {
                return checkDestructuringAssignment(left, checkExpression(right, contextualMapper), contextualMapper);
            }
            let leftType = checkExpression(left, contextualMapper);
            let rightType = checkExpression(right, contextualMapper);
            switch (operator) {
                case SyntaxKind.AsteriskToken:
                case SyntaxKind.AsteriskAsteriskToken:
                case SyntaxKind.AsteriskEqualsToken:
                case SyntaxKind.AsteriskAsteriskEqualsToken:
                case SyntaxKind.SlashToken:
                case SyntaxKind.SlashEqualsToken:
                case SyntaxKind.PercentToken:
                case SyntaxKind.PercentEqualsToken:
                case SyntaxKind.MinusToken:
                case SyntaxKind.MinusEqualsToken:
                case SyntaxKind.LessThanLessThanToken:
                case SyntaxKind.LessThanLessThanEqualsToken:
                case SyntaxKind.GreaterThanGreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanEqualsToken:
                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanGreaterThanEqualsToken:
                case SyntaxKind.BarToken:
                case SyntaxKind.BarEqualsToken:
                case SyntaxKind.CaretToken:
                case SyntaxKind.CaretEqualsToken:
                case SyntaxKind.AmpersandToken:
                case SyntaxKind.AmpersandEqualsToken:
                    // TypeScript 1.0 spec (April 2014): 4.19.1
                    // These operators require their operands to be of type Any, the Number primitive type,
                    // or an enum type. Operands of an enum type are treated
                    // as having the primitive type Number. If one operand is the null or undefined value,
                    // it is treated as having the type of the other operand.
                    // The result is always of the Number primitive type.
                    if (leftType.flags & TypeFlags.Nullable) leftType = rightType;
                    if (rightType.flags & TypeFlags.Nullable) rightType = leftType;

                    leftType = getNonNullableType(leftType);
                    rightType = getNonNullableType(rightType);

                    let suggestedOperator: SyntaxKind;
                    // if a user tries to apply a bitwise operator to 2 boolean operands
                    // try and return them a helpful suggestion
                    if ((leftType.flags & TypeFlags.BooleanLike) &&
                        (rightType.flags & TypeFlags.BooleanLike) &&
                        (suggestedOperator = getSuggestedBooleanOperator(operatorToken.kind)) !== undefined) {
                        error(errorNode || operatorToken, Diagnostics.The_0_operator_is_not_allowed_for_boolean_types_Consider_using_1_instead, tokenToString(operatorToken.kind), tokenToString(suggestedOperator));
                    }
                    else {
                        // otherwise just check each operand separately and report errors as normal
                        const leftOk = checkArithmeticOperandType(left, leftType, Diagnostics.The_left_hand_side_of_an_arithmetic_operation_must_be_of_type_any_number_or_an_enum_type);
                        const rightOk = checkArithmeticOperandType(right, rightType, Diagnostics.The_right_hand_side_of_an_arithmetic_operation_must_be_of_type_any_number_or_an_enum_type);
                        if (leftOk && rightOk) {
                            checkAssignmentOperator(numberType);
                        }
                    }

                    return numberType;
                case SyntaxKind.PlusToken:
                case SyntaxKind.PlusEqualsToken:
                    // TypeScript 1.0 spec (April 2014): 4.19.2
                    // The binary + operator requires both operands to be of the Number primitive type or an enum type,
                    // or at least one of the operands to be of type Any or the String primitive type.

                    // If one operand is the null or undefined value, it is treated as having the type of the other operand.
                    if (leftType.flags & TypeFlags.Nullable) leftType = rightType;
                    if (rightType.flags & TypeFlags.Nullable) rightType = leftType;

                    leftType = getNonNullableType(leftType);
                    rightType = getNonNullableType(rightType);

                    let resultType: Type;
                    if (isTypeOfKind(leftType, TypeFlags.NumberLike) && isTypeOfKind(rightType, TypeFlags.NumberLike)) {
                        // Operands of an enum type are treated as having the primitive type Number.
                        // If both operands are of the Number primitive type, the result is of the Number primitive type.
                        resultType = numberType;
                    }
                    else {
                        if (isTypeOfKind(leftType, TypeFlags.StringLike) || isTypeOfKind(rightType, TypeFlags.StringLike)) {
                            // If one or both operands are of the String primitive type, the result is of the String primitive type.
                            resultType = stringType;
                        }
                        else if (isTypeAny(leftType) || isTypeAny(rightType)) {
                            // Otherwise, the result is of type Any.
                            // NOTE: unknown type here denotes error type. Old compiler treated this case as any type so do we.
                            resultType = leftType === unknownType || rightType === unknownType ? unknownType : anyType;
                        }

                        // Symbols are not allowed at all in arithmetic expressions
                        if (resultType && !checkForDisallowedESSymbolOperand(operator)) {
                            return resultType;
                        }
                    }

                    if (!resultType) {
                        reportOperatorError();
                        return anyType;
                    }

                    if (operator === SyntaxKind.PlusEqualsToken) {
                        checkAssignmentOperator(resultType);
                    }
                    return resultType;
                case SyntaxKind.LessThanToken:
                case SyntaxKind.GreaterThanToken:
                case SyntaxKind.LessThanEqualsToken:
                case SyntaxKind.GreaterThanEqualsToken:
                    if (checkForDisallowedESSymbolOperand(operator)) {
                        if (!isTypeComparableTo(leftType, rightType) && !isTypeComparableTo(rightType, leftType)) {
                            reportOperatorError();
                        }
                    }
                    return booleanType;
                case SyntaxKind.EqualsEqualsToken:
                case SyntaxKind.ExclamationEqualsToken:
                case SyntaxKind.EqualsEqualsEqualsToken:
                case SyntaxKind.ExclamationEqualsEqualsToken:
                    const leftIsUnit = isUnitUnionType(leftType);
                    const rightIsUnit = isUnitUnionType(rightType);
                    if (!leftIsUnit || !rightIsUnit) {
                        leftType = leftIsUnit ? getBaseTypeOfUnitType(leftType) : leftType;
                        rightType = rightIsUnit ? getBaseTypeOfUnitType(rightType) : rightType;
                    }
                    if (!isTypeEqualityComparableTo(leftType, rightType) && !isTypeEqualityComparableTo(rightType, leftType)) {
                        reportOperatorError();
                    }
                    return booleanType;
                case SyntaxKind.InstanceOfKeyword:
                    return checkInstanceOfExpression(left, right, leftType, rightType);
                case SyntaxKind.InKeyword:
                    return checkInExpression(left, right, leftType, rightType);
                case SyntaxKind.AmpersandAmpersandToken:
                    return getTypeFacts(leftType) & TypeFacts.Truthy ?
                        includeFalsyTypes(rightType, getFalsyFlags(strictNullChecks ? leftType : getBaseTypeOfUnitType(rightType))) :
                        leftType;
                case SyntaxKind.BarBarToken:
                    return getTypeFacts(leftType) & TypeFacts.Falsy ?
                        getBestChoiceType(removeDefinitelyFalsyTypes(leftType), rightType) :
                        leftType;
                case SyntaxKind.EqualsToken:
                    checkAssignmentOperator(rightType);
                    return getRegularTypeOfObjectLiteral(rightType);
                case SyntaxKind.CommaToken:
                    return rightType;
            }

            // Return true if there was no error, false if there was an error.
            function checkForDisallowedESSymbolOperand(operator: SyntaxKind): boolean {
                const offendingSymbolOperand =
                    maybeTypeOfKind(leftType, TypeFlags.ESSymbol) ? left :
                        maybeTypeOfKind(rightType, TypeFlags.ESSymbol) ? right :
                            undefined;
                if (offendingSymbolOperand) {
                    error(offendingSymbolOperand, Diagnostics.The_0_operator_cannot_be_applied_to_type_symbol, tokenToString(operator));
                    return false;
                }

                return true;
            }

            function getSuggestedBooleanOperator(operator: SyntaxKind): SyntaxKind {
                switch (operator) {
                    case SyntaxKind.BarToken:
                    case SyntaxKind.BarEqualsToken:
                        return SyntaxKind.BarBarToken;
                    case SyntaxKind.CaretToken:
                    case SyntaxKind.CaretEqualsToken:
                        return SyntaxKind.ExclamationEqualsEqualsToken;
                    case SyntaxKind.AmpersandToken:
                    case SyntaxKind.AmpersandEqualsToken:
                        return SyntaxKind.AmpersandAmpersandToken;
                    default:
                        return undefined;
                }
            }

            function checkAssignmentOperator(valueType: Type): void {
                if (produceDiagnostics && operator >= SyntaxKind.FirstAssignment && operator <= SyntaxKind.LastAssignment) {
                    // TypeScript 1.0 spec (April 2014): 4.17
                    // An assignment of the form
                    //    VarExpr = ValueExpr
                    // requires VarExpr to be classified as a reference
                    // A compound assignment furthermore requires VarExpr to be classified as a reference (section 4.1)
                    // and the type of the non - compound operation to be assignable to the type of VarExpr.
                    const ok = checkReferenceExpression(left,
                        Diagnostics.Invalid_left_hand_side_of_assignment_expression,
                        Diagnostics.Left_hand_side_of_assignment_expression_cannot_be_a_constant_or_a_read_only_property);
                    // Use default messages
                    if (ok) {
                        // to avoid cascading errors check assignability only if 'isReference' check succeeded and no errors were reported
                        checkTypeAssignableTo(valueType, leftType, left, /*headMessage*/ undefined);
                    }
                }
            }

            function reportOperatorError() {
                error(errorNode || operatorToken, Diagnostics.Operator_0_cannot_be_applied_to_types_1_and_2, tokenToString(operatorToken.kind), typeToString(leftType), typeToString(rightType));
            }
        }

        function isYieldExpressionInClass(node: YieldExpression): boolean {
            let current: Node = node;
            let parent = node.parent;
            while (parent) {
                if (isFunctionLike(parent) && current === (<FunctionLikeDeclaration>parent).body) {
                    return false;
                }
                else if (isClassLike(current)) {
                    return true;
                }

                current = parent;
                parent = parent.parent;
            }

            return false;
        }

        function checkYieldExpression(node: YieldExpression): Type {
            // Grammar checking
            if (produceDiagnostics) {
                if (!(node.flags & NodeFlags.YieldContext) || isYieldExpressionInClass(node)) {
                    grammarErrorOnFirstToken(node, Diagnostics.A_yield_expression_is_only_allowed_in_a_generator_body);
                }

                if (isInParameterInitializerBeforeContainingFunction(node)) {
                    error(node, Diagnostics.yield_expressions_cannot_be_used_in_a_parameter_initializer);
                }
            }

            if (node.expression) {
                const func = getContainingFunction(node);
                // If the user's code is syntactically correct, the func should always have a star. After all,
                // we are in a yield context.
                if (func && func.asteriskToken) {
                    const expressionType = checkExpressionCached(node.expression, /*contextualMapper*/ undefined);
                    let expressionElementType: Type;
                    const nodeIsYieldStar = !!node.asteriskToken;
                    if (nodeIsYieldStar) {
                        expressionElementType = checkElementTypeOfIterable(expressionType, node.expression);
                    }
                    // There is no point in doing an assignability check if the function
                    // has no explicit return type because the return type is directly computed
                    // from the yield expressions.
                    if (func.type) {
                        const signatureElementType = getElementTypeOfIterableIterator(getTypeFromTypeNode(func.type)) || anyType;
                        if (nodeIsYieldStar) {
                            checkTypeAssignableTo(expressionElementType, signatureElementType, node.expression, /*headMessage*/ undefined);
                        }
                        else {
                            checkTypeAssignableTo(expressionType, signatureElementType, node.expression, /*headMessage*/ undefined);
                        }
                    }
                }
            }

            // Both yield and yield* expressions have type 'any'
            return anyType;
        }

        function checkConditionalExpression(node: ConditionalExpression, contextualMapper?: TypeMapper): Type {
            checkExpression(node.condition);
            const type1 = checkExpression(node.whenTrue, contextualMapper);
            const type2 = checkExpression(node.whenFalse, contextualMapper);
            return getBestChoiceType(type1, type2);
        }

        function typeContainsLiteralFromEnum(type: Type, enumType: EnumType) {
            if (type.flags & TypeFlags.Union) {
                for (const t of (<UnionType>type).types) {
                    if (t.flags & TypeFlags.EnumLiteral && (<EnumLiteralType>t).baseType === enumType) {
                        return true;
                    }
                }
            }
            if (type.flags & TypeFlags.EnumLiteral) {
                return (<EnumLiteralType>type).baseType === enumType;
            }
            return false;
        }

        function isLiteralContextForType(node: Expression, type: Type) {
            if (isLiteralTypeLocation(node)) {
                return true;
            }
            let contextualType = getContextualType(node);
            if (contextualType) {
                if (contextualType.flags & TypeFlags.TypeParameter) {
                    const apparentType = getApparentTypeOfTypeParameter(<TypeParameter>contextualType);
                    // If the type parameter is constrained to the base primitive type we're checking for,
                    // consider this a literal context. For example, given a type parameter 'T extends string',
                    // this causes us to infer string literal types for T.
                    if (type === apparentType) {
                        return true;
                    }
                    contextualType = apparentType;
                }
                if (type.flags & TypeFlags.String) {
                    return maybeTypeOfKind(contextualType, TypeFlags.StringLiteral);
                }
                if (type.flags & TypeFlags.Number) {
                    return maybeTypeOfKind(contextualType, (TypeFlags.NumberLiteral | TypeFlags.EnumLiteral));
                }
                if (type.flags & TypeFlags.Boolean) {
                    return maybeTypeOfKind(contextualType, TypeFlags.BooleanLiteral) && !isTypeAssignableTo(booleanType, contextualType);
                }
                if (type.flags & TypeFlags.Enum) {
                    return typeContainsLiteralFromEnum(contextualType, <EnumType>type);
                }
            }
            return false;
        }

        function checkLiteralExpression(node: Expression): Type {
            if (node.kind === SyntaxKind.NumericLiteral) {
                checkGrammarNumericLiteral(<LiteralExpression>node);
            }
            switch (node.kind) {
                case SyntaxKind.StringLiteral:
                    return isLiteralContextForType(node, stringType) ? getLiteralTypeForText(TypeFlags.StringLiteral, (<LiteralExpression>node).text) : stringType;
                case SyntaxKind.NumericLiteral:
                    return isLiteralContextForType(node, numberType) ? getLiteralTypeForText(TypeFlags.NumberLiteral, (<LiteralExpression>node).text) : numberType;
                case SyntaxKind.TrueKeyword:
                case SyntaxKind.FalseKeyword:
                    return isLiteralContextForType(node, booleanType) ? node.kind === SyntaxKind.TrueKeyword ? trueType : falseType : booleanType;
            }
        }

        function checkTemplateExpression(node: TemplateExpression): Type {
            // We just want to check each expressions, but we are unconcerned with
            // the type of each expression, as any value may be coerced into a string.
            // It is worth asking whether this is what we really want though.
            // A place where we actually *are* concerned with the expressions' types are
            // in tagged templates.
            forEach((<TemplateExpression>node).templateSpans, templateSpan => {
                checkExpression(templateSpan.expression);
            });

            return stringType;
        }

        function checkExpressionWithContextualType(node: Expression, contextualType: Type, contextualMapper?: TypeMapper): Type {
            const saveContextualType = node.contextualType;
            node.contextualType = contextualType;
            const result = checkExpression(node, contextualMapper);
            node.contextualType = saveContextualType;
            return result;
        }

        function checkExpressionCached(node: Expression, contextualMapper?: TypeMapper): Type {
            const links = getNodeLinks(node);
            if (!links.resolvedType) {
                // When computing a type that we're going to cache, we need to ignore any ongoing control flow
                // analysis because variables may have transient types in indeterminable states. Moving flowLoopStart
                // to the top of the stack ensures all transient types are computed from a known point.
                const saveFlowLoopStart = flowLoopStart;
                flowLoopStart = flowLoopCount;
                links.resolvedType = checkExpression(node, contextualMapper);
                flowLoopStart = saveFlowLoopStart;
            }
            return links.resolvedType;
        }

        function checkPropertyAssignment(node: PropertyAssignment, contextualMapper?: TypeMapper): Type {
            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(<ComputedPropertyName>node.name);
            }

            return checkExpression((<PropertyAssignment>node).initializer, contextualMapper);
        }

        function checkObjectLiteralMethod(node: MethodDeclaration, contextualMapper?: TypeMapper): Type {
            // Grammar checking
            checkGrammarMethod(node);

            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(<ComputedPropertyName>node.name);
            }

            const uninstantiatedType = checkFunctionExpressionOrObjectLiteralMethod(node, contextualMapper);
            return instantiateTypeWithSingleGenericCallSignature(node, uninstantiatedType, contextualMapper);
        }

        function instantiateTypeWithSingleGenericCallSignature(node: Expression | MethodDeclaration, type: Type, contextualMapper?: TypeMapper) {
            if (isInferentialContext(contextualMapper)) {
                const signature = getSingleCallSignature(type);
                if (signature && signature.typeParameters) {
                    const contextualType = getApparentTypeOfContextualType(<Expression>node);
                    if (contextualType) {
                        const contextualSignature = getSingleCallSignature(contextualType);
                        if (contextualSignature && !contextualSignature.typeParameters) {
                            return getOrCreateTypeFromSignature(instantiateSignatureInContextOf(signature, contextualSignature, contextualMapper));
                        }
                    }
                }
            }

            return type;
        }

        // Checks an expression and returns its type. The contextualMapper parameter serves two purposes: When
        // contextualMapper is not undefined and not equal to the identityMapper function object it indicates that the
        // expression is being inferentially typed (section 4.15.2 in spec) and provides the type mapper to use in
        // conjunction with the generic contextual type. When contextualMapper is equal to the identityMapper function
        // object, it serves as an indicator that all contained function and arrow expressions should be considered to
        // have the wildcard function type; this form of type check is used during overload resolution to exclude
        // contextually typed function and arrow expressions in the initial phase.
        function checkExpression(node: Expression | QualifiedName, contextualMapper?: TypeMapper): Type {
            let type: Type;
            if (node.kind === SyntaxKind.QualifiedName) {
                type = checkQualifiedName(<QualifiedName>node);
            }
            else {
                const uninstantiatedType = checkExpressionWorker(<Expression>node, contextualMapper);
                type = instantiateTypeWithSingleGenericCallSignature(<Expression>node, uninstantiatedType, contextualMapper);
            }

            if (isConstEnumObjectType(type)) {
                // enum object type for const enums are only permitted in:
                // - 'left' in property access
                // - 'object' in indexed access
                // - target in rhs of import statement
                const ok =
                    (node.parent.kind === SyntaxKind.PropertyAccessExpression && (<PropertyAccessExpression>node.parent).expression === node) ||
                    (node.parent.kind === SyntaxKind.ElementAccessExpression && (<ElementAccessExpression>node.parent).expression === node) ||
                    ((node.kind === SyntaxKind.Identifier || node.kind === SyntaxKind.QualifiedName) && isInRightSideOfImportOrExportAssignment(<Identifier>node));

                if (!ok) {
                    error(node, Diagnostics.const_enums_can_only_be_used_in_property_or_index_access_expressions_or_the_right_hand_side_of_an_import_declaration_or_export_assignment);
                }
            }
            return type;
        }

        function checkExpressionWorker(node: Expression, contextualMapper: TypeMapper): Type {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                    return checkIdentifier(<Identifier>node);
                case SyntaxKind.ThisKeyword:
                    return checkThisExpression(node);
                case SyntaxKind.SuperKeyword:
                    return checkSuperExpression(node);
                case SyntaxKind.NullKeyword:
                    return nullWideningType;
                case SyntaxKind.StringLiteral:
                case SyntaxKind.NumericLiteral:
                case SyntaxKind.TrueKeyword:
                case SyntaxKind.FalseKeyword:
                    return checkLiteralExpression(node);
                case SyntaxKind.TemplateExpression:
                    return checkTemplateExpression(<TemplateExpression>node);
                case SyntaxKind.NoSubstitutionTemplateLiteral:
                    return stringType;
                case SyntaxKind.RegularExpressionLiteral:
                    return globalRegExpType;
                case SyntaxKind.ArrayLiteralExpression:
                    return checkArrayLiteral(<ArrayLiteralExpression>node, contextualMapper);
                case SyntaxKind.ObjectLiteralExpression:
                    return checkObjectLiteral(<ObjectLiteralExpression>node, contextualMapper);
                case SyntaxKind.PropertyAccessExpression:
                    return checkPropertyAccessExpression(<PropertyAccessExpression>node);
                case SyntaxKind.ElementAccessExpression:
                    return checkIndexedAccess(<ElementAccessExpression>node);
                case SyntaxKind.CallExpression:
                case SyntaxKind.NewExpression:
                    return checkCallExpression(<CallExpression>node);
                case SyntaxKind.TaggedTemplateExpression:
                    return checkTaggedTemplateExpression(<TaggedTemplateExpression>node);
                case SyntaxKind.ParenthesizedExpression:
                    return checkExpression((<ParenthesizedExpression>node).expression, contextualMapper);
                case SyntaxKind.ClassExpression:
                    return checkClassExpression(<ClassExpression>node);
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    return checkFunctionExpressionOrObjectLiteralMethod(<FunctionExpression>node, contextualMapper);
                case SyntaxKind.TypeOfExpression:
                    return checkTypeOfExpression(<TypeOfExpression>node);
                case SyntaxKind.TypeAssertionExpression:
                case SyntaxKind.AsExpression:
                    return checkAssertion(<AssertionExpression>node);
                case SyntaxKind.NonNullExpression:
                    return checkNonNullAssertion(<NonNullExpression>node);
                case SyntaxKind.DeleteExpression:
                    return checkDeleteExpression(<DeleteExpression>node);
                case SyntaxKind.VoidExpression:
                    return checkVoidExpression(<VoidExpression>node);
                case SyntaxKind.AwaitExpression:
                    return checkAwaitExpression(<AwaitExpression>node);
                case SyntaxKind.PrefixUnaryExpression:
                    return checkPrefixUnaryExpression(<PrefixUnaryExpression>node);
                case SyntaxKind.PostfixUnaryExpression:
                    return checkPostfixUnaryExpression(<PostfixUnaryExpression>node);
                case SyntaxKind.BinaryExpression:
                    return checkBinaryExpression(<BinaryExpression>node, contextualMapper);
                case SyntaxKind.ConditionalExpression:
                    return checkConditionalExpression(<ConditionalExpression>node, contextualMapper);
                case SyntaxKind.SpreadElementExpression:
                    return checkSpreadElementExpression(<SpreadElementExpression>node, contextualMapper);
                case SyntaxKind.OmittedExpression:
                    return undefinedWideningType;
                case SyntaxKind.YieldExpression:
                    return checkYieldExpression(<YieldExpression>node);
                case SyntaxKind.JsxExpression:
                    return checkJsxExpression(<JsxExpression>node);
                case SyntaxKind.JsxElement:
                    return checkJsxElement(<JsxElement>node);
                case SyntaxKind.JsxSelfClosingElement:
                    return checkJsxSelfClosingElement(<JsxSelfClosingElement>node);
                case SyntaxKind.JsxOpeningElement:
                    Debug.fail("Shouldn't ever directly check a JsxOpeningElement");
            }
            return unknownType;
        }

        // DECLARATION AND STATEMENT TYPE CHECKING

        function checkTypeParameter(node: TypeParameterDeclaration) {
            // Grammar Checking
            if (node.expression) {
                grammarErrorOnFirstToken(node.expression, Diagnostics.Type_expected);
            }

            checkSourceElement(node.constraint);
            getConstraintOfTypeParameter(getDeclaredTypeOfTypeParameter(getSymbolOfNode(node)));
            if (produceDiagnostics) {
                checkTypeNameIsReserved(node.name, Diagnostics.Type_parameter_name_cannot_be_0);
            }
        }

        function checkParameter(node: ParameterDeclaration) {
            // Grammar checking
            // It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the
            // Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code
            // or if its FunctionBody is strict code(11.1.5).

            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node);

            checkVariableLikeDeclaration(node);
            let func = getContainingFunction(node);
            if (node.flags & NodeFlags.ParameterPropertyModifier) {
                func = getContainingFunction(node);
                if (!(func.kind === SyntaxKind.Constructor && nodeIsPresent(func.body))) {
                    error(node, Diagnostics.A_parameter_property_is_only_allowed_in_a_constructor_implementation);
                }
            }
            if (node.questionToken && isBindingPattern(node.name) && func.body) {
                error(node, Diagnostics.A_binding_pattern_parameter_cannot_be_optional_in_an_implementation_signature);
            }
            if ((<Identifier>node.name).text === "this") {
                if (indexOf(func.parameters, node) !== 0) {
                    error(node, Diagnostics.A_this_parameter_must_be_the_first_parameter);
                }
                if (func.kind === SyntaxKind.Constructor || func.kind === SyntaxKind.ConstructSignature || func.kind === SyntaxKind.ConstructorType) {
                    error(node, Diagnostics.A_constructor_cannot_have_a_this_parameter);
                }
            }

            // Only check rest parameter type if it's not a binding pattern. Since binding patterns are
            // not allowed in a rest parameter, we already have an error from checkGrammarParameterList.
            if (node.dotDotDotToken && !isBindingPattern(node.name) && !isArrayType(getTypeOfSymbol(node.symbol))) {
                error(node, Diagnostics.A_rest_parameter_must_be_of_an_array_type);
            }
        }

        function isSyntacticallyValidGenerator(node: SignatureDeclaration): boolean {
            if (!(<FunctionLikeDeclaration>node).asteriskToken || !(<FunctionLikeDeclaration>node).body) {
                return false;
            }

            return node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.FunctionDeclaration ||
                node.kind === SyntaxKind.FunctionExpression;
        }

        function getTypePredicateParameterIndex(parameterList: NodeArray<ParameterDeclaration>, parameter: Identifier): number {
            if (parameterList) {
                for (let i = 0; i < parameterList.length; i++) {
                    const param = parameterList[i];
                    if (param.name.kind === SyntaxKind.Identifier &&
                        (<Identifier>param.name).text === parameter.text) {

                        return i;
                    }
                }
            }
            return -1;
        }

        function checkTypePredicate(node: TypePredicateNode): void {
            const parent = getTypePredicateParent(node);
            if (!parent) {
                // The parent must not be valid.
                error(node, Diagnostics.A_type_predicate_is_only_allowed_in_return_type_position_for_functions_and_methods);
                return;
            }

            const typePredicate = getSignatureFromDeclaration(parent).typePredicate;
            if (!typePredicate) {
                return;
            }

            const { parameterName } = node;
            if (isThisTypePredicate(typePredicate)) {
                getTypeFromThisTypeNode(parameterName as ThisTypeNode);
            }
            else {
                if (typePredicate.parameterIndex >= 0) {
                    if (parent.parameters[typePredicate.parameterIndex].dotDotDotToken) {
                        error(parameterName,
                            Diagnostics.A_type_predicate_cannot_reference_a_rest_parameter);
                    }
                    else {
                        const leadingError = chainDiagnosticMessages(undefined, Diagnostics.A_type_predicate_s_type_must_be_assignable_to_its_parameter_s_type);
                        checkTypeAssignableTo(typePredicate.type,
                            getTypeOfNode(parent.parameters[typePredicate.parameterIndex]),
                            node.type,
                            /*headMessage*/ undefined,
                            leadingError);
                    }
                }
                else if (parameterName) {
                    let hasReportedError = false;
                    for (const { name } of parent.parameters) {
                        if (isBindingPattern(name) &&
                                checkIfTypePredicateVariableIsDeclaredInBindingPattern(name, parameterName, typePredicate.parameterName)) {
                            hasReportedError = true;
                            break;
                        }
                    }
                    if (!hasReportedError) {
                        error(node.parameterName, Diagnostics.Cannot_find_parameter_0, typePredicate.parameterName);
                    }
                }
            }
        }

        function getTypePredicateParent(node: Node): SignatureDeclaration {
            switch (node.parent.kind) {
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.CallSignature:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.FunctionType:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                    const parent = <SignatureDeclaration>node.parent;
                    if (node === parent.type) {
                        return parent;
                    }
            }
        }

        function checkIfTypePredicateVariableIsDeclaredInBindingPattern(
            pattern: BindingPattern,
            predicateVariableNode: Node,
            predicateVariableName: string) {
            for (const { name } of pattern.elements) {
                if (name.kind === SyntaxKind.Identifier &&
                    (<Identifier>name).text === predicateVariableName) {
                    error(predicateVariableNode,
                        Diagnostics.A_type_predicate_cannot_reference_element_0_in_a_binding_pattern,
                        predicateVariableName);
                    return true;
                }
                else if (name.kind === SyntaxKind.ArrayBindingPattern ||
                    name.kind === SyntaxKind.ObjectBindingPattern) {
                    if (checkIfTypePredicateVariableIsDeclaredInBindingPattern(
                        <BindingPattern>name,
                        predicateVariableNode,
                        predicateVariableName)) {
                        return true;
                    }
                }
            }
        }

        function checkSignatureDeclaration(node: SignatureDeclaration) {
            // Grammar checking
            if (node.kind === SyntaxKind.IndexSignature) {
                checkGrammarIndexSignature(<SignatureDeclaration>node);
            }
            // TODO (yuisu): Remove this check in else-if when SyntaxKind.Construct is moved and ambient context is handled
            else if (node.kind === SyntaxKind.FunctionType || node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.ConstructorType ||
                node.kind === SyntaxKind.CallSignature || node.kind === SyntaxKind.Constructor ||
                node.kind === SyntaxKind.ConstructSignature) {
                checkGrammarFunctionLikeDeclaration(<FunctionLikeDeclaration>node);
            }

            checkTypeParameters(node.typeParameters);

            forEach(node.parameters, checkParameter);

            if (node.type) {
                checkSourceElement(node.type);
            }

            if (produceDiagnostics) {
                checkCollisionWithArgumentsInGeneratedCode(node);
                if (compilerOptions.noImplicitAny && !node.type) {
                    switch (node.kind) {
                        case SyntaxKind.ConstructSignature:
                            error(node, Diagnostics.Construct_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type);
                            break;
                        case SyntaxKind.CallSignature:
                            error(node, Diagnostics.Call_signature_which_lacks_return_type_annotation_implicitly_has_an_any_return_type);
                            break;
                    }
                }

                if (node.type) {
                    if (languageVersion >= ScriptTarget.ES6 && isSyntacticallyValidGenerator(node)) {
                        const returnType = getTypeFromTypeNode(node.type);
                        if (returnType === voidType) {
                            error(node.type, Diagnostics.A_generator_cannot_have_a_void_type_annotation);
                        }
                        else {
                            const generatorElementType = getElementTypeOfIterableIterator(returnType) || anyType;
                            const iterableIteratorInstantiation = createIterableIteratorType(generatorElementType);

                            // Naively, one could check that IterableIterator<any> is assignable to the return type annotation.
                            // However, that would not catch the error in the following case.
                            //
                            //    interface BadGenerator extends Iterable<number>, Iterator<string> { }
                            //    function* g(): BadGenerator { } // Iterable and Iterator have different types!
                            //
                            checkTypeAssignableTo(iterableIteratorInstantiation, returnType, node.type);
                        }
                    }
                    else if (isAsyncFunctionLike(node)) {
                        checkAsyncFunctionReturnType(<FunctionLikeDeclaration>node);
                    }
                }
                if (noUnusedIdentifiers && !(<FunctionDeclaration>node).body) {
                    checkUnusedTypeParameters(node);
                }
            }
        }

        function checkClassForDuplicateDeclarations(node: ClassLikeDeclaration) {
            const getter = 1, setter = 2, property = getter | setter;

            const instanceNames = createMap<number>();
            const staticNames = createMap<number>();
            for (const member of node.members) {
                if (member.kind === SyntaxKind.Constructor) {
                    for (const param of (member as ConstructorDeclaration).parameters) {
                        if (isParameterPropertyDeclaration(param)) {
                            addName(instanceNames, param.name, (param.name as Identifier).text, property);
                        }
                    }
                }
                else {
                    const isStatic = forEach(member.modifiers, m => m.kind === SyntaxKind.StaticKeyword);
                    const names = isStatic ? staticNames : instanceNames;

                    const memberName = member.name && getPropertyNameForPropertyNameNode(member.name);
                    if (memberName) {
                        switch (member.kind) {
                            case SyntaxKind.GetAccessor:
                                addName(names, member.name, memberName, getter);
                                break;

                            case SyntaxKind.SetAccessor:
                                addName(names, member.name, memberName, setter);
                                break;

                            case SyntaxKind.PropertyDeclaration:
                                addName(names, member.name, memberName, property);
                                break;
                        }
                    }
                }
            }

            function addName(names: Map<number>, location: Node, name: string, meaning: number) {
                const prev = names[name];
                if (prev) {
                    if (prev & meaning) {
                        error(location, Diagnostics.Duplicate_identifier_0, getTextOfNode(location));
                    }
                    else {
                        names[name] = prev | meaning;
                    }
                }
                else {
                    names[name] = meaning;
                }
            }
        }

        function checkObjectTypeForDuplicateDeclarations(node: TypeLiteralNode | InterfaceDeclaration) {
            const names = createMap<boolean>();
            for (const member of node.members) {
                if (member.kind == SyntaxKind.PropertySignature) {
                    let memberName: string;
                    switch (member.name.kind) {
                        case SyntaxKind.StringLiteral:
                        case SyntaxKind.NumericLiteral:
                        case SyntaxKind.Identifier:
                            memberName = (member.name as LiteralExpression | Identifier).text;
                            break;
                        default:
                            continue;
                    }

                    if (names[memberName]) {
                        error(member.symbol.valueDeclaration.name, Diagnostics.Duplicate_identifier_0, memberName);
                        error(member.name, Diagnostics.Duplicate_identifier_0, memberName);
                    }
                    else {
                        names[memberName] = true;
                    }
                }
            }
        }

        function checkTypeForDuplicateIndexSignatures(node: Node) {
            if (node.kind === SyntaxKind.InterfaceDeclaration) {
                const nodeSymbol = getSymbolOfNode(node);
                // in case of merging interface declaration it is possible that we'll enter this check procedure several times for every declaration
                // to prevent this run check only for the first declaration of a given kind
                if (nodeSymbol.declarations.length > 0 && nodeSymbol.declarations[0] !== node) {
                    return;
                }
            }

            // TypeScript 1.0 spec (April 2014)
            // 3.7.4: An object type can contain at most one string index signature and one numeric index signature.
            // 8.5: A class declaration can have at most one string index member declaration and one numeric index member declaration
            const indexSymbol = getIndexSymbol(getSymbolOfNode(node));
            if (indexSymbol) {
                let seenNumericIndexer = false;
                let seenStringIndexer = false;
                for (const decl of indexSymbol.declarations) {
                    const declaration = <SignatureDeclaration>decl;
                    if (declaration.parameters.length === 1 && declaration.parameters[0].type) {
                        switch (declaration.parameters[0].type.kind) {
                            case SyntaxKind.StringKeyword:
                                if (!seenStringIndexer) {
                                    seenStringIndexer = true;
                                }
                                else {
                                    error(declaration, Diagnostics.Duplicate_string_index_signature);
                                }
                                break;
                            case SyntaxKind.NumberKeyword:
                                if (!seenNumericIndexer) {
                                    seenNumericIndexer = true;
                                }
                                else {
                                    error(declaration, Diagnostics.Duplicate_number_index_signature);
                                }
                                break;
                        }
                    }
                }
            }
        }

        function checkPropertyDeclaration(node: PropertyDeclaration) {
            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node) || checkGrammarProperty(node) || checkGrammarComputedPropertyName(node.name);

            checkVariableLikeDeclaration(node);
        }

        function checkMethodDeclaration(node: MethodDeclaration) {
            // Grammar checking
            checkGrammarMethod(node) || checkGrammarComputedPropertyName(node.name);

            // Grammar checking for modifiers is done inside the function checkGrammarFunctionLikeDeclaration
            checkFunctionOrMethodDeclaration(node);

            // Abstract methods cannot have an implementation.
            // Extra checks are to avoid reporting multiple errors relating to the "abstractness" of the node.
            if (node.flags & NodeFlags.Abstract && node.body) {
                error(node, Diagnostics.Method_0_cannot_have_an_implementation_because_it_is_marked_abstract, declarationNameToString(node.name));
            }
        }

        function checkConstructorDeclaration(node: ConstructorDeclaration) {
            // Grammar check on signature of constructor and modifier of the constructor is done in checkSignatureDeclaration function.
            checkSignatureDeclaration(node);
            // Grammar check for checking only related to constructorDeclaration
            checkGrammarConstructorTypeParameters(node) || checkGrammarConstructorTypeAnnotation(node);

            checkSourceElement(node.body);
            registerForUnusedIdentifiersCheck(node);

            const symbol = getSymbolOfNode(node);
            const firstDeclaration = getDeclarationOfKind(symbol, node.kind);

            // Only type check the symbol once
            if (node === firstDeclaration) {
                checkFunctionOrConstructorSymbol(symbol);
            }

            // exit early in the case of signature - super checks are not relevant to them
            if (nodeIsMissing(node.body)) {
                return;
            }

            if (!produceDiagnostics) {
                return;
            }

            function containsSuperCallAsComputedPropertyName(n: Declaration): boolean {
                return n.name && containsSuperCall(n.name);
            }

            function containsSuperCall(n: Node): boolean {
                if (isSuperCallExpression(n)) {
                    return true;
                }
                else if (isFunctionLike(n)) {
                    return false;
                }
                else if (isClassLike(n)) {
                    return forEach((<ClassLikeDeclaration>n).members, containsSuperCallAsComputedPropertyName);
                }
                return forEachChild(n, containsSuperCall);
            }

            function markThisReferencesAsErrors(n: Node): void {
                if (n.kind === SyntaxKind.ThisKeyword) {
                    error(n, Diagnostics.this_cannot_be_referenced_in_current_location);
                }
                else if (n.kind !== SyntaxKind.FunctionExpression && n.kind !== SyntaxKind.FunctionDeclaration) {
                    forEachChild(n, markThisReferencesAsErrors);
                }
            }

            function isInstancePropertyWithInitializer(n: Node): boolean {
                return n.kind === SyntaxKind.PropertyDeclaration &&
                    !(n.flags & NodeFlags.Static) &&
                    !!(<PropertyDeclaration>n).initializer;
            }

            // TS 1.0 spec (April 2014): 8.3.2
            // Constructors of classes with no extends clause may not contain super calls, whereas
            // constructors of derived classes must contain at least one super call somewhere in their function body.
            const containingClassDecl = <ClassDeclaration>node.parent;
            if (getClassExtendsHeritageClauseElement(containingClassDecl)) {
                const classExtendsNull = classDeclarationExtendsNull(containingClassDecl);
                const superCall = getSuperCallInConstructor(node);
                if (superCall) {
                    if (classExtendsNull) {
                        error(superCall, Diagnostics.A_constructor_cannot_contain_a_super_call_when_its_class_extends_null);
                    }

                    // The first statement in the body of a constructor (excluding prologue directives) must be a super call
                    // if both of the following are true:
                    // - The containing class is a derived class.
                    // - The constructor declares parameter properties
                    //   or the containing class declares instance member variables with initializers.
                    const superCallShouldBeFirst =
                        forEach((<ClassDeclaration>node.parent).members, isInstancePropertyWithInitializer) ||
                        forEach(node.parameters, p => p.flags & NodeFlags.ParameterPropertyModifier);

                    // Skip past any prologue directives to find the first statement
                    // to ensure that it was a super call.
                    if (superCallShouldBeFirst) {
                        const statements = (<Block>node.body).statements;
                        let superCallStatement: ExpressionStatement;

                        for (const statement of statements) {
                            if (statement.kind === SyntaxKind.ExpressionStatement && isSuperCallExpression((<ExpressionStatement>statement).expression)) {
                                superCallStatement = <ExpressionStatement>statement;
                                break;
                            }
                            if (!isPrologueDirective(statement)) {
                                break;
                            }
                        }
                        if (!superCallStatement) {
                            error(node, Diagnostics.A_super_call_must_be_the_first_statement_in_the_constructor_when_a_class_contains_initialized_properties_or_has_parameter_properties);
                        }
                    }
                }
                else if (!classExtendsNull) {
                    error(node, Diagnostics.Constructors_for_derived_classes_must_contain_a_super_call);
                }
            }
        }

        function checkAccessorDeclaration(node: AccessorDeclaration) {
            if (produceDiagnostics) {
                // Grammar checking accessors
                checkGrammarFunctionLikeDeclaration(node) || checkGrammarAccessor(node) || checkGrammarComputedPropertyName(node.name);

                checkDecorators(node);
                checkSignatureDeclaration(node);
                if (node.kind === SyntaxKind.GetAccessor) {
                    if (!isInAmbientContext(node) && nodeIsPresent(node.body) && (node.flags & NodeFlags.HasImplicitReturn)) {
                        if (node.flags & NodeFlags.HasExplicitReturn) {
                            if (compilerOptions.noImplicitReturns) {
                                error(node.name, Diagnostics.Not_all_code_paths_return_a_value);
                            }
                        }
                        else {
                            error(node.name, Diagnostics.A_get_accessor_must_return_a_value);
                        }
                    }
                }
                // Do not use hasDynamicName here, because that returns false for well known symbols.
                // We want to perform checkComputedPropertyName for all computed properties, including
                // well known symbols.
                if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(<ComputedPropertyName>node.name);
                }
                if (!hasDynamicName(node)) {
                    // TypeScript 1.0 spec (April 2014): 8.4.3
                    // Accessors for the same member name must specify the same accessibility.
                    const otherKind = node.kind === SyntaxKind.GetAccessor ? SyntaxKind.SetAccessor : SyntaxKind.GetAccessor;
                    const otherAccessor = <AccessorDeclaration>getDeclarationOfKind(node.symbol, otherKind);
                    if (otherAccessor) {
                        if (((node.flags & NodeFlags.AccessibilityModifier) !== (otherAccessor.flags & NodeFlags.AccessibilityModifier))) {
                            error(node.name, Diagnostics.Getter_and_setter_accessors_do_not_agree_in_visibility);
                        }
                        if (((node.flags & NodeFlags.Abstract) !== (otherAccessor.flags & NodeFlags.Abstract))) {
                            error(node.name, Diagnostics.Accessors_must_both_be_abstract_or_non_abstract);
                        }

                        // TypeScript 1.0 spec (April 2014): 4.5
                        // If both accessors include type annotations, the specified types must be identical.
                        checkAccessorDeclarationTypesIdentical(node, otherAccessor, getAnnotatedAccessorType, Diagnostics.get_and_set_accessor_must_have_the_same_type);
                        checkAccessorDeclarationTypesIdentical(node, otherAccessor, getThisTypeOfDeclaration, Diagnostics.get_and_set_accessor_must_have_the_same_this_type);
                    }
                }
                getTypeOfAccessors(getSymbolOfNode(node));
            }
            if (node.parent.kind !== SyntaxKind.ObjectLiteralExpression) {
                checkSourceElement(node.body);
                registerForUnusedIdentifiersCheck(node);
            }
            else {
                checkNodeDeferred(node);
            }
        }

        function checkAccessorDeclarationTypesIdentical(first: AccessorDeclaration, second: AccessorDeclaration, getAnnotatedType: (a: AccessorDeclaration) => Type, message: DiagnosticMessage) {
            const firstType = getAnnotatedType(first);
            const secondType = getAnnotatedType(second);
            if (firstType && secondType && !isTypeIdenticalTo(firstType, secondType)) {
                error(first, message);
            }
        }

        function checkAccessorDeferred(node: AccessorDeclaration) {
            checkSourceElement(node.body);
            registerForUnusedIdentifiersCheck(node);
        }

        function checkMissingDeclaration(node: Node) {
            checkDecorators(node);
        }

        function checkTypeArgumentConstraints(typeParameters: TypeParameter[], typeArgumentNodes: TypeNode[]): boolean {
            let typeArguments: Type[];
            let mapper: TypeMapper;
            let result = true;
            for (let i = 0; i < typeParameters.length; i++) {
                const constraint = getConstraintOfTypeParameter(typeParameters[i]);
                if (constraint) {
                    if (!typeArguments) {
                        typeArguments = map(typeArgumentNodes, getTypeFromTypeNode);
                        mapper = createTypeMapper(typeParameters, typeArguments);
                    }
                    const typeArgument = typeArguments[i];
                    result = result && checkTypeAssignableTo(
                        typeArgument,
                        getTypeWithThisArgument(instantiateType(constraint, mapper), typeArgument),
                        typeArgumentNodes[i],
                        Diagnostics.Type_0_does_not_satisfy_the_constraint_1);
                }
            }
            return result;
        }

        function checkTypeReferenceNode(node: TypeReferenceNode | ExpressionWithTypeArguments) {
            checkGrammarTypeArguments(node, node.typeArguments);
            const type = getTypeFromTypeReference(node);
            if (type !== unknownType) {
                if (node.typeArguments) {
                    // Do type argument local checks only if referenced type is successfully resolved
                    forEach(node.typeArguments, checkSourceElement);
                    if (produceDiagnostics) {
                        const symbol = getNodeLinks(node).resolvedSymbol;
                        const typeParameters = symbol.flags & SymbolFlags.TypeAlias ? getSymbolLinks(symbol).typeParameters : (<TypeReference>type).target.localTypeParameters;
                        checkTypeArgumentConstraints(typeParameters, node.typeArguments);
                    }
                }
                if (type.flags & TypeFlags.Enum && !(<EnumType>type).memberTypes && getNodeLinks(node).resolvedSymbol.flags & SymbolFlags.EnumMember) {
                    error(node, Diagnostics.Enum_type_0_has_members_with_initializers_that_are_not_literals, typeToString(type));
                }
            }
        }

        function checkTypeQuery(node: TypeQueryNode) {
            getTypeFromTypeQueryNode(node);
        }

        function checkTypeLiteral(node: TypeLiteralNode) {
            forEach(node.members, checkSourceElement);
            if (produceDiagnostics) {
                const type = getTypeFromTypeLiteralOrFunctionOrConstructorTypeNode(node);
                checkIndexConstraints(type);
                checkTypeForDuplicateIndexSignatures(node);
                checkObjectTypeForDuplicateDeclarations(node);
            }
        }

        function checkArrayType(node: ArrayTypeNode) {
            checkSourceElement(node.elementType);
        }

        function checkTupleType(node: TupleTypeNode) {
            // Grammar checking
            const hasErrorFromDisallowedTrailingComma = checkGrammarForDisallowedTrailingComma(node.elementTypes);
            if (!hasErrorFromDisallowedTrailingComma && node.elementTypes.length === 0) {
                grammarErrorOnNode(node, Diagnostics.A_tuple_type_element_list_cannot_be_empty);
            }

            forEach(node.elementTypes, checkSourceElement);
        }

        function checkUnionOrIntersectionType(node: UnionOrIntersectionTypeNode) {
            forEach(node.types, checkSourceElement);
        }

        function isPrivateWithinAmbient(node: Node): boolean {
            return (node.flags & NodeFlags.Private) && isInAmbientContext(node);
        }

        function getEffectiveDeclarationFlags(n: Node, flagsToCheck: NodeFlags): NodeFlags {
            let flags = getCombinedNodeFlags(n);

            // children of classes (even ambient classes) should not be marked as ambient or export
            // because those flags have no useful semantics there.
            if (n.parent.kind !== SyntaxKind.InterfaceDeclaration &&
                n.parent.kind !== SyntaxKind.ClassDeclaration &&
                n.parent.kind !== SyntaxKind.ClassExpression &&
                isInAmbientContext(n)) {
                if (!(flags & NodeFlags.Ambient)) {
                    // It is nested in an ambient context, which means it is automatically exported
                    flags |= NodeFlags.Export;
                }
                flags |= NodeFlags.Ambient;
            }

            return flags & flagsToCheck;
        }

        function checkFunctionOrConstructorSymbol(symbol: Symbol): void {
            if (!produceDiagnostics) {
                return;
            }

            function getCanonicalOverload(overloads: Declaration[], implementation: FunctionLikeDeclaration) {
                // Consider the canonical set of flags to be the flags of the bodyDeclaration or the first declaration
                // Error on all deviations from this canonical set of flags
                // The caveat is that if some overloads are defined in lib.d.ts, we don't want to
                // report the errors on those. To achieve this, we will say that the implementation is
                // the canonical signature only if it is in the same container as the first overload
                const implementationSharesContainerWithFirstOverload = implementation !== undefined && implementation.parent === overloads[0].parent;
                return implementationSharesContainerWithFirstOverload ? implementation : overloads[0];
            }

            function checkFlagAgreementBetweenOverloads(overloads: Declaration[], implementation: FunctionLikeDeclaration, flagsToCheck: NodeFlags, someOverloadFlags: NodeFlags, allOverloadFlags: NodeFlags): void {
                // Error if some overloads have a flag that is not shared by all overloads. To find the
                // deviations, we XOR someOverloadFlags with allOverloadFlags
                const someButNotAllOverloadFlags = someOverloadFlags ^ allOverloadFlags;
                if (someButNotAllOverloadFlags !== 0) {
                    const canonicalFlags = getEffectiveDeclarationFlags(getCanonicalOverload(overloads, implementation), flagsToCheck);

                    forEach(overloads, o => {
                        const deviation = getEffectiveDeclarationFlags(o, flagsToCheck) ^ canonicalFlags;
                        if (deviation & NodeFlags.Export) {
                            error(o.name, Diagnostics.Overload_signatures_must_all_be_exported_or_non_exported);
                        }
                        else if (deviation & NodeFlags.Ambient) {
                            error(o.name, Diagnostics.Overload_signatures_must_all_be_ambient_or_non_ambient);
                        }
                        else if (deviation & (NodeFlags.Private | NodeFlags.Protected)) {
                            error(o.name || o, Diagnostics.Overload_signatures_must_all_be_public_private_or_protected);
                        }
                        else if (deviation & NodeFlags.Abstract) {
                            error(o.name, Diagnostics.Overload_signatures_must_all_be_abstract_or_non_abstract);
                        }
                    });
                }
            }

            function checkQuestionTokenAgreementBetweenOverloads(overloads: Declaration[], implementation: FunctionLikeDeclaration, someHaveQuestionToken: boolean, allHaveQuestionToken: boolean): void {
                if (someHaveQuestionToken !== allHaveQuestionToken) {
                    const canonicalHasQuestionToken = hasQuestionToken(getCanonicalOverload(overloads, implementation));
                    forEach(overloads, o => {
                        const deviation = hasQuestionToken(o) !== canonicalHasQuestionToken;
                        if (deviation) {
                            error(o.name, Diagnostics.Overload_signatures_must_all_be_optional_or_required);
                        }
                    });
                }
            }

            const flagsToCheck: NodeFlags = NodeFlags.Export | NodeFlags.Ambient | NodeFlags.Private | NodeFlags.Protected | NodeFlags.Abstract;
            let someNodeFlags: NodeFlags = 0;
            let allNodeFlags = flagsToCheck;
            let someHaveQuestionToken = false;
            let allHaveQuestionToken = true;
            let hasOverloads = false;
            let bodyDeclaration: FunctionLikeDeclaration;
            let lastSeenNonAmbientDeclaration: FunctionLikeDeclaration;
            let previousDeclaration: FunctionLikeDeclaration;

            const declarations = symbol.declarations;
            const isConstructor = (symbol.flags & SymbolFlags.Constructor) !== 0;

            function reportImplementationExpectedError(node: FunctionLikeDeclaration): void {
                if (node.name && nodeIsMissing(node.name)) {
                    return;
                }

                let seen = false;
                const subsequentNode = forEachChild(node.parent, c => {
                    if (seen) {
                        return c;
                    }
                    else {
                        seen = c === node;
                    }
                });
                // We may be here because of some extra nodes between overloads that could not be parsed into a valid node.
                // In this case the subsequent node is not really consecutive (.pos !== node.end), and we must ignore it here.
                if (subsequentNode && subsequentNode.pos === node.end) {
                    if (subsequentNode.kind === node.kind) {
                        const errorNode: Node = (<FunctionLikeDeclaration>subsequentNode).name || subsequentNode;
                        // TODO(jfreeman): These are methods, so handle computed name case
                        if (node.name && (<FunctionLikeDeclaration>subsequentNode).name && (<Identifier>node.name).text === (<Identifier>(<FunctionLikeDeclaration>subsequentNode).name).text) {
                            const reportError =
                                (node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.MethodSignature) &&
                                (node.flags & NodeFlags.Static) !== (subsequentNode.flags & NodeFlags.Static);
                            // we can get here in two cases
                            // 1. mixed static and instance class members
                            // 2. something with the same name was defined before the set of overloads that prevents them from merging
                            // here we'll report error only for the first case since for second we should already report error in binder
                            if (reportError) {
                                const diagnostic = node.flags & NodeFlags.Static ? Diagnostics.Function_overload_must_be_static : Diagnostics.Function_overload_must_not_be_static;
                                error(errorNode, diagnostic);
                            }
                            return;
                        }
                        else if (nodeIsPresent((<FunctionLikeDeclaration>subsequentNode).body)) {
                            error(errorNode, Diagnostics.Function_implementation_name_must_be_0, declarationNameToString(node.name));
                            return;
                        }
                    }
                }
                const errorNode: Node = node.name || node;
                if (isConstructor) {
                    error(errorNode, Diagnostics.Constructor_implementation_is_missing);
                }
                else {
                    // Report different errors regarding non-consecutive blocks of declarations depending on whether
                    // the node in question is abstract.
                    if (node.flags & NodeFlags.Abstract) {
                        error(errorNode, Diagnostics.All_declarations_of_an_abstract_method_must_be_consecutive);
                    }
                    else {
                        error(errorNode, Diagnostics.Function_implementation_is_missing_or_not_immediately_following_the_declaration);
                    }
                }
            }

            let duplicateFunctionDeclaration = false;
            let multipleConstructorImplementation = false;
            for (const current of declarations) {
                const node = <FunctionLikeDeclaration>current;
                const inAmbientContext = isInAmbientContext(node);
                const inAmbientContextOrInterface = node.parent.kind === SyntaxKind.InterfaceDeclaration || node.parent.kind === SyntaxKind.TypeLiteral || inAmbientContext;
                if (inAmbientContextOrInterface) {
                    // check if declarations are consecutive only if they are non-ambient
                    // 1. ambient declarations can be interleaved
                    // i.e. this is legal
                    //     declare function foo();
                    //     declare function bar();
                    //     declare function foo();
                    // 2. mixing ambient and non-ambient declarations is a separate error that will be reported - do not want to report an extra one
                    previousDeclaration = undefined;
                }

                if (node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.MethodSignature || node.kind === SyntaxKind.Constructor) {
                    const currentNodeFlags = getEffectiveDeclarationFlags(node, flagsToCheck);
                    someNodeFlags |= currentNodeFlags;
                    allNodeFlags &= currentNodeFlags;
                    someHaveQuestionToken = someHaveQuestionToken || hasQuestionToken(node);
                    allHaveQuestionToken = allHaveQuestionToken && hasQuestionToken(node);

                    if (nodeIsPresent(node.body) && bodyDeclaration) {
                        if (isConstructor) {
                            multipleConstructorImplementation = true;
                        }
                        else {
                            duplicateFunctionDeclaration = true;
                        }
                    }
                    else if (previousDeclaration && previousDeclaration.parent === node.parent && previousDeclaration.end !== node.pos) {
                        reportImplementationExpectedError(previousDeclaration);
                    }

                    if (nodeIsPresent(node.body)) {
                        if (!bodyDeclaration) {
                            bodyDeclaration = node;
                        }
                    }
                    else {
                        hasOverloads = true;
                    }

                    previousDeclaration = node;

                    if (!inAmbientContextOrInterface) {
                        lastSeenNonAmbientDeclaration = node;
                    }
                }
            }

            if (multipleConstructorImplementation) {
                forEach(declarations, declaration => {
                    error(declaration, Diagnostics.Multiple_constructor_implementations_are_not_allowed);
                });
            }

            if (duplicateFunctionDeclaration) {
                forEach(declarations, declaration => {
                    error(declaration.name, Diagnostics.Duplicate_function_implementation);
                });
            }

            // Abstract methods can't have an implementation -- in particular, they don't need one.
            if (lastSeenNonAmbientDeclaration && !lastSeenNonAmbientDeclaration.body &&
                !(lastSeenNonAmbientDeclaration.flags & NodeFlags.Abstract) && !lastSeenNonAmbientDeclaration.questionToken) {
                reportImplementationExpectedError(lastSeenNonAmbientDeclaration);
            }

            if (hasOverloads) {
                checkFlagAgreementBetweenOverloads(declarations, bodyDeclaration, flagsToCheck, someNodeFlags, allNodeFlags);
                checkQuestionTokenAgreementBetweenOverloads(declarations, bodyDeclaration, someHaveQuestionToken, allHaveQuestionToken);

                if (bodyDeclaration) {
                    const signatures = getSignaturesOfSymbol(symbol);
                    const bodySignature = getSignatureFromDeclaration(bodyDeclaration);
                    for (const signature of signatures) {
                        if (!isImplementationCompatibleWithOverload(bodySignature, signature)) {
                            error(signature.declaration, Diagnostics.Overload_signature_is_not_compatible_with_function_implementation);
                            break;
                        }
                    }
                }
            }
        }

        function checkExportsOnMergedDeclarations(node: Node): void {
            if (!produceDiagnostics) {
                return;
            }

            // if localSymbol is defined on node then node itself is exported - check is required
            let symbol = node.localSymbol;
            if (!symbol) {
                // local symbol is undefined => this declaration is non-exported.
                // however symbol might contain other declarations that are exported
                symbol = getSymbolOfNode(node);
                if (!(symbol.flags & SymbolFlags.Export)) {
                    // this is a pure local symbol (all declarations are non-exported) - no need to check anything
                    return;
                }
            }

            // run the check only for the first declaration in the list
            if (getDeclarationOfKind(symbol, node.kind) !== node) {
                return;
            }

            // we use SymbolFlags.ExportValue, SymbolFlags.ExportType and SymbolFlags.ExportNamespace
            // to denote disjoint declarationSpaces (without making new enum type).
            let exportedDeclarationSpaces = SymbolFlags.None;
            let nonExportedDeclarationSpaces = SymbolFlags.None;
            let defaultExportedDeclarationSpaces = SymbolFlags.None;
            for (const d of symbol.declarations) {
                const declarationSpaces = getDeclarationSpaces(d);
                const effectiveDeclarationFlags = getEffectiveDeclarationFlags(d, NodeFlags.Export | NodeFlags.Default);

                if (effectiveDeclarationFlags & NodeFlags.Export) {
                    if (effectiveDeclarationFlags & NodeFlags.Default) {
                        defaultExportedDeclarationSpaces |= declarationSpaces;
                    }
                    else {
                        exportedDeclarationSpaces |= declarationSpaces;
                    }
                }
                else {
                    nonExportedDeclarationSpaces |= declarationSpaces;
                }
            }

            // Spaces for anything not declared a 'default export'.
            const nonDefaultExportedDeclarationSpaces = exportedDeclarationSpaces | nonExportedDeclarationSpaces;

            const commonDeclarationSpacesForExportsAndLocals = exportedDeclarationSpaces & nonExportedDeclarationSpaces;
            const commonDeclarationSpacesForDefaultAndNonDefault = defaultExportedDeclarationSpaces & nonDefaultExportedDeclarationSpaces;

            if (commonDeclarationSpacesForExportsAndLocals || commonDeclarationSpacesForDefaultAndNonDefault) {
                // declaration spaces for exported and non-exported declarations intersect
                for (const d of symbol.declarations) {
                    const declarationSpaces = getDeclarationSpaces(d);

                    // Only error on the declarations that contributed to the intersecting spaces.
                    if (declarationSpaces & commonDeclarationSpacesForDefaultAndNonDefault) {
                        error(d.name, Diagnostics.Merged_declaration_0_cannot_include_a_default_export_declaration_Consider_adding_a_separate_export_default_0_declaration_instead, declarationNameToString(d.name));
                    }
                    else if (declarationSpaces & commonDeclarationSpacesForExportsAndLocals) {
                        error(d.name, Diagnostics.Individual_declarations_in_merged_declaration_0_must_be_all_exported_or_all_local, declarationNameToString(d.name));
                    }
                }
            }

            function getDeclarationSpaces(d: Declaration): SymbolFlags {
                switch (d.kind) {
                    case SyntaxKind.InterfaceDeclaration:
                        return SymbolFlags.ExportType;
                    case SyntaxKind.ModuleDeclaration:
                        return isAmbientModule(d) || getModuleInstanceState(d) !== ModuleInstanceState.NonInstantiated
                            ? SymbolFlags.ExportNamespace | SymbolFlags.ExportValue
                            : SymbolFlags.ExportNamespace;
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.EnumDeclaration:
                        return SymbolFlags.ExportType | SymbolFlags.ExportValue;
                    case SyntaxKind.ImportEqualsDeclaration:
                        let result: SymbolFlags = 0;
                        const target = resolveAlias(getSymbolOfNode(d));
                        forEach(target.declarations, d => { result |= getDeclarationSpaces(d); });
                        return result;
                    default:
                        return SymbolFlags.ExportValue;
                }
            }
        }

        function checkNonThenableType(type: Type, location?: Node, message?: DiagnosticMessage) {
            type = getWidenedType(type);
            if (!isTypeAny(type) && !isTypeNever(type) && isTypeAssignableTo(type, getGlobalThenableType())) {
                if (location) {
                    if (!message) {
                        message = Diagnostics.Operand_for_await_does_not_have_a_valid_callable_then_member;
                    }

                    error(location, message);
                }

                return unknownType;
            }

            return type;
        }

        /**
          * Gets the "promised type" of a promise.
          * @param type The type of the promise.
          * @remarks The "promised type" of a type is the type of the "value" parameter of the "onfulfilled" callback.
          */
        function getPromisedType(promise: Type): Type {
            //
            //  { // promise
            //      then( // thenFunction
            //          onfulfilled: ( // onfulfilledParameterType
            //              value: T // valueParameterType
            //          ) => any
            //      ): any;
            //  }
            //

            if (isTypeAny(promise)) {
                return undefined;
            }

            if (promise.flags & TypeFlags.Reference) {
                if ((<GenericType>promise).target === tryGetGlobalPromiseType()
                    || (<GenericType>promise).target === getGlobalPromiseLikeType()) {
                    return (<GenericType>promise).typeArguments[0];
                }
            }

            const globalPromiseLikeType = getInstantiatedGlobalPromiseLikeType();
            if (globalPromiseLikeType === emptyObjectType || !isTypeAssignableTo(promise, globalPromiseLikeType)) {
                return undefined;
            }

            const thenFunction = getTypeOfPropertyOfType(promise, "then");
            if (!thenFunction || isTypeAny(thenFunction)) {
                return undefined;
            }

            const thenSignatures = getSignaturesOfType(thenFunction, SignatureKind.Call);
            if (thenSignatures.length === 0) {
                return undefined;
            }

            const onfulfilledParameterType = getTypeWithFacts(getUnionType(map(thenSignatures, getTypeOfFirstParameterOfSignature)), TypeFacts.NEUndefined);
            if (isTypeAny(onfulfilledParameterType)) {
                return undefined;
            }

            const onfulfilledParameterSignatures = getSignaturesOfType(onfulfilledParameterType, SignatureKind.Call);
            if (onfulfilledParameterSignatures.length === 0) {
                return undefined;
            }

            return getUnionType(map(onfulfilledParameterSignatures, getTypeOfFirstParameterOfSignature), /*subtypeReduction*/ true);
        }

        function getTypeOfFirstParameterOfSignature(signature: Signature) {
            return signature.parameters.length > 0 ? getTypeAtPosition(signature, 0) : neverType;
        }

        /**
          * Gets the "awaited type" of a type.
          * @param type The type to await.
          * @remarks The "awaited type" of an expression is its "promised type" if the expression is a
          * Promise-like type; otherwise, it is the type of the expression. This is used to reflect
          * The runtime behavior of the `await` keyword.
          */
        function getAwaitedType(type: Type) {
            return checkAwaitedType(type, /*location*/ undefined, /*message*/ undefined);
        }

        function checkAwaitedType(type: Type, location?: Node, message?: DiagnosticMessage) {
            return checkAwaitedTypeWorker(type);

            function checkAwaitedTypeWorker(type: Type): Type {
                if (type.flags & TypeFlags.Union) {
                    const types: Type[] = [];
                    for (const constituentType of (<UnionType>type).types) {
                        types.push(checkAwaitedTypeWorker(constituentType));
                    }

                    return getUnionType(types, /*subtypeReduction*/ true);
                }
                else {
                    const promisedType = getPromisedType(type);
                    if (promisedType === undefined) {
                        // The type was not a PromiseLike, so it could not be unwrapped any further.
                        // As long as the type does not have a callable "then" property, it is
                        // safe to return the type; otherwise, an error will have been reported in
                        // the call to checkNonThenableType and we will return unknownType.
                        //
                        // An example of a non-promise "thenable" might be:
                        //
                        //  await { then(): void {} }
                        //
                        // The "thenable" does not match the minimal definition for a PromiseLike. When
                        // a Promise/A+-compatible or ES6 promise tries to adopt this value, the promise
                        // will never settle. We treat this as an error to help flag an early indicator
                        // of a runtime problem. If the user wants to return this value from an async
                        // function, they would need to wrap it in some other value. If they want it to
                        // be treated as a promise, they can cast to <any>.
                        return checkNonThenableType(type, location, message);
                    }
                    else {
                        if (type.id === promisedType.id || indexOf(awaitedTypeStack, promisedType.id) >= 0) {
                            // We have a bad actor in the form of a promise whose promised type is
                            // the same promise type, or a mutually recursive promise. Return the
                            // unknown type as we cannot guess the shape. If this were the actual
                            // case in the JavaScript, this Promise would never resolve.
                            //
                            // An example of a bad actor with a singly-recursive promise type might
                            // be:
                            //
                            //  interface BadPromise {
                            //      then(
                            //          onfulfilled: (value: BadPromise) => any,
                            //          onrejected: (error: any) => any): BadPromise;
                            //  }
                            //
                            // The above interface will pass the PromiseLike check, and return a
                            // promised type of `BadPromise`. Since this is a self reference, we
                            // don't want to keep recursing ad infinitum.
                            //
                            // An example of a bad actor in the form of a mutually-recursive
                            // promise type might be:
                            //
                            //  interface BadPromiseA {
                            //      then(
                            //          onfulfilled: (value: BadPromiseB) => any,
                            //          onrejected: (error: any) => any): BadPromiseB;
                            //  }
                            //
                            //  interface BadPromiseB {
                            //      then(
                            //          onfulfilled: (value: BadPromiseA) => any,
                            //          onrejected: (error: any) => any): BadPromiseA;
                            //  }
                            //
                            if (location) {
                                error(
                                    location,
                                    Diagnostics._0_is_referenced_directly_or_indirectly_in_the_fulfillment_callback_of_its_own_then_method,
                                    symbolToString(type.symbol));
                            }

                            return unknownType;
                        }

                        // Keep track of the type we're about to unwrap to avoid bad recursive promise types.
                        // See the comments above for more information.
                        awaitedTypeStack.push(type.id);
                        const awaitedType = checkAwaitedTypeWorker(promisedType);
                        awaitedTypeStack.pop();
                        return awaitedType;
                    }
                }
            }
        }

        /**
         * Checks that the return type provided is an instantiation of the global Promise<T> type
         * and returns the awaited type of the return type.
         *
         * @param returnType The return type of a FunctionLikeDeclaration
         * @param location The node on which to report the error.
         */
        function checkCorrectPromiseType(returnType: Type, location: Node) {
            if (returnType === unknownType) {
                // The return type already had some other error, so we ignore and return
                // the unknown type.
                return unknownType;
            }

            const globalPromiseType = getGlobalPromiseType();
            if (globalPromiseType === emptyGenericType
                || globalPromiseType === getTargetType(returnType)) {
                // Either we couldn't resolve the global promise type, which would have already
                // reported an error, or we could resolve it and the return type is a valid type
                // reference to the global type. In either case, we return the awaited type for
                // the return type.
                return checkAwaitedType(returnType, location, Diagnostics.An_async_function_or_method_must_have_a_valid_awaitable_return_type);
            }

            // The promise type was not a valid type reference to the global promise type, so we
            // report an error and return the unknown type.
            error(location, Diagnostics.The_return_type_of_an_async_function_or_method_must_be_the_global_Promise_T_type);
            return unknownType;
        }

        /**
          * Checks the return type of an async function to ensure it is a compatible
          * Promise implementation.
          * @param node The signature to check
          * @param returnType The return type for the function
          * @remarks
          * This checks that an async function has a valid Promise-compatible return type,
          * and returns the *awaited type* of the promise. An async function has a valid
          * Promise-compatible return type if the resolved value of the return type has a
          * construct signature that takes in an `initializer` function that in turn supplies
          * a `resolve` function as one of its arguments and results in an object with a
          * callable `then` signature.
          */
        function checkAsyncFunctionReturnType(node: FunctionLikeDeclaration): Type {
            if (languageVersion >= ScriptTarget.ES6) {
                const returnType = getTypeFromTypeNode(node.type);
                return checkCorrectPromiseType(returnType, node.type);
            }

            const globalPromiseConstructorLikeType = getGlobalPromiseConstructorLikeType();
            if (globalPromiseConstructorLikeType === emptyObjectType) {
                // If we couldn't resolve the global PromiseConstructorLike type we cannot verify
                // compatibility with __awaiter.
                return unknownType;
            }

            // As part of our emit for an async function, we will need to emit the entity name of
            // the return type annotation as an expression. To meet the necessary runtime semantics
            // for __awaiter, we must also check that the type of the declaration (e.g. the static
            // side or "constructor" of the promise type) is compatible `PromiseConstructorLike`.
            //
            // An example might be (from lib.es6.d.ts):
            //
            //  interface Promise<T> { ... }
            //  interface PromiseConstructor {
            //      new <T>(...): Promise<T>;
            //  }
            //  declare var Promise: PromiseConstructor;
            //
            // When an async function declares a return type annotation of `Promise<T>`, we
            // need to get the type of the `Promise` variable declaration above, which would
            // be `PromiseConstructor`.
            //
            // The same case applies to a class:
            //
            //  declare class Promise<T> {
            //      constructor(...);
            //      then<U>(...): Promise<U>;
            //  }
            //
            // When we get the type of the `Promise` symbol here, we get the type of the static
            // side of the `Promise` class, which would be `{ new <T>(...): Promise<T> }`.

            const promiseType = getTypeFromTypeNode(node.type);
            if (promiseType === unknownType && compilerOptions.isolatedModules) {
                // If we are compiling with isolatedModules, we may not be able to resolve the
                // type as a value. As such, we will just return unknownType;
                return unknownType;
            }

            const promiseConstructor = getNodeLinks(node.type).resolvedSymbol;
            if (!promiseConstructor || !symbolIsValue(promiseConstructor)) {
                const typeName = promiseConstructor
                    ? symbolToString(promiseConstructor)
                    : typeToString(promiseType);
                error(node, Diagnostics.Type_0_is_not_a_valid_async_function_return_type, typeName);
                return unknownType;
            }

            // If the Promise constructor, resolved locally, is an alias symbol we should mark it as referenced.
            checkReturnTypeAnnotationAsExpression(node);

            // Validate the promise constructor type.
            const promiseConstructorType = getTypeOfSymbol(promiseConstructor);
            if (!checkTypeAssignableTo(promiseConstructorType, globalPromiseConstructorLikeType, node, Diagnostics.Type_0_is_not_a_valid_async_function_return_type)) {
                return unknownType;
            }

            // Verify there is no local declaration that could collide with the promise constructor.
            const promiseName = getEntityNameFromTypeNode(node.type);
            const promiseNameOrNamespaceRoot = getFirstIdentifier(promiseName);
            const rootSymbol = getSymbol(node.locals, promiseNameOrNamespaceRoot.text, SymbolFlags.Value);
            if (rootSymbol) {
                error(rootSymbol.valueDeclaration, Diagnostics.Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions,
                    promiseNameOrNamespaceRoot.text,
                    getFullyQualifiedName(promiseConstructor));
                return unknownType;
            }

            // Get and return the awaited type of the return type.
            return checkAwaitedType(promiseType, node, Diagnostics.An_async_function_or_method_must_have_a_valid_awaitable_return_type);
        }

        /** Check a decorator */
        function checkDecorator(node: Decorator): void {
            const signature = getResolvedSignature(node);
            const returnType = getReturnTypeOfSignature(signature);
            if (returnType.flags & TypeFlags.Any) {
                return;
            }

            let expectedReturnType: Type;
            const headMessage = getDiagnosticHeadMessageForDecoratorResolution(node);
            let errorInfo: DiagnosticMessageChain;
            switch (node.parent.kind) {
                case SyntaxKind.ClassDeclaration:
                    const classSymbol = getSymbolOfNode(node.parent);
                    const classConstructorType = getTypeOfSymbol(classSymbol);
                    expectedReturnType = getUnionType([classConstructorType, voidType]);
                    break;

                case SyntaxKind.Parameter:
                    expectedReturnType = voidType;
                    errorInfo = chainDiagnosticMessages(
                        errorInfo,
                        Diagnostics.The_return_type_of_a_parameter_decorator_function_must_be_either_void_or_any);

                    break;

                case SyntaxKind.PropertyDeclaration:
                    expectedReturnType = voidType;
                    errorInfo = chainDiagnosticMessages(
                        errorInfo,
                        Diagnostics.The_return_type_of_a_property_decorator_function_must_be_either_void_or_any);
                    break;

                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    const methodType = getTypeOfNode(node.parent);
                    const descriptorType = createTypedPropertyDescriptorType(methodType);
                    expectedReturnType = getUnionType([descriptorType, voidType]);
                    break;
            }

            checkTypeAssignableTo(
                returnType,
                expectedReturnType,
                node,
                headMessage,
                errorInfo);
        }

        /** Checks a type reference node as an expression. */
        function checkTypeNodeAsExpression(node: TypeNode) {
            // When we are emitting type metadata for decorators, we need to try to check the type
            // as if it were an expression so that we can emit the type in a value position when we
            // serialize the type metadata.
            if (node && node.kind === SyntaxKind.TypeReference) {
                const root = getFirstIdentifier((<TypeReferenceNode>node).typeName);
                const meaning = root.parent.kind === SyntaxKind.TypeReference ? SymbolFlags.Type : SymbolFlags.Namespace;
                // Resolve type so we know which symbol is referenced
                const rootSymbol = resolveName(root, root.text, meaning | SymbolFlags.Alias, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined);
                // Resolved symbol is alias
                if (rootSymbol && rootSymbol.flags & SymbolFlags.Alias) {
                    const aliasTarget = resolveAlias(rootSymbol);
                    // If alias has value symbol - mark alias as referenced
                    if (aliasTarget.flags & SymbolFlags.Value && !isConstEnumOrConstEnumOnlyModule(resolveAlias(rootSymbol))) {
                        markAliasSymbolAsReferenced(rootSymbol);
                    }
                }
            }
        }

        /**
          * Checks the type annotation of an accessor declaration or property declaration as
          * an expression if it is a type reference to a type with a value declaration.
          */
        function checkTypeAnnotationAsExpression(node: VariableLikeDeclaration) {
            checkTypeNodeAsExpression((<PropertyDeclaration>node).type);
        }

        function checkReturnTypeAnnotationAsExpression(node: FunctionLikeDeclaration) {
            checkTypeNodeAsExpression(node.type);
        }

        /** Checks the type annotation of the parameters of a function/method or the constructor of a class as expressions */
        function checkParameterTypeAnnotationsAsExpressions(node: FunctionLikeDeclaration) {
            // ensure all type annotations with a value declaration are checked as an expression
            for (const parameter of node.parameters) {
                checkTypeAnnotationAsExpression(parameter);
            }
        }

        /** Check the decorators of a node */
        function checkDecorators(node: Node): void {
            if (!node.decorators) {
                return;
            }

            // skip this check for nodes that cannot have decorators. These should have already had an error reported by
            // checkGrammarDecorators.
            if (!nodeCanBeDecorated(node)) {
                return;
            }

            if (!compilerOptions.experimentalDecorators) {
                error(node, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Set_the_experimentalDecorators_option_to_remove_this_warning);
            }

            if (compilerOptions.emitDecoratorMetadata) {
                // we only need to perform these checks if we are emitting serialized type metadata for the target of a decorator.
                switch (node.kind) {
                    case SyntaxKind.ClassDeclaration:
                        const constructor = getFirstConstructorWithBody(<ClassDeclaration>node);
                        if (constructor) {
                            checkParameterTypeAnnotationsAsExpressions(constructor);
                        }
                        break;

                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        checkParameterTypeAnnotationsAsExpressions(<FunctionLikeDeclaration>node);
                        checkReturnTypeAnnotationAsExpression(<FunctionLikeDeclaration>node);
                        break;

                    case SyntaxKind.PropertyDeclaration:
                    case SyntaxKind.Parameter:
                        checkTypeAnnotationAsExpression(<PropertyDeclaration | ParameterDeclaration>node);
                        break;
                }
            }

            forEach(node.decorators, checkDecorator);
        }

        function checkFunctionDeclaration(node: FunctionDeclaration): void {
            if (produceDiagnostics) {
                checkFunctionOrMethodDeclaration(node) || checkGrammarForGenerator(node);

                checkCollisionWithCapturedSuperVariable(node, node.name);
                checkCollisionWithCapturedThisVariable(node, node.name);
                checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
            }
        }

        function checkFunctionOrMethodDeclaration(node: FunctionDeclaration | MethodDeclaration): void {
            checkDecorators(node);
            checkSignatureDeclaration(node);
            const isAsync = isAsyncFunctionLike(node);

            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name && node.name.kind === SyntaxKind.ComputedPropertyName) {
                // This check will account for methods in class/interface declarations,
                // as well as accessors in classes/object literals
                checkComputedPropertyName(<ComputedPropertyName>node.name);
            }

            if (!hasDynamicName(node)) {
                // first we want to check the local symbol that contain this declaration
                // - if node.localSymbol !== undefined - this is current declaration is exported and localSymbol points to the local symbol
                // - if node.localSymbol === undefined - this node is non-exported so we can just pick the result of getSymbolOfNode
                const symbol = getSymbolOfNode(node);
                const localSymbol = node.localSymbol || symbol;

                // Since the javascript won't do semantic analysis like typescript,
                // if the javascript file comes before the typescript file and both contain same name functions,
                // checkFunctionOrConstructorSymbol wouldn't be called if we didnt ignore javascript function.
                const firstDeclaration = forEach(localSymbol.declarations,
                    // Get first non javascript function declaration
                    declaration => declaration.kind === node.kind && !isSourceFileJavaScript(getSourceFileOfNode(declaration)) ?
                        declaration : undefined);

                // Only type check the symbol once
                if (node === firstDeclaration) {
                    checkFunctionOrConstructorSymbol(localSymbol);
                }

                if (symbol.parent) {
                    // run check once for the first declaration
                    if (getDeclarationOfKind(symbol, node.kind) === node) {
                        // run check on export symbol to check that modifiers agree across all exported declarations
                        checkFunctionOrConstructorSymbol(symbol);
                    }
                }
            }

            checkSourceElement(node.body);

            if (!node.asteriskToken) {
                const returnOrPromisedType = node.type && (isAsync ? checkAsyncFunctionReturnType(node) : getTypeFromTypeNode(node.type));
                checkAllCodePathsInNonVoidFunctionReturnOrThrow(node, returnOrPromisedType);
            }

            if (produceDiagnostics && !node.type) {
                // Report an implicit any error if there is no body, no explicit return type, and node is not a private method
                // in an ambient context
                if (compilerOptions.noImplicitAny && nodeIsMissing(node.body) && !isPrivateWithinAmbient(node)) {
                    reportImplicitAnyError(node, anyType);
                }

                if (node.asteriskToken && nodeIsPresent(node.body)) {
                    // A generator with a body and no type annotation can still cause errors. It can error if the
                    // yielded values have no common supertype, or it can give an implicit any error if it has no
                    // yielded values. The only way to trigger these errors is to try checking its return type.
                    getReturnTypeOfSignature(getSignatureFromDeclaration(node));
                }
            }

            registerForUnusedIdentifiersCheck(node);
        }

        function registerForUnusedIdentifiersCheck(node: Node) {
            if (deferredUnusedIdentifierNodes) {
                deferredUnusedIdentifierNodes.push(node);
            }
        }

        function checkUnusedIdentifiers() {
            if (deferredUnusedIdentifierNodes) {
                for (const node of deferredUnusedIdentifierNodes) {
                    switch (node.kind) {
                        case SyntaxKind.SourceFile:
                        case SyntaxKind.ModuleDeclaration:
                            checkUnusedModuleMembers(<ModuleDeclaration | SourceFile>node);
                            break;
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.ClassExpression:
                            checkUnusedClassMembers(<ClassDeclaration | ClassExpression>node);
                            checkUnusedTypeParameters(<ClassDeclaration | ClassExpression>node);
                            break;
                        case SyntaxKind.InterfaceDeclaration:
                            checkUnusedTypeParameters(<InterfaceDeclaration>node);
                            break;
                        case SyntaxKind.Block:
                        case SyntaxKind.CaseBlock:
                        case SyntaxKind.ForStatement:
                        case SyntaxKind.ForInStatement:
                        case SyntaxKind.ForOfStatement:
                            checkUnusedLocalsAndParameters(node);
                            break;
                        case SyntaxKind.Constructor:
                        case SyntaxKind.FunctionExpression:
                        case SyntaxKind.FunctionDeclaration:
                        case SyntaxKind.ArrowFunction:
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                            if ((<FunctionLikeDeclaration>node).body) {
                                checkUnusedLocalsAndParameters(<FunctionLikeDeclaration>node);
                            }
                            checkUnusedTypeParameters(<FunctionLikeDeclaration>node);
                            break;
                        case SyntaxKind.MethodSignature:
                        case SyntaxKind.CallSignature:
                        case SyntaxKind.ConstructSignature:
                        case SyntaxKind.IndexSignature:
                        case SyntaxKind.FunctionType:
                        case SyntaxKind.ConstructorType:
                            checkUnusedTypeParameters(<FunctionLikeDeclaration>node);
                            break;
                    };
                }
            }
        }

        function checkUnusedLocalsAndParameters(node: Node): void {
            if (node.parent.kind !== SyntaxKind.InterfaceDeclaration && noUnusedIdentifiers && !isInAmbientContext(node)) {
                for (const key in node.locals) {
                    const local = node.locals[key];
                    if (!local.isReferenced) {
                        if (local.valueDeclaration && local.valueDeclaration.kind === SyntaxKind.Parameter) {
                            const parameter = <ParameterDeclaration>local.valueDeclaration;
                            if (compilerOptions.noUnusedParameters &&
                                !isParameterPropertyDeclaration(parameter) &&
                                !parameterIsThisKeyword(parameter) &&
                                !parameterNameStartsWithUnderscore(parameter)) {
                                error(local.valueDeclaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
                            }
                        }
                        else if (compilerOptions.noUnusedLocals) {
                            forEach(local.declarations, d => error(d.name || d, Diagnostics._0_is_declared_but_never_used, local.name));
                        }
                    }
                }
            }
        }

        function parameterIsThisKeyword(parameter: ParameterDeclaration) {
            return parameter.name && (<Identifier>parameter.name).originalKeywordKind === SyntaxKind.ThisKeyword;
        }

        function parameterNameStartsWithUnderscore(parameter: ParameterDeclaration) {
            return parameter.name && parameter.name.kind === SyntaxKind.Identifier && (<Identifier>parameter.name).text.charCodeAt(0) === CharacterCodes._;
        }

        function checkUnusedClassMembers(node: ClassDeclaration | ClassExpression): void {
            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
                if (node.members) {
                    for (const member of node.members) {
                        if (member.kind === SyntaxKind.MethodDeclaration || member.kind === SyntaxKind.PropertyDeclaration) {
                            if (!member.symbol.isReferenced && member.flags & NodeFlags.Private) {
                                error(member.name, Diagnostics._0_is_declared_but_never_used, member.symbol.name);
                            }
                        }
                        else if (member.kind === SyntaxKind.Constructor) {
                            for (const parameter of (<ConstructorDeclaration>member).parameters) {
                                if (!parameter.symbol.isReferenced && parameter.flags & NodeFlags.Private) {
                                    error(parameter.name, Diagnostics._0_is_declared_but_never_used, parameter.symbol.name);
                                }
                            }
                        }
                    }
                }
            }
        }

        function checkUnusedTypeParameters(node: ClassDeclaration | ClassExpression | FunctionDeclaration | MethodDeclaration | FunctionExpression | ArrowFunction | ConstructorDeclaration | SignatureDeclaration | InterfaceDeclaration) {
            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
                if (node.typeParameters) {
                    // Only report errors on the last declaration for the type parameter container;
                    // this ensures that all uses have been accounted for.
                    const symbol = getSymbolOfNode(node);
                    const lastDeclaration = symbol && symbol.declarations && lastOrUndefined(symbol.declarations);
                    if (lastDeclaration !== node) {
                        return;
                    }
                    for (const typeParameter of node.typeParameters) {
                        if (!getMergedSymbol(typeParameter.symbol).isReferenced) {
                            error(typeParameter.name, Diagnostics._0_is_declared_but_never_used, typeParameter.symbol.name);
                        }
                    }
                }
            }
        }

        function checkUnusedModuleMembers(node: ModuleDeclaration | SourceFile): void {
            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
                for (const key in node.locals) {
                    const local = node.locals[key];
                    if (!local.isReferenced && !local.exportSymbol) {
                        for (const declaration of local.declarations) {
                            if (!isAmbientModule(declaration)) {
                                error(declaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
                            }
                        }
                    }
                }
            }
        }

        function checkBlock(node: Block) {
            // Grammar checking for SyntaxKind.Block
            if (node.kind === SyntaxKind.Block) {
                checkGrammarStatementInAmbientContext(node);
            }
            forEach(node.statements, checkSourceElement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkCollisionWithArgumentsInGeneratedCode(node: SignatureDeclaration) {
            // no rest parameters \ declaration context \ overload - no codegen impact
            if (!hasDeclaredRestParameter(node) || isInAmbientContext(node) || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
                return;
            }

            forEach(node.parameters, p => {
                if (p.name && !isBindingPattern(p.name) && (<Identifier>p.name).text === argumentsSymbol.name) {
                    error(p, Diagnostics.Duplicate_identifier_arguments_Compiler_uses_arguments_to_initialize_rest_parameters);
                }
            });
        }

        function needCollisionCheckForIdentifier(node: Node, identifier: Identifier, name: string): boolean {
            if (!(identifier && identifier.text === name)) {
                return false;
            }

            if (node.kind === SyntaxKind.PropertyDeclaration ||
                node.kind === SyntaxKind.PropertySignature ||
                node.kind === SyntaxKind.MethodDeclaration ||
                node.kind === SyntaxKind.MethodSignature ||
                node.kind === SyntaxKind.GetAccessor ||
                node.kind === SyntaxKind.SetAccessor) {
                // it is ok to have member named '_super' or '_this' - member access is always qualified
                return false;
            }

            if (isInAmbientContext(node)) {
                // ambient context - no codegen impact
                return false;
            }

            const root = getRootDeclaration(node);
            if (root.kind === SyntaxKind.Parameter && nodeIsMissing((<FunctionLikeDeclaration>root.parent).body)) {
                // just an overload - no codegen impact
                return false;
            }

            return true;
        }

        function checkCollisionWithCapturedThisVariable(node: Node, name: Identifier): void {
            if (needCollisionCheckForIdentifier(node, name, "_this")) {
                potentialThisCollisions.push(node);
            }
        }

        // this function will run after checking the source file so 'CaptureThis' is correct for all nodes
        function checkIfThisIsCapturedInEnclosingScope(node: Node): void {
            let current = node;
            while (current) {
                if (getNodeCheckFlags(current) & NodeCheckFlags.CaptureThis) {
                    const isDeclaration = node.kind !== SyntaxKind.Identifier;
                    if (isDeclaration) {
                        error((<Declaration>node).name, Diagnostics.Duplicate_identifier_this_Compiler_uses_variable_declaration_this_to_capture_this_reference);
                    }
                    else {
                        error(node, Diagnostics.Expression_resolves_to_variable_declaration_this_that_compiler_uses_to_capture_this_reference);
                    }
                    return;
                }
                current = current.parent;
            }
        }

        function checkCollisionWithCapturedSuperVariable(node: Node, name: Identifier) {
            if (!needCollisionCheckForIdentifier(node, name, "_super")) {
                return;
            }

            // bubble up and find containing type
            const enclosingClass = getContainingClass(node);
            // if containing type was not found or it is ambient - exit (no codegen)
            if (!enclosingClass || isInAmbientContext(enclosingClass)) {
                return;
            }

            if (getClassExtendsHeritageClauseElement(enclosingClass)) {
                const isDeclaration = node.kind !== SyntaxKind.Identifier;
                if (isDeclaration) {
                    error(node, Diagnostics.Duplicate_identifier_super_Compiler_uses_super_to_capture_base_class_reference);
                }
                else {
                    error(node, Diagnostics.Expression_resolves_to_super_that_compiler_uses_to_capture_base_class_reference);
                }
            }
        }

        function checkCollisionWithRequireExportsInGeneratedCode(node: Node, name: Identifier) {
            if (!needCollisionCheckForIdentifier(node, name, "require") && !needCollisionCheckForIdentifier(node, name, "exports")) {
                return;
            }

            // Uninstantiated modules shouldnt do this check
            if (node.kind === SyntaxKind.ModuleDeclaration && getModuleInstanceState(node) !== ModuleInstanceState.Instantiated) {
                return;
            }

            // In case of variable declaration, node.parent is variable statement so look at the variable statement's parent
            const parent = getDeclarationContainer(node);
            if (parent.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>parent)) {
                // If the declaration happens to be in external module, report error that require and exports are reserved keywords
                error(name, Diagnostics.Duplicate_identifier_0_Compiler_reserves_name_1_in_top_level_scope_of_a_module,
                    declarationNameToString(name), declarationNameToString(name));
            }
        }

        function checkCollisionWithGlobalPromiseInGeneratedCode(node: Node, name: Identifier): void {
            if (!needCollisionCheckForIdentifier(node, name, "Promise")) {
                return;
            }

            // Uninstantiated modules shouldnt do this check
            if (node.kind === SyntaxKind.ModuleDeclaration && getModuleInstanceState(node) !== ModuleInstanceState.Instantiated) {
                return;
            }

            // In case of variable declaration, node.parent is variable statement so look at the variable statement's parent
            const parent = getDeclarationContainer(node);
            if (parent.kind === SyntaxKind.SourceFile && isExternalOrCommonJsModule(<SourceFile>parent) && parent.flags & NodeFlags.HasAsyncFunctions) {
                // If the declaration happens to be in external module, report error that Promise is a reserved identifier.
                error(name, Diagnostics.Duplicate_identifier_0_Compiler_reserves_name_1_in_top_level_scope_of_a_module_containing_async_functions,
                    declarationNameToString(name), declarationNameToString(name));
            }
        }

        function checkVarDeclaredNamesNotShadowed(node: VariableDeclaration | BindingElement) {
            // - ScriptBody : StatementList
            // It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList
            // also occurs in the VarDeclaredNames of StatementList.

            // - Block : { StatementList }
            // It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList
            // also occurs in the VarDeclaredNames of StatementList.

            // Variable declarations are hoisted to the top of their function scope. They can shadow
            // block scoped declarations, which bind tighter. this will not be flagged as duplicate definition
            // by the binder as the declaration scope is different.
            // A non-initialized declaration is a no-op as the block declaration will resolve before the var
            // declaration. the problem is if the declaration has an initializer. this will act as a write to the
            // block declared value. this is fine for let, but not const.
            // Only consider declarations with initializers, uninitialized const declarations will not
            // step on a let/const variable.
            // Do not consider const and const declarations, as duplicate block-scoped declarations
            // are handled by the binder.
            // We are only looking for const declarations that step on let\const declarations from a
            // different scope. e.g.:
            //      {
            //          const x = 0; // localDeclarationSymbol obtained after name resolution will correspond to this declaration
            //          const x = 0; // symbol for this declaration will be 'symbol'
            //      }

            // skip block-scoped variables and parameters
            if ((getCombinedNodeFlags(node) & NodeFlags.BlockScoped) !== 0 || isParameterDeclaration(node)) {
                return;
            }

            // skip variable declarations that don't have initializers
            // NOTE: in ES6 spec initializer is required in variable declarations where name is binding pattern
            // so we'll always treat binding elements as initialized
            if (node.kind === SyntaxKind.VariableDeclaration && !node.initializer) {
                return;
            }

            const symbol = getSymbolOfNode(node);
            if (symbol.flags & SymbolFlags.FunctionScopedVariable) {
                const localDeclarationSymbol = resolveName(node, (<Identifier>node.name).text, SymbolFlags.Variable, /*nodeNotFoundErrorMessage*/ undefined, /*nameArg*/ undefined);
                if (localDeclarationSymbol &&
                    localDeclarationSymbol !== symbol &&
                    localDeclarationSymbol.flags & SymbolFlags.BlockScopedVariable) {
                    if (getDeclarationFlagsFromSymbol(localDeclarationSymbol) & NodeFlags.BlockScoped) {
                        const varDeclList = getAncestor(localDeclarationSymbol.valueDeclaration, SyntaxKind.VariableDeclarationList);
                        const container =
                            varDeclList.parent.kind === SyntaxKind.VariableStatement && varDeclList.parent.parent
                                ? varDeclList.parent.parent
                                : undefined;

                        // names of block-scoped and function scoped variables can collide only
                        // if block scoped variable is defined in the function\module\source file scope (because of variable hoisting)
                        const namesShareScope =
                            container &&
                            (container.kind === SyntaxKind.Block && isFunctionLike(container.parent) ||
                                container.kind === SyntaxKind.ModuleBlock ||
                                container.kind === SyntaxKind.ModuleDeclaration ||
                                container.kind === SyntaxKind.SourceFile);

                        // here we know that function scoped variable is shadowed by block scoped one
                        // if they are defined in the same scope - binder has already reported redeclaration error
                        // otherwise if variable has an initializer - show error that initialization will fail
                        // since LHS will be block scoped name instead of function scoped
                        if (!namesShareScope) {
                            const name = symbolToString(localDeclarationSymbol);
                            error(node, Diagnostics.Cannot_initialize_outer_scoped_variable_0_in_the_same_scope_as_block_scoped_declaration_1, name, name);
                        }
                    }
                }
            }
        }

        // Check that a parameter initializer contains no references to parameters declared to the right of itself
        function checkParameterInitializer(node: VariableLikeDeclaration): void {
            if (getRootDeclaration(node).kind !== SyntaxKind.Parameter) {
                return;
            }

            const func = getContainingFunction(node);
            visit(node.initializer);

            function visit(n: Node): void {
                if (isTypeNode(n) || isDeclarationName(n)) {
                    // do not dive in types
                    // skip declaration names (i.e. in object literal expressions)
                    return;
                }
                if (n.kind === SyntaxKind.PropertyAccessExpression) {
                    // skip property names in property access expression
                    return visit((<PropertyAccessExpression>n).expression);
                }
                else if (n.kind === SyntaxKind.Identifier) {
                    // check FunctionLikeDeclaration.locals (stores parameters\function local variable)
                    // if it contains entry with a specified name
                    const symbol = resolveName(n, (<Identifier>n).text, SymbolFlags.Value | SymbolFlags.Alias, /*nameNotFoundMessage*/undefined, /*nameArg*/undefined);
                    if (!symbol || symbol === unknownSymbol || !symbol.valueDeclaration) {
                        return;
                    }
                    if (symbol.valueDeclaration === node) {
                        error(n, Diagnostics.Parameter_0_cannot_be_referenced_in_its_initializer, declarationNameToString(node.name));
                        return;
                    }
                    // locals map for function contain both parameters and function locals
                    // so we need to do a bit of extra work to check if reference is legal
                    const enclosingContainer = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
                    if (enclosingContainer === func) {
                        if (symbol.valueDeclaration.kind === SyntaxKind.Parameter) {
                            // it is ok to reference parameter in initializer if either
                            // - parameter is located strictly on the left of current parameter declaration
                            if (symbol.valueDeclaration.pos < node.pos) {
                                return;
                            }
                            // - parameter is wrapped in function-like entity
                            let current = n;
                            while (current !== node.initializer) {
                                if (isFunctionLike(current.parent)) {
                                    return;
                                }
                                // computed property names/initializers in instance property declaration of class like entities
                                // are executed in constructor and thus deferred
                                if (current.parent.kind === SyntaxKind.PropertyDeclaration &&
                                    !(current.parent.flags & NodeFlags.Static) &&
                                    isClassLike(current.parent.parent)) {
                                    return;
                                }
                                current = current.parent;
                            }
                            // fall through to report error
                        }
                        error(n, Diagnostics.Initializer_of_parameter_0_cannot_reference_identifier_1_declared_after_it, declarationNameToString(node.name), declarationNameToString(<Identifier>n));
                    }
                }
                else {
                    return forEachChild(n, visit);
                }
            }
        }

        // Check variable, parameter, or property declaration
        function checkVariableLikeDeclaration(node: VariableLikeDeclaration) {
            checkDecorators(node);
            checkSourceElement(node.type);
            // For a computed property, just check the initializer and exit
            // Do not use hasDynamicName here, because that returns false for well known symbols.
            // We want to perform checkComputedPropertyName for all computed properties, including
            // well known symbols.
            if (node.name.kind === SyntaxKind.ComputedPropertyName) {
                checkComputedPropertyName(<ComputedPropertyName>node.name);
                if (node.initializer) {
                    checkExpressionCached(node.initializer);
                }
            }

            if (node.kind === SyntaxKind.BindingElement) {
                // check computed properties inside property names of binding elements
                if (node.propertyName && node.propertyName.kind === SyntaxKind.ComputedPropertyName) {
                    checkComputedPropertyName(<ComputedPropertyName>node.propertyName);
                }

                // check private/protected variable access
                const parent = <VariableLikeDeclaration>(<BindingPattern>node.parent).parent;
                const parentType = getTypeForBindingElementParent(parent);
                const name = node.propertyName || <Identifier>node.name;
                const property = getPropertyOfType(parentType, getTextOfPropertyName(name));
                if (parent.initializer && property && getParentOfSymbol(property)) {
                    checkClassPropertyAccess(parent, parent.initializer, parentType, property);
                }
            }

            // For a binding pattern, check contained binding elements
            if (isBindingPattern(node.name)) {
                forEach((<BindingPattern>node.name).elements, checkSourceElement);
            }
            // For a parameter declaration with an initializer, error and exit if the containing function doesn't have a body
            if (node.initializer && getRootDeclaration(node).kind === SyntaxKind.Parameter && nodeIsMissing(getContainingFunction(node).body)) {
                error(node, Diagnostics.A_parameter_initializer_is_only_allowed_in_a_function_or_constructor_implementation);
                return;
            }
            // For a binding pattern, validate the initializer and exit
            if (isBindingPattern(node.name)) {
                // Don't validate for-in initializer as it is already an error
                if (node.initializer && node.parent.parent.kind !== SyntaxKind.ForInStatement) {
                    checkTypeAssignableTo(checkExpressionCached(node.initializer), getWidenedTypeForVariableLikeDeclaration(node), node, /*headMessage*/ undefined);
                    checkParameterInitializer(node);
                }
                return;
            }
            const symbol = getSymbolOfNode(node);
            const type = getTypeOfVariableOrParameterOrProperty(symbol);
            if (node === symbol.valueDeclaration) {
                // Node is the primary declaration of the symbol, just validate the initializer
                // Don't validate for-in initializer as it is already an error
                if (node.initializer && node.parent.parent.kind !== SyntaxKind.ForInStatement) {
                    checkTypeAssignableTo(checkExpressionCached(node.initializer), type, node, /*headMessage*/ undefined);
                    checkParameterInitializer(node);
                }
            }
            else {
                // Node is a secondary declaration, check that type is identical to primary declaration and check that
                // initializer is consistent with type associated with the node
                const declarationType = getWidenedTypeForVariableLikeDeclaration(node);
                if (type !== unknownType && declarationType !== unknownType && !isTypeIdenticalTo(type, declarationType)) {
                    error(node.name, Diagnostics.Subsequent_variable_declarations_must_have_the_same_type_Variable_0_must_be_of_type_1_but_here_has_type_2, declarationNameToString(node.name), typeToString(type), typeToString(declarationType));
                }
                if (node.initializer) {
                    checkTypeAssignableTo(checkExpressionCached(node.initializer), declarationType, node, /*headMessage*/ undefined);
                }
                if (!areDeclarationFlagsIdentical(node, symbol.valueDeclaration)) {
                    error(symbol.valueDeclaration.name, Diagnostics.All_declarations_of_0_must_have_identical_modifiers, declarationNameToString(node.name));
                    error(node.name, Diagnostics.All_declarations_of_0_must_have_identical_modifiers, declarationNameToString(node.name));
                }
            }
            if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature) {
                // We know we don't have a binding pattern or computed name here
                checkExportsOnMergedDeclarations(node);
                if (node.kind === SyntaxKind.VariableDeclaration || node.kind === SyntaxKind.BindingElement) {
                    checkVarDeclaredNamesNotShadowed(<VariableDeclaration | BindingElement>node);
                }
                checkCollisionWithCapturedSuperVariable(node, <Identifier>node.name);
                checkCollisionWithCapturedThisVariable(node, <Identifier>node.name);
                checkCollisionWithRequireExportsInGeneratedCode(node, <Identifier>node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, <Identifier>node.name);
            }
        }

        function areDeclarationFlagsIdentical(left: Declaration, right: Declaration) {
            if ((left.kind === SyntaxKind.Parameter && right.kind === SyntaxKind.VariableDeclaration) ||
                (left.kind === SyntaxKind.VariableDeclaration && right.kind === SyntaxKind.Parameter)) {
                // Differences in optionality between parameters and variables are allowed.
                return true;
            }

            if (hasQuestionToken(left) !== hasQuestionToken(right)) {
                return false;
            }

            const interestingFlags = NodeFlags.Private |
                NodeFlags.Protected |
                NodeFlags.Async |
                NodeFlags.Abstract |
                NodeFlags.Readonly |
                NodeFlags.Static;

            return (left.flags & interestingFlags) === (right.flags & interestingFlags);
        }

        function checkVariableDeclaration(node: VariableDeclaration) {
            checkGrammarVariableDeclaration(node);
            return checkVariableLikeDeclaration(node);
        }

        function checkBindingElement(node: BindingElement) {
            checkGrammarBindingElement(<BindingElement>node);
            return checkVariableLikeDeclaration(node);
        }

        function checkVariableStatement(node: VariableStatement) {
            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node) || checkGrammarVariableDeclarationList(node.declarationList) || checkGrammarForDisallowedLetOrConstStatement(node);

            forEach(node.declarationList.declarations, checkSourceElement);
        }

        function checkGrammarDisallowedModifiersOnObjectLiteralExpressionMethod(node: Node) {
            // We only disallow modifier on a method declaration if it is a property of object-literal-expression
            if (node.modifiers && node.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                if (isAsyncFunctionLike(node)) {
                    if (node.modifiers.length > 1) {
                        return grammarErrorOnFirstToken(node, Diagnostics.Modifiers_cannot_appear_here);
                    }
                }
                else {
                    return grammarErrorOnFirstToken(node, Diagnostics.Modifiers_cannot_appear_here);
                }
            }
        }

        function checkExpressionStatement(node: ExpressionStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkExpression(node.expression);
        }

        function checkIfStatement(node: IfStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkExpression(node.expression);
            checkSourceElement(node.thenStatement);

            if (node.thenStatement.kind === SyntaxKind.EmptyStatement) {
                error(node.thenStatement, Diagnostics.The_body_of_an_if_statement_cannot_be_the_empty_statement);
            }

            checkSourceElement(node.elseStatement);
        }

        function checkDoStatement(node: DoStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkSourceElement(node.statement);
            checkExpression(node.expression);
        }

        function checkWhileStatement(node: WhileStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkExpression(node.expression);
            checkSourceElement(node.statement);
        }

        function checkForStatement(node: ForStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.initializer && node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                    checkGrammarVariableDeclarationList(<VariableDeclarationList>node.initializer);
                }
            }

            if (node.initializer) {
                if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                    forEach((<VariableDeclarationList>node.initializer).declarations, checkVariableDeclaration);
                }
                else {
                    checkExpression(<Expression>node.initializer);
                }
            }

            if (node.condition) checkExpression(node.condition);
            if (node.incrementor) checkExpression(node.incrementor);
            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForOfStatement(node: ForOfStatement): void {
            checkGrammarForInOrForOfStatement(node);

            // Check the LHS and RHS
            // If the LHS is a declaration, just check it as a variable declaration, which will in turn check the RHS
            // via checkRightHandSideOfForOf.
            // If the LHS is an expression, check the LHS, as a destructuring assignment or as a reference.
            // Then check that the RHS is assignable to it.
            if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                checkForInOrForOfVariableDeclaration(node);
            }
            else {
                const varExpr = <Expression>node.initializer;
                const iteratedType = checkRightHandSideOfForOf(node.expression);

                // There may be a destructuring assignment on the left side
                if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
                    // iteratedType may be undefined. In this case, we still want to check the structure of
                    // varExpr, in particular making sure it's a valid LeftHandSideExpression. But we'd like
                    // to short circuit the type relation checking as much as possible, so we pass the unknownType.
                    checkDestructuringAssignment(varExpr, iteratedType || unknownType);
                }
                else {
                    const leftType = checkExpression(varExpr);
                    checkReferenceExpression(varExpr, /*invalidReferenceMessage*/ Diagnostics.Invalid_left_hand_side_in_for_of_statement,
                        /*constantVariableMessage*/ Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_be_a_constant_or_a_read_only_property);

                    // iteratedType will be undefined if the rightType was missing properties/signatures
                    // required to get its iteratedType (like [Symbol.iterator] or next). This may be
                    // because we accessed properties from anyType, or it may have led to an error inside
                    // getElementTypeOfIterable.
                    if (iteratedType) {
                        checkTypeAssignableTo(iteratedType, leftType, varExpr, /*headMessage*/ undefined);
                    }
                }
            }

            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForInStatement(node: ForInStatement) {
            // Grammar checking
            checkGrammarForInOrForOfStatement(node);

            // TypeScript 1.0 spec  (April 2014): 5.4
            // In a 'for-in' statement of the form
            // for (let VarDecl in Expr) Statement
            //   VarDecl must be a variable declaration without a type annotation that declares a variable of type Any,
            //   and Expr must be an expression of type Any, an object type, or a type parameter type.
            if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variable = (<VariableDeclarationList>node.initializer).declarations[0];
                if (variable && isBindingPattern(variable.name)) {
                    error(variable.name, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
                }

                checkForInOrForOfVariableDeclaration(node);
            }
            else {
                // In a 'for-in' statement of the form
                // for (Var in Expr) Statement
                //   Var must be an expression classified as a reference of type Any or the String primitive type,
                //   and Expr must be an expression of type Any, an object type, or a type parameter type.
                const varExpr = <Expression>node.initializer;
                const leftType = checkExpression(varExpr);
                if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
                    error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
                }
                else if (!isTypeAnyOrAllConstituentTypesHaveKind(leftType, TypeFlags.StringLike)) {
                    error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_must_be_of_type_string_or_any);
                }
                else {
                    // run check only former check succeeded to avoid cascading errors
                    checkReferenceExpression(varExpr, Diagnostics.Invalid_left_hand_side_in_for_in_statement,
                        Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_constant_or_a_read_only_property);
                }
            }

            const rightType = checkNonNullExpression(node.expression);
            // unknownType is returned i.e. if node.expression is identifier whose name cannot be resolved
            // in this case error about missing name is already reported - do not report extra one
            if (!isTypeAnyOrAllConstituentTypesHaveKind(rightType, TypeFlags.ObjectType | TypeFlags.TypeParameter)) {
                error(node.expression, Diagnostics.The_right_hand_side_of_a_for_in_statement_must_be_of_type_any_an_object_type_or_a_type_parameter);
            }

            checkSourceElement(node.statement);
            if (node.locals) {
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkForInOrForOfVariableDeclaration(iterationStatement: ForInStatement | ForOfStatement): void {
            const variableDeclarationList = <VariableDeclarationList>iterationStatement.initializer;
            // checkGrammarForInOrForOfStatement will check that there is exactly one declaration.
            if (variableDeclarationList.declarations.length >= 1) {
                const decl = variableDeclarationList.declarations[0];
                checkVariableDeclaration(decl);
            }
        }

        function checkRightHandSideOfForOf(rhsExpression: Expression): Type {
            const expressionType = checkNonNullExpression(rhsExpression);
            return checkIteratedTypeOrElementType(expressionType, rhsExpression, /*allowStringInput*/ true);
        }

        function checkIteratedTypeOrElementType(inputType: Type, errorNode: Node, allowStringInput: boolean): Type {
            if (isTypeAny(inputType)) {
                return inputType;
            }
            if (languageVersion >= ScriptTarget.ES6) {
                return checkElementTypeOfIterable(inputType, errorNode);
            }
            if (allowStringInput) {
                return checkElementTypeOfArrayOrString(inputType, errorNode);
            }
            if (isArrayLikeType(inputType)) {
                const indexType = getIndexTypeOfType(inputType, IndexKind.Number);
                if (indexType) {
                    return indexType;
                }
            }
            if (errorNode) {
                error(errorNode, Diagnostics.Type_0_is_not_an_array_type, typeToString(inputType));
            }
            return unknownType;
        }

        /**
         * When errorNode is undefined, it means we should not report any errors.
         */
        function checkElementTypeOfIterable(iterable: Type, errorNode: Node): Type {
            const elementType = getElementTypeOfIterable(iterable, errorNode);
            // Now even though we have extracted the iteratedType, we will have to validate that the type
            // passed in is actually an Iterable.
            if (errorNode && elementType) {
                checkTypeAssignableTo(iterable, createIterableType(elementType), errorNode);
            }

            return elementType || anyType;
        }

        /**
         * We want to treat type as an iterable, and get the type it is an iterable of. The iterable
         * must have the following structure (annotated with the names of the variables below):
         *
         * { // iterable
         *     [Symbol.iterator]: { // iteratorFunction
         *         (): Iterator<T>
         *     }
         * }
         *
         * T is the type we are after. At every level that involves analyzing return types
         * of signatures, we union the return types of all the signatures.
         *
         * Another thing to note is that at any step of this process, we could run into a dead end,
         * meaning either the property is missing, or we run into the anyType. If either of these things
         * happens, we return undefined to signal that we could not find the iterated type. If a property
         * is missing, and the previous step did not result in 'any', then we also give an error if the
         * caller requested it. Then the caller can decide what to do in the case where there is no iterated
         * type. This is different from returning anyType, because that would signify that we have matched the
         * whole pattern and that T (above) is 'any'.
         */
        function getElementTypeOfIterable(type: Type, errorNode: Node): Type {
            if (isTypeAny(type)) {
                return undefined;
            }

            const typeAsIterable = <IterableOrIteratorType>type;
            if (!typeAsIterable.iterableElementType) {
                // As an optimization, if the type is instantiated directly using the globalIterableType (Iterable<number>),
                // then just grab its type argument.
                if ((type.flags & TypeFlags.Reference) && (<GenericType>type).target === getGlobalIterableType()) {
                    typeAsIterable.iterableElementType = (<GenericType>type).typeArguments[0];
                }
                else {
                    const iteratorFunction = getTypeOfPropertyOfType(type, getPropertyNameForKnownSymbolName("iterator"));
                    if (isTypeAny(iteratorFunction)) {
                        return undefined;
                    }

                    const iteratorFunctionSignatures = iteratorFunction ? getSignaturesOfType(iteratorFunction, SignatureKind.Call) : emptyArray;
                    if (iteratorFunctionSignatures.length === 0) {
                        if (errorNode) {
                            error(errorNode, Diagnostics.Type_must_have_a_Symbol_iterator_method_that_returns_an_iterator);
                        }
                        return undefined;
                    }

                    typeAsIterable.iterableElementType = getElementTypeOfIterator(getUnionType(map(iteratorFunctionSignatures, getReturnTypeOfSignature), /*subtypeReduction*/ true), errorNode);
                }
            }

            return typeAsIterable.iterableElementType;
        }

        /**
         * This function has very similar logic as getElementTypeOfIterable, except that it operates on
         * Iterators instead of Iterables. Here is the structure:
         *
         *  { // iterator
         *      next: { // iteratorNextFunction
         *          (): { // iteratorNextResult
         *              value: T // iteratorNextValue
         *          }
         *      }
         *  }
         *
         */
        function getElementTypeOfIterator(type: Type, errorNode: Node): Type {
            if (isTypeAny(type)) {
                return undefined;
            }

            const typeAsIterator = <IterableOrIteratorType>type;
            if (!typeAsIterator.iteratorElementType) {
                // As an optimization, if the type is instantiated directly using the globalIteratorType (Iterator<number>),
                // then just grab its type argument.
                if ((type.flags & TypeFlags.Reference) && (<GenericType>type).target === getGlobalIteratorType()) {
                    typeAsIterator.iteratorElementType = (<GenericType>type).typeArguments[0];
                }
                else {
                    const iteratorNextFunction = getTypeOfPropertyOfType(type, "next");
                    if (isTypeAny(iteratorNextFunction)) {
                        return undefined;
                    }

                    const iteratorNextFunctionSignatures = iteratorNextFunction ? getSignaturesOfType(iteratorNextFunction, SignatureKind.Call) : emptyArray;
                    if (iteratorNextFunctionSignatures.length === 0) {
                        if (errorNode) {
                            error(errorNode, Diagnostics.An_iterator_must_have_a_next_method);
                        }
                        return undefined;
                    }

                    const iteratorNextResult = getUnionType(map(iteratorNextFunctionSignatures, getReturnTypeOfSignature), /*subtypeReduction*/ true);
                    if (isTypeAny(iteratorNextResult)) {
                        return undefined;
                    }

                    const iteratorNextValue = getTypeOfPropertyOfType(iteratorNextResult, "value");
                    if (!iteratorNextValue) {
                        if (errorNode) {
                            error(errorNode, Diagnostics.The_type_returned_by_the_next_method_of_an_iterator_must_have_a_value_property);
                        }
                        return undefined;
                    }

                    typeAsIterator.iteratorElementType = iteratorNextValue;
                }
            }

            return typeAsIterator.iteratorElementType;
        }

        function getElementTypeOfIterableIterator(type: Type): Type {
            if (isTypeAny(type)) {
                return undefined;
            }

            // As an optimization, if the type is instantiated directly using the globalIterableIteratorType (IterableIterator<number>),
            // then just grab its type argument.
            if ((type.flags & TypeFlags.Reference) && (<GenericType>type).target === getGlobalIterableIteratorType()) {
                return (<GenericType>type).typeArguments[0];
            }

            return getElementTypeOfIterable(type, /*errorNode*/ undefined) ||
                getElementTypeOfIterator(type, /*errorNode*/ undefined);
        }

        /**
         * This function does the following steps:
         *   1. Break up arrayOrStringType (possibly a union) into its string constituents and array constituents.
         *   2. Take the element types of the array constituents.
         *   3. Return the union of the element types, and string if there was a string constituent.
         *
         * For example:
         *     string -> string
         *     number[] -> number
         *     string[] | number[] -> string | number
         *     string | number[] -> string | number
         *     string | string[] | number[] -> string | number
         *
         * It also errors if:
         *   1. Some constituent is neither a string nor an array.
         *   2. Some constituent is a string and target is less than ES5 (because in ES3 string is not indexable).
         */
        function checkElementTypeOfArrayOrString(arrayOrStringType: Type, errorNode: Node): Type {
            Debug.assert(languageVersion < ScriptTarget.ES6);

            // After we remove all types that are StringLike, we will know if there was a string constituent
            // based on whether the remaining type is the same as the initial type.
            let arrayType = arrayOrStringType;
            if (arrayOrStringType.flags & TypeFlags.Union) {
                arrayType = getUnionType(filter((arrayOrStringType as UnionType).types, t => !(t.flags & TypeFlags.StringLike)), /*subtypeReduction*/ true);
            }
            else if (arrayOrStringType.flags & TypeFlags.StringLike) {
                arrayType = neverType;
            }
            const hasStringConstituent = arrayOrStringType !== arrayType;
            let reportedError = false;
            if (hasStringConstituent) {
                if (languageVersion < ScriptTarget.ES5) {
                    error(errorNode, Diagnostics.Using_a_string_in_a_for_of_statement_is_only_supported_in_ECMAScript_5_and_higher);
                    reportedError = true;
                }

                // Now that we've removed all the StringLike types, if no constituents remain, then the entire
                // arrayOrStringType was a string.
                if (arrayType === neverType) {
                    return stringType;
                }
            }

            if (!isArrayLikeType(arrayType)) {
                if (!reportedError) {
                    // Which error we report depends on whether there was a string constituent. For example,
                    // if the input type is number | string, we want to say that number is not an array type.
                    // But if the input was just number, we want to say that number is not an array type
                    // or a string type.
                    const diagnostic = hasStringConstituent
                        ? Diagnostics.Type_0_is_not_an_array_type
                        : Diagnostics.Type_0_is_not_an_array_type_or_a_string_type;
                    error(errorNode, diagnostic, typeToString(arrayType));
                }
                return hasStringConstituent ? stringType : unknownType;
            }

            const arrayElementType = getIndexTypeOfType(arrayType, IndexKind.Number) || unknownType;
            if (hasStringConstituent) {
                // This is just an optimization for the case where arrayOrStringType is string | string[]
                if (arrayElementType.flags & TypeFlags.StringLike) {
                    return stringType;
                }

                return getUnionType([arrayElementType, stringType], /*subtypeReduction*/ true);
            }

            return arrayElementType;
        }

        function checkBreakOrContinueStatement(node: BreakOrContinueStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node) || checkGrammarBreakOrContinueStatement(node);

            // TODO: Check that target label is valid
        }

        function isGetAccessorWithAnnotatedSetAccessor(node: FunctionLikeDeclaration) {
            return !!(node.kind === SyntaxKind.GetAccessor && getSetAccessorTypeAnnotationNode(<AccessorDeclaration>getDeclarationOfKind(node.symbol, SyntaxKind.SetAccessor)));
        }

        function isUnwrappedReturnTypeVoidOrAny(func: FunctionLikeDeclaration, returnType: Type): boolean {
            const unwrappedReturnType = isAsyncFunctionLike(func) ? getPromisedType(returnType) : returnType;
            return unwrappedReturnType && maybeTypeOfKind(unwrappedReturnType, TypeFlags.Void | TypeFlags.Any);
        }

        function checkReturnStatement(node: ReturnStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                const functionBlock = getContainingFunction(node);
                if (!functionBlock) {
                    grammarErrorOnFirstToken(node, Diagnostics.A_return_statement_can_only_be_used_within_a_function_body);
                }
            }

            const func = getContainingFunction(node);
            if (func) {
                const signature = getSignatureFromDeclaration(func);
                const returnType = getReturnTypeOfSignature(signature);
                if (strictNullChecks || node.expression || returnType === neverType) {
                    const exprType = node.expression ? checkExpressionCached(node.expression) : undefinedType;

                    if (func.asteriskToken) {
                        // A generator does not need its return expressions checked against its return type.
                        // Instead, the yield expressions are checked against the element type.
                        // TODO: Check return expressions of generators when return type tracking is added
                        // for generators.
                        return;
                    }

                    if (func.kind === SyntaxKind.SetAccessor) {
                        if (node.expression) {
                            error(node.expression, Diagnostics.Setters_cannot_return_a_value);
                        }
                    }
                    else if (func.kind === SyntaxKind.Constructor) {
                        if (node.expression && !checkTypeAssignableTo(exprType, returnType, node.expression)) {
                            error(node.expression, Diagnostics.Return_type_of_constructor_signature_must_be_assignable_to_the_instance_type_of_the_class);
                        }
                    }
                    else if (func.type || isGetAccessorWithAnnotatedSetAccessor(func)) {
                        if (isAsyncFunctionLike(func)) {
                            const promisedType = getPromisedType(returnType);
                            const awaitedType = checkAwaitedType(exprType, node.expression || node, Diagnostics.Return_expression_in_async_function_does_not_have_a_valid_callable_then_member);
                            if (promisedType) {
                                // If the function has a return type, but promisedType is
                                // undefined, an error will be reported in checkAsyncFunctionReturnType
                                // so we don't need to report one here.
                                checkTypeAssignableTo(awaitedType, promisedType, node.expression || node);
                            }
                        }
                        else {
                            checkTypeAssignableTo(exprType, returnType, node.expression || node);
                        }
                    }
                }
                else if (func.kind !== SyntaxKind.Constructor && compilerOptions.noImplicitReturns && !isUnwrappedReturnTypeVoidOrAny(func, returnType)) {
                    // The function has a return type, but the return statement doesn't have an expression.
                    error(node, Diagnostics.Not_all_code_paths_return_a_value);
                }
            }
        }

        function checkWithStatement(node: WithStatement) {
            // Grammar checking for withStatement
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.flags & NodeFlags.AwaitContext) {
                    grammarErrorOnFirstToken(node, Diagnostics.with_statements_are_not_allowed_in_an_async_function_block);
                }
            }

            checkExpression(node.expression);
            error(node.expression, Diagnostics.All_symbols_within_a_with_block_will_be_resolved_to_any);
        }

        function checkSwitchStatement(node: SwitchStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            let firstDefaultClause: CaseOrDefaultClause;
            let hasDuplicateDefaultClause = false;

            const expressionType = checkExpression(node.expression);
            forEach(node.caseBlock.clauses, clause => {
                // Grammar check for duplicate default clauses, skip if we already report duplicate default clause
                if (clause.kind === SyntaxKind.DefaultClause && !hasDuplicateDefaultClause) {
                    if (firstDefaultClause === undefined) {
                        firstDefaultClause = clause;
                    }
                    else {
                        const sourceFile = getSourceFileOfNode(node);
                        const start = skipTrivia(sourceFile.text, clause.pos);
                        const end = clause.statements.length > 0 ? clause.statements[0].pos : clause.end;
                        grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.A_default_clause_cannot_appear_more_than_once_in_a_switch_statement);
                        hasDuplicateDefaultClause = true;
                    }
                }

                if (produceDiagnostics && clause.kind === SyntaxKind.CaseClause) {
                    const caseClause = <CaseClause>clause;
                    // TypeScript 1.0 spec (April 2014): 5.9
                    // In a 'switch' statement, each 'case' expression must be of a type that is comparable
                    // to or from the type of the 'switch' expression.
                    const caseType = checkExpression(caseClause.expression);
                    if (!isTypeEqualityComparableTo(expressionType, caseType)) {
                        // expressionType is not comparable to caseType, try the reversed check and report errors if it fails
                        checkTypeComparableTo(caseType, expressionType, caseClause.expression, /*headMessage*/ undefined);
                    }
                }
                forEach(clause.statements, checkSourceElement);
            });
            if (node.caseBlock.locals) {
                registerForUnusedIdentifiersCheck(node.caseBlock);
            }
        }

        function checkLabeledStatement(node: LabeledStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                let current = node.parent;
                while (current) {
                    if (isFunctionLike(current)) {
                        break;
                    }
                    if (current.kind === SyntaxKind.LabeledStatement && (<LabeledStatement>current).label.text === node.label.text) {
                        const sourceFile = getSourceFileOfNode(node);
                        grammarErrorOnNode(node.label, Diagnostics.Duplicate_label_0, getTextOfNodeFromSourceText(sourceFile.text, node.label));
                        break;
                    }
                    current = current.parent;
                }
            }

            // ensure that label is unique
            checkSourceElement(node.statement);
        }

        function checkThrowStatement(node: ThrowStatement) {
            // Grammar checking
            if (!checkGrammarStatementInAmbientContext(node)) {
                if (node.expression === undefined) {
                    grammarErrorAfterFirstToken(node, Diagnostics.Line_break_not_permitted_here);
                }
            }

            if (node.expression) {
                checkExpression(node.expression);
            }
        }

        function checkTryStatement(node: TryStatement) {
            // Grammar checking
            checkGrammarStatementInAmbientContext(node);

            checkBlock(node.tryBlock);
            const catchClause = node.catchClause;
            if (catchClause) {
                // Grammar checking
                if (catchClause.variableDeclaration) {
                    if (catchClause.variableDeclaration.name.kind !== SyntaxKind.Identifier) {
                        grammarErrorOnFirstToken(catchClause.variableDeclaration.name, Diagnostics.Catch_clause_variable_name_must_be_an_identifier);
                    }
                    else if (catchClause.variableDeclaration.type) {
                        grammarErrorOnFirstToken(catchClause.variableDeclaration.type, Diagnostics.Catch_clause_variable_cannot_have_a_type_annotation);
                    }
                    else if (catchClause.variableDeclaration.initializer) {
                        grammarErrorOnFirstToken(catchClause.variableDeclaration.initializer, Diagnostics.Catch_clause_variable_cannot_have_an_initializer);
                    }
                    else {
                        const identifierName = (<Identifier>catchClause.variableDeclaration.name).text;
                        const locals = catchClause.block.locals;
                        if (locals) {
                            const localSymbol = locals[identifierName];
                            if (localSymbol && (localSymbol.flags & SymbolFlags.BlockScopedVariable) !== 0) {
                                grammarErrorOnNode(localSymbol.valueDeclaration, Diagnostics.Cannot_redeclare_identifier_0_in_catch_clause, identifierName);
                            }
                        }
                    }
                }

                checkBlock(catchClause.block);
            }

            if (node.finallyBlock) {
                checkBlock(node.finallyBlock);
            }
        }

        function checkIndexConstraints(type: Type) {
            const declaredNumberIndexer = getIndexDeclarationOfSymbol(type.symbol, IndexKind.Number);
            const declaredStringIndexer = getIndexDeclarationOfSymbol(type.symbol, IndexKind.String);

            const stringIndexType = getIndexTypeOfType(type, IndexKind.String);
            const numberIndexType = getIndexTypeOfType(type, IndexKind.Number);

            if (stringIndexType || numberIndexType) {
                forEach(getPropertiesOfObjectType(type), prop => {
                    const propType = getTypeOfSymbol(prop);
                    checkIndexConstraintForProperty(prop, propType, type, declaredStringIndexer, stringIndexType, IndexKind.String);
                    checkIndexConstraintForProperty(prop, propType, type, declaredNumberIndexer, numberIndexType, IndexKind.Number);
                });

                if (type.flags & TypeFlags.Class && isClassLike(type.symbol.valueDeclaration)) {
                    const classDeclaration = <ClassLikeDeclaration>type.symbol.valueDeclaration;
                    for (const member of classDeclaration.members) {
                        // Only process instance properties with computed names here.
                        // Static properties cannot be in conflict with indexers,
                        // and properties with literal names were already checked.
                        if (!(member.flags & NodeFlags.Static) && hasDynamicName(member)) {
                            const propType = getTypeOfSymbol(member.symbol);
                            checkIndexConstraintForProperty(member.symbol, propType, type, declaredStringIndexer, stringIndexType, IndexKind.String);
                            checkIndexConstraintForProperty(member.symbol, propType, type, declaredNumberIndexer, numberIndexType, IndexKind.Number);
                        }
                    }
                }
            }

            let errorNode: Node;
            if (stringIndexType && numberIndexType) {
                errorNode = declaredNumberIndexer || declaredStringIndexer;
                // condition 'errorNode === undefined' may appear if types does not declare nor string neither number indexer
                if (!errorNode && (type.flags & TypeFlags.Interface)) {
                    const someBaseTypeHasBothIndexers = forEach(getBaseTypes(<InterfaceType>type), base => getIndexTypeOfType(base, IndexKind.String) && getIndexTypeOfType(base, IndexKind.Number));
                    errorNode = someBaseTypeHasBothIndexers ? undefined : type.symbol.declarations[0];
                }
            }

            if (errorNode && !isTypeAssignableTo(numberIndexType, stringIndexType)) {
                error(errorNode, Diagnostics.Numeric_index_type_0_is_not_assignable_to_string_index_type_1,
                    typeToString(numberIndexType), typeToString(stringIndexType));
            }

            function checkIndexConstraintForProperty(
                prop: Symbol,
                propertyType: Type,
                containingType: Type,
                indexDeclaration: Declaration,
                indexType: Type,
                indexKind: IndexKind): void {

                if (!indexType) {
                    return;
                }

                // index is numeric and property name is not valid numeric literal
                if (indexKind === IndexKind.Number && !isNumericName(prop.valueDeclaration.name)) {
                    return;
                }

                // perform property check if property or indexer is declared in 'type'
                // this allows to rule out cases when both property and indexer are inherited from the base class
                let errorNode: Node;
                if (prop.valueDeclaration.name.kind === SyntaxKind.ComputedPropertyName || prop.parent === containingType.symbol) {
                    errorNode = prop.valueDeclaration;
                }
                else if (indexDeclaration) {
                    errorNode = indexDeclaration;
                }
                else if (containingType.flags & TypeFlags.Interface) {
                    // for interfaces property and indexer might be inherited from different bases
                    // check if any base class already has both property and indexer.
                    // check should be performed only if 'type' is the first type that brings property\indexer together
                    const someBaseClassHasBothPropertyAndIndexer = forEach(getBaseTypes(<InterfaceType>containingType), base => getPropertyOfObjectType(base, prop.name) && getIndexTypeOfType(base, indexKind));
                    errorNode = someBaseClassHasBothPropertyAndIndexer ? undefined : containingType.symbol.declarations[0];
                }

                if (errorNode && !isTypeAssignableTo(propertyType, indexType)) {
                    const errorMessage =
                        indexKind === IndexKind.String
                            ? Diagnostics.Property_0_of_type_1_is_not_assignable_to_string_index_type_2
                            : Diagnostics.Property_0_of_type_1_is_not_assignable_to_numeric_index_type_2;
                    error(errorNode, errorMessage, symbolToString(prop), typeToString(propertyType), typeToString(indexType));
                }
            }
        }

        function checkTypeNameIsReserved(name: DeclarationName, message: DiagnosticMessage): void {
            // TS 1.0 spec (April 2014): 3.6.1
            // The predefined type keywords are reserved and cannot be used as names of user defined types.
            switch ((<Identifier>name).text) {
                case "any":
                case "number":
                case "boolean":
                case "string":
                case "symbol":
                case "void":
                    error(name, message, (<Identifier>name).text);
            }
        }

        /** Check each type parameter and check that type parameters have no duplicate type parameter declarations */
        function checkTypeParameters(typeParameterDeclarations: TypeParameterDeclaration[]) {
            if (typeParameterDeclarations) {
                for (let i = 0, n = typeParameterDeclarations.length; i < n; i++) {
                    const node = typeParameterDeclarations[i];
                    checkTypeParameter(node);

                    if (produceDiagnostics) {
                        for (let j = 0; j < i; j++) {
                            if (typeParameterDeclarations[j].symbol === node.symbol) {
                                error(node.name, Diagnostics.Duplicate_identifier_0, declarationNameToString(node.name));
                            }
                        }
                    }
                }
            }
        }

        /** Check that type parameter lists are identical across multiple declarations */
        function checkTypeParameterListsIdentical(node: ClassLikeDeclaration | InterfaceDeclaration, symbol: Symbol) {
            if (symbol.declarations.length === 1) {
                return;
            }
            let firstDecl: ClassLikeDeclaration | InterfaceDeclaration;
            for (const declaration of symbol.declarations) {
                if (declaration.kind === SyntaxKind.ClassDeclaration || declaration.kind === SyntaxKind.InterfaceDeclaration) {
                    if (!firstDecl) {
                        firstDecl = <ClassLikeDeclaration | InterfaceDeclaration>declaration;
                    }
                    else if (!areTypeParametersIdentical(firstDecl.typeParameters, node.typeParameters)) {
                        error(node.name, Diagnostics.All_declarations_of_0_must_have_identical_type_parameters, node.name.text);
                    }
                }
            }
        }

        function checkClassExpression(node: ClassExpression): Type {
            checkClassLikeDeclaration(node);
            checkNodeDeferred(node);
            return getTypeOfSymbol(getSymbolOfNode(node));
        }

        function checkClassExpressionDeferred(node: ClassExpression) {
            forEach(node.members, checkSourceElement);
            registerForUnusedIdentifiersCheck(node);
        }

        function checkClassDeclaration(node: ClassDeclaration) {
            if (!node.name && !(node.flags & NodeFlags.Default)) {
                grammarErrorOnFirstToken(node, Diagnostics.A_class_declaration_without_the_default_modifier_must_have_a_name);
            }
            checkClassLikeDeclaration(node);
            forEach(node.members, checkSourceElement);

            registerForUnusedIdentifiersCheck(node);
        }

        function checkClassLikeDeclaration(node: ClassLikeDeclaration) {
            checkGrammarClassDeclarationHeritageClauses(node);
            checkDecorators(node);
            if (node.name) {
                checkTypeNameIsReserved(node.name, Diagnostics.Class_name_cannot_be_0);
                checkCollisionWithCapturedThisVariable(node, node.name);
                checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
            }
            checkTypeParameters(node.typeParameters);
            checkExportsOnMergedDeclarations(node);
            const symbol = getSymbolOfNode(node);
            const type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
            const typeWithThis = getTypeWithThisArgument(type);
            const staticType = <ObjectType>getTypeOfSymbol(symbol);
            checkTypeParameterListsIdentical(node, symbol);
            checkClassForDuplicateDeclarations(node);

            const baseTypeNode = getClassExtendsHeritageClauseElement(node);
            if (baseTypeNode) {
                const baseTypes = getBaseTypes(type);
                if (baseTypes.length && produceDiagnostics) {
                    const baseType = baseTypes[0];
                    const staticBaseType = getBaseConstructorTypeOfClass(type);
                    checkBaseTypeAccessibility(staticBaseType, baseTypeNode);
                    checkSourceElement(baseTypeNode.expression);
                    if (baseTypeNode.typeArguments) {
                        forEach(baseTypeNode.typeArguments, checkSourceElement);
                        for (const constructor of getConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments)) {
                            if (!checkTypeArgumentConstraints(constructor.typeParameters, baseTypeNode.typeArguments)) {
                                break;
                            }
                        }
                    }
                    checkTypeAssignableTo(typeWithThis, getTypeWithThisArgument(baseType, type.thisType), node.name || node, Diagnostics.Class_0_incorrectly_extends_base_class_1);
                    checkTypeAssignableTo(staticType, getTypeWithoutSignatures(staticBaseType), node.name || node,
                        Diagnostics.Class_static_side_0_incorrectly_extends_base_class_static_side_1);

                    if (baseType.symbol.valueDeclaration && !isInAmbientContext(baseType.symbol.valueDeclaration)) {
                        if (!isBlockScopedNameDeclaredBeforeUse(baseType.symbol.valueDeclaration, node)) {
                            error(baseTypeNode, Diagnostics.A_class_must_be_declared_after_its_base_class);
                        }
                    }

                    if (!(staticBaseType.symbol && staticBaseType.symbol.flags & SymbolFlags.Class)) {
                        // When the static base type is a "class-like" constructor function (but not actually a class), we verify
                        // that all instantiated base constructor signatures return the same type. We can simply compare the type
                        // references (as opposed to checking the structure of the types) because elsewhere we have already checked
                        // that the base type is a class or interface type (and not, for example, an anonymous object type).
                        const constructors = getInstantiatedConstructorsForTypeArguments(staticBaseType, baseTypeNode.typeArguments);
                        if (forEach(constructors, sig => getReturnTypeOfSignature(sig) !== baseType)) {
                            error(baseTypeNode.expression, Diagnostics.Base_constructors_must_all_have_the_same_return_type);
                        }
                    }
                    checkKindsOfPropertyMemberOverrides(type, baseType);
                }
            }

            const implementedTypeNodes = getClassImplementsHeritageClauseElements(node);
            if (implementedTypeNodes) {
                for (const typeRefNode of implementedTypeNodes) {
                    if (!isEntityNameExpression(typeRefNode.expression)) {
                        error(typeRefNode.expression, Diagnostics.A_class_can_only_implement_an_identifier_Slashqualified_name_with_optional_type_arguments);
                    }
                    checkTypeReferenceNode(typeRefNode);
                    if (produceDiagnostics) {
                        const t = getTypeFromTypeNode(typeRefNode);
                        if (t !== unknownType) {
                            const declaredType = (t.flags & TypeFlags.Reference) ? (<TypeReference>t).target : t;
                            if (declaredType.flags & (TypeFlags.Class | TypeFlags.Interface)) {
                                checkTypeAssignableTo(typeWithThis, getTypeWithThisArgument(t, type.thisType), node.name || node, Diagnostics.Class_0_incorrectly_implements_interface_1);
                            }
                            else {
                                error(typeRefNode, Diagnostics.A_class_may_only_implement_another_class_or_interface);
                            }
                        }
                    }
                }
            }

            if (produceDiagnostics) {
                checkIndexConstraints(type);
                checkTypeForDuplicateIndexSignatures(node);
            }
        }

        function checkBaseTypeAccessibility(type: ObjectType, node: ExpressionWithTypeArguments) {
            const signatures = getSignaturesOfType(type, SignatureKind.Construct);
            if (signatures.length) {
                const declaration = signatures[0].declaration;
                if (declaration && declaration.flags & NodeFlags.Private) {
                    const typeClassDeclaration = <ClassLikeDeclaration>getClassLikeDeclarationOfSymbol(type.symbol);
                    if (!isNodeWithinClass(node, typeClassDeclaration)) {
                        error(node, Diagnostics.Cannot_extend_a_class_0_Class_constructor_is_marked_as_private, (<Identifier>node.expression).text);
                    }
                }
            }
        }

        function getTargetSymbol(s: Symbol) {
            // if symbol is instantiated its flags are not copied from the 'target'
            // so we'll need to get back original 'target' symbol to work with correct set of flags
            return s.flags & SymbolFlags.Instantiated ? getSymbolLinks(s).target : s;
        }

        function getClassLikeDeclarationOfSymbol(symbol: Symbol): Declaration {
            return forEach(symbol.declarations, d => isClassLike(d) ? d : undefined);
        }

        function checkKindsOfPropertyMemberOverrides(type: InterfaceType, baseType: ObjectType): void {

            // TypeScript 1.0 spec (April 2014): 8.2.3
            // A derived class inherits all members from its base class it doesn't override.
            // Inheritance means that a derived class implicitly contains all non - overridden members of the base class.
            // Both public and private property members are inherited, but only public property members can be overridden.
            // A property member in a derived class is said to override a property member in a base class
            // when the derived class property member has the same name and kind(instance or static)
            // as the base class property member.
            // The type of an overriding property member must be assignable(section 3.8.4)
            // to the type of the overridden property member, or otherwise a compile - time error occurs.
            // Base class instance member functions can be overridden by derived class instance member functions,
            // but not by other kinds of members.
            // Base class instance member variables and accessors can be overridden by
            // derived class instance member variables and accessors, but not by other kinds of members.

            // NOTE: assignability is checked in checkClassDeclaration
            const baseProperties = getPropertiesOfObjectType(baseType);
            for (const baseProperty of baseProperties) {
                const base = getTargetSymbol(baseProperty);

                if (base.flags & SymbolFlags.Prototype) {
                    continue;
                }

                const derived = getTargetSymbol(getPropertyOfObjectType(type, base.name));
                const baseDeclarationFlags = getDeclarationFlagsFromSymbol(base);

                Debug.assert(!!derived, "derived should point to something, even if it is the base class' declaration.");

                if (derived) {
                    // In order to resolve whether the inherited method was overridden in the base class or not,
                    // we compare the Symbols obtained. Since getTargetSymbol returns the symbol on the *uninstantiated*
                    // type declaration, derived and base resolve to the same symbol even in the case of generic classes.
                    if (derived === base) {
                        // derived class inherits base without override/redeclaration

                        const derivedClassDecl = getClassLikeDeclarationOfSymbol(type.symbol);

                        // It is an error to inherit an abstract member without implementing it or being declared abstract.
                        // If there is no declaration for the derived class (as in the case of class expressions),
                        // then the class cannot be declared abstract.
                        if (baseDeclarationFlags & NodeFlags.Abstract && (!derivedClassDecl || !(derivedClassDecl.flags & NodeFlags.Abstract))) {
                            if (derivedClassDecl.kind === SyntaxKind.ClassExpression) {
                                error(derivedClassDecl, Diagnostics.Non_abstract_class_expression_does_not_implement_inherited_abstract_member_0_from_class_1,
                                    symbolToString(baseProperty), typeToString(baseType));
                            }
                            else {
                                error(derivedClassDecl, Diagnostics.Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2,
                                    typeToString(type), symbolToString(baseProperty), typeToString(baseType));
                            }
                        }
                    }
                    else {
                        // derived overrides base.
                        const derivedDeclarationFlags = getDeclarationFlagsFromSymbol(derived);
                        if ((baseDeclarationFlags & NodeFlags.Private) || (derivedDeclarationFlags & NodeFlags.Private)) {
                            // either base or derived property is private - not override, skip it
                            continue;
                        }

                        if ((baseDeclarationFlags & NodeFlags.Static) !== (derivedDeclarationFlags & NodeFlags.Static)) {
                            // value of 'static' is not the same for properties - not override, skip it
                            continue;
                        }

                        if ((base.flags & derived.flags & SymbolFlags.Method) || ((base.flags & SymbolFlags.PropertyOrAccessor) && (derived.flags & SymbolFlags.PropertyOrAccessor))) {
                            // method is overridden with method or property/accessor is overridden with property/accessor - correct case
                            continue;
                        }

                        let errorMessage: DiagnosticMessage;
                        if (base.flags & SymbolFlags.Method) {
                            if (derived.flags & SymbolFlags.Accessor) {
                                errorMessage = Diagnostics.Class_0_defines_instance_member_function_1_but_extended_class_2_defines_it_as_instance_member_accessor;
                            }
                            else {
                                Debug.assert((derived.flags & SymbolFlags.Property) !== 0);
                                errorMessage = Diagnostics.Class_0_defines_instance_member_function_1_but_extended_class_2_defines_it_as_instance_member_property;
                            }
                        }
                        else if (base.flags & SymbolFlags.Property) {
                            Debug.assert((derived.flags & SymbolFlags.Method) !== 0);
                            errorMessage = Diagnostics.Class_0_defines_instance_member_property_1_but_extended_class_2_defines_it_as_instance_member_function;
                        }
                        else {
                            Debug.assert((base.flags & SymbolFlags.Accessor) !== 0);
                            Debug.assert((derived.flags & SymbolFlags.Method) !== 0);
                            errorMessage = Diagnostics.Class_0_defines_instance_member_accessor_1_but_extended_class_2_defines_it_as_instance_member_function;
                        }

                        error(derived.valueDeclaration.name, errorMessage, typeToString(baseType), symbolToString(base), typeToString(type));
                    }
                }
            }
        }

        function isAccessor(kind: SyntaxKind): boolean {
            return kind === SyntaxKind.GetAccessor || kind === SyntaxKind.SetAccessor;
        }

        function areTypeParametersIdentical(list1: TypeParameterDeclaration[], list2: TypeParameterDeclaration[]) {
            if (!list1 && !list2) {
                return true;
            }
            if (!list1 || !list2 || list1.length !== list2.length) {
                return false;
            }
            // TypeScript 1.0 spec (April 2014):
            // When a generic interface has multiple declarations,  all declarations must have identical type parameter
            // lists, i.e. identical type parameter names with identical constraints in identical order.
            for (let i = 0, len = list1.length; i < len; i++) {
                const tp1 = list1[i];
                const tp2 = list2[i];
                if (tp1.name.text !== tp2.name.text) {
                    return false;
                }
                if (!tp1.constraint && !tp2.constraint) {
                    continue;
                }
                if (!tp1.constraint || !tp2.constraint) {
                    return false;
                }
                if (!isTypeIdenticalTo(getTypeFromTypeNode(tp1.constraint), getTypeFromTypeNode(tp2.constraint))) {
                    return false;
                }
            }
            return true;
        }

        function checkInheritedPropertiesAreIdentical(type: InterfaceType, typeNode: Node): boolean {
            const baseTypes = getBaseTypes(type);
            if (baseTypes.length < 2) {
                return true;
            }

            const seen = createMap<{ prop: Symbol; containingType: Type }>();
            forEach(resolveDeclaredMembers(type).declaredProperties, p => { seen[p.name] = { prop: p, containingType: type }; });
            let ok = true;

            for (const base of baseTypes) {
                const properties = getPropertiesOfObjectType(getTypeWithThisArgument(base, type.thisType));
                for (const prop of properties) {
                    const existing = seen[prop.name];
                    if (!existing) {
                        seen[prop.name] = { prop: prop, containingType: base };
                    }
                    else {
                        const isInheritedProperty = existing.containingType !== type;
                        if (isInheritedProperty && !isPropertyIdenticalTo(existing.prop, prop)) {
                            ok = false;

                            const typeName1 = typeToString(existing.containingType);
                            const typeName2 = typeToString(base);

                            let errorInfo = chainDiagnosticMessages(undefined, Diagnostics.Named_property_0_of_types_1_and_2_are_not_identical, symbolToString(prop), typeName1, typeName2);
                            errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Interface_0_cannot_simultaneously_extend_types_1_and_2, typeToString(type), typeName1, typeName2);
                            diagnostics.add(createDiagnosticForNodeFromMessageChain(typeNode, errorInfo));
                        }
                    }
                }
            }

            return ok;
        }

        function checkInterfaceDeclaration(node: InterfaceDeclaration) {
            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node) || checkGrammarInterfaceDeclaration(node);

            checkTypeParameters(node.typeParameters);
            if (produceDiagnostics) {
                checkTypeNameIsReserved(node.name, Diagnostics.Interface_name_cannot_be_0);

                checkExportsOnMergedDeclarations(node);
                const symbol = getSymbolOfNode(node);
                checkTypeParameterListsIdentical(node, symbol);

                // Only check this symbol once
                const firstInterfaceDecl = <InterfaceDeclaration>getDeclarationOfKind(symbol, SyntaxKind.InterfaceDeclaration);
                if (node === firstInterfaceDecl) {
                    const type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
                    const typeWithThis = getTypeWithThisArgument(type);
                    // run subsequent checks only if first set succeeded
                    if (checkInheritedPropertiesAreIdentical(type, node.name)) {
                        for (const baseType of getBaseTypes(type)) {
                            checkTypeAssignableTo(typeWithThis, getTypeWithThisArgument(baseType, type.thisType), node.name, Diagnostics.Interface_0_incorrectly_extends_interface_1);
                        }
                        checkIndexConstraints(type);
                    }
                }
                checkObjectTypeForDuplicateDeclarations(node);
            }
            forEach(getInterfaceBaseTypeNodes(node), heritageElement => {
                if (!isEntityNameExpression(heritageElement.expression)) {
                    error(heritageElement.expression, Diagnostics.An_interface_can_only_extend_an_identifier_Slashqualified_name_with_optional_type_arguments);
                }
                checkTypeReferenceNode(heritageElement);
            });

            forEach(node.members, checkSourceElement);

            if (produceDiagnostics) {
                checkTypeForDuplicateIndexSignatures(node);
                registerForUnusedIdentifiersCheck(node);
            }
        }

        function checkTypeAliasDeclaration(node: TypeAliasDeclaration) {
            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node);

            checkTypeNameIsReserved(node.name, Diagnostics.Type_alias_name_cannot_be_0);
            checkSourceElement(node.type);
        }

        function computeEnumMemberValues(node: EnumDeclaration) {
            const nodeLinks = getNodeLinks(node);

            if (!(nodeLinks.flags & NodeCheckFlags.EnumValuesComputed)) {
                const enumSymbol = getSymbolOfNode(node);
                const enumType = getDeclaredTypeOfSymbol(enumSymbol);
                let autoValue = 0; // set to undefined when enum member is non-constant
                const ambient = isInAmbientContext(node);
                const enumIsConst = isConst(node);

                for (const member of node.members) {
                    if (isComputedNonLiteralName(<PropertyName>member.name)) {
                        error(member.name, Diagnostics.Computed_property_names_are_not_allowed_in_enums);
                    }
                    else {
                        const text = getTextOfPropertyName(<PropertyName>member.name);
                        if (isNumericLiteralName(text)) {
                            error(member.name, Diagnostics.An_enum_member_cannot_have_a_numeric_name);
                        }
                    }

                    const previousEnumMemberIsNonConstant = autoValue === undefined;

                    const initializer = member.initializer;
                    if (initializer) {
                        autoValue = computeConstantValueForEnumMemberInitializer(initializer, enumType, enumIsConst, ambient);
                    }
                    else if (ambient && !enumIsConst) {
                        // In ambient enum declarations that specify no const modifier, enum member declarations
                        // that omit a value are considered computed members (as opposed to having auto-incremented values assigned).
                        autoValue = undefined;
                    }
                    else if (previousEnumMemberIsNonConstant) {
                        // If the member declaration specifies no value, the member is considered a constant enum member.
                        // If the member is the first member in the enum declaration, it is assigned the value zero.
                        // Otherwise, it is assigned the value of the immediately preceding member plus one,
                        // and an error occurs if the immediately preceding member is not a constant enum member
                        error(member.name, Diagnostics.Enum_member_must_have_initializer);
                    }

                    if (autoValue !== undefined) {
                        getNodeLinks(member).enumMemberValue = autoValue;
                        autoValue++;
                    }
                }

                nodeLinks.flags |= NodeCheckFlags.EnumValuesComputed;
            }

            function computeConstantValueForEnumMemberInitializer(initializer: Expression, enumType: Type, enumIsConst: boolean, ambient: boolean): number {
                // Controls if error should be reported after evaluation of constant value is completed
                // Can be false if another more precise error was already reported during evaluation.
                let reportError = true;
                const value = evalConstant(initializer);

                if (reportError) {
                    if (value === undefined) {
                        if (enumIsConst) {
                            error(initializer, Diagnostics.In_const_enum_declarations_member_initializer_must_be_constant_expression);
                        }
                        else if (ambient) {
                            error(initializer, Diagnostics.In_ambient_enum_declarations_member_initializer_must_be_constant_expression);
                        }
                        else {
                            // Only here do we need to check that the initializer is assignable to the enum type.
                            checkTypeAssignableTo(checkExpression(initializer), enumType, initializer, /*headMessage*/ undefined);
                        }
                    }
                    else if (enumIsConst) {
                        if (isNaN(value)) {
                            error(initializer, Diagnostics.const_enum_member_initializer_was_evaluated_to_disallowed_value_NaN);
                        }
                        else if (!isFinite(value)) {
                            error(initializer, Diagnostics.const_enum_member_initializer_was_evaluated_to_a_non_finite_value);
                        }
                    }
                }

                return value;

                function evalConstant(e: Node): number {
                    switch (e.kind) {
                        case SyntaxKind.PrefixUnaryExpression:
                            const value = evalConstant((<PrefixUnaryExpression>e).operand);
                            if (value === undefined) {
                                return undefined;
                            }
                            switch ((<PrefixUnaryExpression>e).operator) {
                                case SyntaxKind.PlusToken: return value;
                                case SyntaxKind.MinusToken: return -value;
                                case SyntaxKind.TildeToken: return ~value;
                            }
                            return undefined;
                        case SyntaxKind.BinaryExpression:
                            const left = evalConstant((<BinaryExpression>e).left);
                            if (left === undefined) {
                                return undefined;
                            }
                            const right = evalConstant((<BinaryExpression>e).right);
                            if (right === undefined) {
                                return undefined;
                            }
                            switch ((<BinaryExpression>e).operatorToken.kind) {
                                case SyntaxKind.BarToken: return left | right;
                                case SyntaxKind.AmpersandToken: return left & right;
                                case SyntaxKind.GreaterThanGreaterThanToken: return left >> right;
                                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken: return left >>> right;
                                case SyntaxKind.LessThanLessThanToken: return left << right;
                                case SyntaxKind.CaretToken: return left ^ right;
                                case SyntaxKind.AsteriskToken: return left * right;
                                case SyntaxKind.SlashToken: return left / right;
                                case SyntaxKind.PlusToken: return left + right;
                                case SyntaxKind.MinusToken: return left - right;
                                case SyntaxKind.PercentToken: return left % right;
                            }
                            return undefined;
                        case SyntaxKind.NumericLiteral:
                            return +(<LiteralExpression>e).text;
                        case SyntaxKind.ParenthesizedExpression:
                            return evalConstant((<ParenthesizedExpression>e).expression);
                        case SyntaxKind.Identifier:
                        case SyntaxKind.ElementAccessExpression:
                        case SyntaxKind.PropertyAccessExpression:
                            const member = initializer.parent;
                            const currentType = getTypeOfSymbol(getSymbolOfNode(member.parent));
                            let enumType: Type;
                            let propertyName: string;

                            if (e.kind === SyntaxKind.Identifier) {
                                // unqualified names can refer to member that reside in different declaration of the enum so just doing name resolution won't work.
                                // instead pick current enum type and later try to fetch member from the type
                                enumType = currentType;
                                propertyName = (<Identifier>e).text;
                            }
                            else {
                                let expression: Expression;
                                if (e.kind === SyntaxKind.ElementAccessExpression) {
                                    if ((<ElementAccessExpression>e).argumentExpression === undefined ||
                                        (<ElementAccessExpression>e).argumentExpression.kind !== SyntaxKind.StringLiteral) {
                                        return undefined;
                                    }
                                    expression = (<ElementAccessExpression>e).expression;
                                    propertyName = (<LiteralExpression>(<ElementAccessExpression>e).argumentExpression).text;
                                }
                                else {
                                    expression = (<PropertyAccessExpression>e).expression;
                                    propertyName = (<PropertyAccessExpression>e).name.text;
                                }

                                // expression part in ElementAccess\PropertyAccess should be either identifier or dottedName
                                let current = expression;
                                while (current) {
                                    if (current.kind === SyntaxKind.Identifier) {
                                        break;
                                    }
                                    else if (current.kind === SyntaxKind.PropertyAccessExpression) {
                                        current = (<ElementAccessExpression>current).expression;
                                    }
                                    else {
                                        return undefined;
                                    }
                                }

                                enumType = checkExpression(expression);
                                // allow references to constant members of other enums
                                if (!(enumType.symbol && (enumType.symbol.flags & SymbolFlags.Enum))) {
                                    return undefined;
                                }
                            }

                            if (propertyName === undefined) {
                                return undefined;
                            }

                            const property = getPropertyOfObjectType(enumType, propertyName);
                            if (!property || !(property.flags & SymbolFlags.EnumMember)) {
                                return undefined;
                            }

                            const propertyDecl = property.valueDeclaration;
                            // self references are illegal
                            if (member === propertyDecl) {
                                return undefined;
                            }

                            // illegal case: forward reference
                            if (!isBlockScopedNameDeclaredBeforeUse(propertyDecl, member)) {
                                reportError = false;
                                error(e, Diagnostics.A_member_initializer_in_a_enum_declaration_cannot_reference_members_declared_after_it_including_members_defined_in_other_enums);
                                return undefined;
                            }

                            return <number>getNodeLinks(propertyDecl).enumMemberValue;
                    }
                }
            }
        }

        function checkEnumDeclaration(node: EnumDeclaration) {
            if (!produceDiagnostics) {
                return;
            }

            // Grammar checking
            checkGrammarDecorators(node) || checkGrammarModifiers(node);

            checkTypeNameIsReserved(node.name, Diagnostics.Enum_name_cannot_be_0);
            checkCollisionWithCapturedThisVariable(node, node.name);
            checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
            checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
            checkExportsOnMergedDeclarations(node);

            computeEnumMemberValues(node);

            const enumIsConst = isConst(node);
            if (compilerOptions.isolatedModules && enumIsConst && isInAmbientContext(node)) {
                error(node.name, Diagnostics.Ambient_const_enums_are_not_allowed_when_the_isolatedModules_flag_is_provided);
            }

            // Spec 2014 - Section 9.3:
            // It isn't possible for one enum declaration to continue the automatic numbering sequence of another,
            // and when an enum type has multiple declarations, only one declaration is permitted to omit a value
            // for the first member.
            //
            // Only perform this check once per symbol
            const enumSymbol = getSymbolOfNode(node);
            const firstDeclaration = getDeclarationOfKind(enumSymbol, node.kind);
            if (node === firstDeclaration) {
                if (enumSymbol.declarations.length > 1) {
                    // check that const is placed\omitted on all enum declarations
                    forEach(enumSymbol.declarations, decl => {
                        if (isConstEnumDeclaration(decl) !== enumIsConst) {
                            error(decl.name, Diagnostics.Enum_declarations_must_all_be_const_or_non_const);
                        }
                    });
                }

                let seenEnumMissingInitialInitializer = false;
                forEach(enumSymbol.declarations, declaration => {
                    // return true if we hit a violation of the rule, false otherwise
                    if (declaration.kind !== SyntaxKind.EnumDeclaration) {
                        return false;
                    }

                    const enumDeclaration = <EnumDeclaration>declaration;
                    if (!enumDeclaration.members.length) {
                        return false;
                    }

                    const firstEnumMember = enumDeclaration.members[0];
                    if (!firstEnumMember.initializer) {
                        if (seenEnumMissingInitialInitializer) {
                            error(firstEnumMember.name, Diagnostics.In_an_enum_with_multiple_declarations_only_one_declaration_can_omit_an_initializer_for_its_first_enum_element);
                        }
                        else {
                            seenEnumMissingInitialInitializer = true;
                        }
                    }
                });
            }
        }

        function getFirstNonAmbientClassOrFunctionDeclaration(symbol: Symbol): Declaration {
            const declarations = symbol.declarations;
            for (const declaration of declarations) {
                if ((declaration.kind === SyntaxKind.ClassDeclaration ||
                    (declaration.kind === SyntaxKind.FunctionDeclaration && nodeIsPresent((<FunctionLikeDeclaration>declaration).body))) &&
                    !isInAmbientContext(declaration)) {
                    return declaration;
                }
            }
            return undefined;
        }

        function inSameLexicalScope(node1: Node, node2: Node) {
            const container1 = getEnclosingBlockScopeContainer(node1);
            const container2 = getEnclosingBlockScopeContainer(node2);
            if (isGlobalSourceFile(container1)) {
                return isGlobalSourceFile(container2);
            }
            else if (isGlobalSourceFile(container2)) {
                return false;
            }
            else {
                return container1 === container2;
            }
        }

        function checkModuleDeclaration(node: ModuleDeclaration) {
            if (produceDiagnostics) {
                // Grammar checking
                const isGlobalAugmentation = isGlobalScopeAugmentation(node);
                const inAmbientContext = isInAmbientContext(node);
                if (isGlobalAugmentation && !inAmbientContext) {
                    error(node.name, Diagnostics.Augmentations_for_the_global_scope_should_have_declare_modifier_unless_they_appear_in_already_ambient_context);
                }

                const isAmbientExternalModule = isAmbientModule(node);
                const contextErrorMessage = isAmbientExternalModule
                    ? Diagnostics.An_ambient_module_declaration_is_only_allowed_at_the_top_level_in_a_file
                    : Diagnostics.A_namespace_declaration_is_only_allowed_in_a_namespace_or_module;
                if (checkGrammarModuleElementContext(node, contextErrorMessage)) {
                    // If we hit a module declaration in an illegal context, just bail out to avoid cascading errors.
                    return;
                }

                if (!checkGrammarDecorators(node) && !checkGrammarModifiers(node)) {
                    if (!inAmbientContext && node.name.kind === SyntaxKind.StringLiteral) {
                        grammarErrorOnNode(node.name, Diagnostics.Only_ambient_modules_can_use_quoted_names);
                    }
                }

                checkCollisionWithCapturedThisVariable(node, node.name);
                checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
                checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
                checkExportsOnMergedDeclarations(node);
                const symbol = getSymbolOfNode(node);

                // The following checks only apply on a non-ambient instantiated module declaration.
                if (symbol.flags & SymbolFlags.ValueModule
                    && symbol.declarations.length > 1
                    && !inAmbientContext
                    && isInstantiatedModule(node, compilerOptions.preserveConstEnums || compilerOptions.isolatedModules)) {
                    const firstNonAmbientClassOrFunc = getFirstNonAmbientClassOrFunctionDeclaration(symbol);
                    if (firstNonAmbientClassOrFunc) {
                        if (getSourceFileOfNode(node) !== getSourceFileOfNode(firstNonAmbientClassOrFunc)) {
                            error(node.name, Diagnostics.A_namespace_declaration_cannot_be_in_a_different_file_from_a_class_or_function_with_which_it_is_merged);
                        }
                        else if (node.pos < firstNonAmbientClassOrFunc.pos) {
                            error(node.name, Diagnostics.A_namespace_declaration_cannot_be_located_prior_to_a_class_or_function_with_which_it_is_merged);
                        }
                    }

                    // if the module merges with a class declaration in the same lexical scope,
                    // we need to track this to ensure the correct emit.
                    const mergedClass = getDeclarationOfKind(symbol, SyntaxKind.ClassDeclaration);
                    if (mergedClass &&
                        inSameLexicalScope(node, mergedClass)) {
                        getNodeLinks(node).flags |= NodeCheckFlags.LexicalModuleMergesWithClass;
                    }
                }

                if (isAmbientExternalModule) {
                    if (isExternalModuleAugmentation(node)) {
                        // body of the augmentation should be checked for consistency only if augmentation was applied to its target (either global scope or module)
                        // otherwise we'll be swamped in cascading errors.
                        // We can detect if augmentation was applied using following rules:
                        // - augmentation for a global scope is always applied
                        // - augmentation for some external module is applied if symbol for augmentation is merged (it was combined with target module).
                        const checkBody = isGlobalAugmentation || (getSymbolOfNode(node).flags & SymbolFlags.Merged);
                        if (checkBody && node.body) {
                            // body of ambient external module is always a module block
                            for (const statement of (<ModuleBlock>node.body).statements) {
                                checkModuleAugmentationElement(statement, isGlobalAugmentation);
                            }
                        }
                    }
                    else if (isGlobalSourceFile(node.parent)) {
                        if (isGlobalAugmentation) {
                            error(node.name, Diagnostics.Augmentations_for_the_global_scope_can_only_be_directly_nested_in_external_modules_or_ambient_module_declarations);
                        }
                        else if (isExternalModuleNameRelative(node.name.text)) {
                            error(node.name, Diagnostics.Ambient_module_declaration_cannot_specify_relative_module_name);
                        }
                    }
                    else {
                        if (isGlobalAugmentation) {
                            error(node.name, Diagnostics.Augmentations_for_the_global_scope_can_only_be_directly_nested_in_external_modules_or_ambient_module_declarations);
                        }
                        else {
                            // Node is not an augmentation and is not located on the script level.
                            // This means that this is declaration of ambient module that is located in other module or namespace which is prohibited.
                            error(node.name, Diagnostics.Ambient_modules_cannot_be_nested_in_other_modules_or_namespaces);
                        }
                    }
                }
            }

            if (compilerOptions.noImplicitAny && !node.body) {
                // Ambient shorthand module is an implicit any
                reportImplicitAnyError(node, anyType);
            }

            if (node.body) {
                checkSourceElement(node.body);
                if (!isGlobalScopeAugmentation(node)) {
                    registerForUnusedIdentifiersCheck(node);
                }
            }
        }

        function checkModuleAugmentationElement(node: Node, isGlobalAugmentation: boolean): void {
            switch (node.kind) {
                case SyntaxKind.VariableStatement:
                    // error each individual name in variable statement instead of marking the entire variable statement
                    for (const decl of (<VariableStatement>node).declarationList.declarations) {
                        checkModuleAugmentationElement(decl, isGlobalAugmentation);
                    }
                    break;
                case SyntaxKind.ExportAssignment:
                case SyntaxKind.ExportDeclaration:
                    grammarErrorOnFirstToken(node, Diagnostics.Exports_and_export_assignments_are_not_permitted_in_module_augmentations);
                    break;
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ImportDeclaration:
                    grammarErrorOnFirstToken(node, Diagnostics.Imports_are_not_permitted_in_module_augmentations_Consider_moving_them_to_the_enclosing_external_module);
                    break;
                case SyntaxKind.BindingElement:
                case SyntaxKind.VariableDeclaration:
                    const name = (<VariableDeclaration | BindingElement>node).name;
                    if (isBindingPattern(name)) {
                        for (const el of name.elements) {
                            // mark individual names in binding pattern
                            checkModuleAugmentationElement(el, isGlobalAugmentation);
                        }
                        break;
                    }
                    // fallthrough
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.EnumDeclaration:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.ModuleDeclaration:
                case SyntaxKind.TypeAliasDeclaration:
                    if (isGlobalAugmentation) {
                        return;
                    }
                    const symbol = getSymbolOfNode(node);
                    if (symbol) {
                        // module augmentations cannot introduce new names on the top level scope of the module
                        // this is done it two steps
                        // 1. quick check - if symbol for node is not merged - this is local symbol to this augmentation - report error
                        // 2. main check - report error if value declaration of the parent symbol is module augmentation)
                        let reportError = !(symbol.flags & SymbolFlags.Merged);
                        if (!reportError) {
                            // symbol should not originate in augmentation
                            reportError = isExternalModuleAugmentation(symbol.parent.declarations[0]);
                        }
                    }
                    break;
            }
        }

        function getFirstIdentifier(node: EntityNameOrEntityNameExpression): Identifier {
            switch (node.kind) {
                case SyntaxKind.Identifier:
                    return <Identifier>node;
                case SyntaxKind.QualifiedName:
                    do {
                        node = (<QualifiedName>node).left;
                    } while (node.kind !== SyntaxKind.Identifier);
                    return <Identifier>node;
                case SyntaxKind.PropertyAccessExpression:
                    do {
                        node = (<PropertyAccessEntityNameExpression>node).expression;
                    } while (node.kind !== SyntaxKind.Identifier);
                    return <Identifier>node;
            }
        }

        function checkExternalImportOrExportDeclaration(node: ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration): boolean {
            const moduleName = getExternalModuleName(node);
            if (!nodeIsMissing(moduleName) && moduleName.kind !== SyntaxKind.StringLiteral) {
                error(moduleName, Diagnostics.String_literal_expected);
                return false;
            }
            const inAmbientExternalModule = node.parent.kind === SyntaxKind.ModuleBlock && isAmbientModule(<ModuleDeclaration>node.parent.parent);
            if (node.parent.kind !== SyntaxKind.SourceFile && !inAmbientExternalModule) {
                error(moduleName, node.kind === SyntaxKind.ExportDeclaration ?
                    Diagnostics.Export_declarations_are_not_permitted_in_a_namespace :
                    Diagnostics.Import_declarations_in_a_namespace_cannot_reference_a_module);
                return false;
            }
            if (inAmbientExternalModule && isExternalModuleNameRelative((<LiteralExpression>moduleName).text)) {
                // we have already reported errors on top level imports\exports in external module augmentations in checkModuleDeclaration
                // no need to do this again.
                if (!isTopLevelInExternalModuleAugmentation(node)) {
                    // TypeScript 1.0 spec (April 2013): 12.1.6
                    // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference
                    // other external modules only through top - level external module names.
                    // Relative external module names are not permitted.
                    error(node, Diagnostics.Import_or_export_declaration_in_an_ambient_module_declaration_cannot_reference_module_through_relative_module_name);
                    return false;
                }
            }
            return true;
        }

        function checkAliasSymbol(node: ImportEqualsDeclaration | ImportClause | NamespaceImport | ImportSpecifier | ExportSpecifier) {
            const symbol = getSymbolOfNode(node);
            const target = resolveAlias(symbol);
            if (target !== unknownSymbol) {
                // For external modules symbol represent local symbol for an alias.
                // This local symbol will merge any other local declarations (excluding other aliases)
                // and symbol.flags will contains combined representation for all merged declaration.
                // Based on symbol.flags we can compute a set of excluded meanings (meaning that resolved alias should not have,
                // otherwise it will conflict with some local declaration). Note that in addition to normal flags we include matching SymbolFlags.Export*
                // in order to prevent collisions with declarations that were exported from the current module (they still contribute to local names).
                const excludedMeanings =
                    (symbol.flags & (SymbolFlags.Value | SymbolFlags.ExportValue) ? SymbolFlags.Value : 0) |
                    (symbol.flags & SymbolFlags.Type ? SymbolFlags.Type : 0) |
                    (symbol.flags & SymbolFlags.Namespace ? SymbolFlags.Namespace : 0);
                if (target.flags & excludedMeanings) {
                    const message = node.kind === SyntaxKind.ExportSpecifier ?
                        Diagnostics.Export_declaration_conflicts_with_exported_declaration_of_0 :
                        Diagnostics.Import_declaration_conflicts_with_local_declaration_of_0;
                    error(node, message, symbolToString(symbol));
                }
            }
        }

        function checkImportBinding(node: ImportEqualsDeclaration | ImportClause | NamespaceImport | ImportSpecifier) {
            checkCollisionWithCapturedThisVariable(node, node.name);
            checkCollisionWithRequireExportsInGeneratedCode(node, node.name);
            checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name);
            checkAliasSymbol(node);
        }

        function checkImportDeclaration(node: ImportDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_import_declaration_can_only_be_used_in_a_namespace_or_module)) {
                // If we hit an import declaration in an illegal context, just bail out to avoid cascading errors.
                return;
            }
            if (!checkGrammarDecorators(node) && !checkGrammarModifiers(node) && (node.flags & NodeFlags.Modifier)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_import_declaration_cannot_have_modifiers);
            }
            if (checkExternalImportOrExportDeclaration(node)) {
                const importClause = node.importClause;
                if (importClause) {
                    if (importClause.name) {
                        checkImportBinding(importClause);
                    }
                    if (importClause.namedBindings) {
                        if (importClause.namedBindings.kind === SyntaxKind.NamespaceImport) {
                            checkImportBinding(<NamespaceImport>importClause.namedBindings);
                        }
                        else {
                            forEach((<NamedImports>importClause.namedBindings).elements, checkImportBinding);
                        }
                    }
                }
            }
        }

        function checkImportEqualsDeclaration(node: ImportEqualsDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_import_declaration_can_only_be_used_in_a_namespace_or_module)) {
                // If we hit an import declaration in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            checkGrammarDecorators(node) || checkGrammarModifiers(node);
            if (isInternalModuleImportEqualsDeclaration(node) || checkExternalImportOrExportDeclaration(node)) {
                checkImportBinding(node);
                if (node.flags & NodeFlags.Export) {
                    markExportAsReferenced(node);
                }
                if (isInternalModuleImportEqualsDeclaration(node)) {
                    const target = resolveAlias(getSymbolOfNode(node));
                    if (target !== unknownSymbol) {
                        if (target.flags & SymbolFlags.Value) {
                            // Target is a value symbol, check that it is not hidden by a local declaration with the same name
                            const moduleName = getFirstIdentifier(<EntityName>node.moduleReference);
                            if (!(resolveEntityName(moduleName, SymbolFlags.Value | SymbolFlags.Namespace).flags & SymbolFlags.Namespace)) {
                                error(moduleName, Diagnostics.Module_0_is_hidden_by_a_local_declaration_with_the_same_name, declarationNameToString(moduleName));
                            }
                        }
                        if (target.flags & SymbolFlags.Type) {
                            checkTypeNameIsReserved(node.name, Diagnostics.Import_name_cannot_be_0);
                        }
                    }
                }
                else {
                    if (modulekind === ModuleKind.ES6 && !isInAmbientContext(node)) {
                        // Import equals declaration is deprecated in es6 or above
                        grammarErrorOnNode(node, Diagnostics.Import_assignment_cannot_be_used_when_targeting_ECMAScript_2015_modules_Consider_using_import_Asterisk_as_ns_from_mod_import_a_from_mod_import_d_from_mod_or_another_module_format_instead);
                    }
                }
            }
        }

        function checkExportDeclaration(node: ExportDeclaration) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_export_declaration_can_only_be_used_in_a_module)) {
                // If we hit an export in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            if (!checkGrammarDecorators(node) && !checkGrammarModifiers(node) && (node.flags & NodeFlags.Modifier)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_export_declaration_cannot_have_modifiers);
            }

            if (!node.moduleSpecifier || checkExternalImportOrExportDeclaration(node)) {
                if (node.exportClause) {
                    // export { x, y }
                    // export { x, y } from "foo"
                    forEach(node.exportClause.elements, checkExportSpecifier);

                    const inAmbientExternalModule = node.parent.kind === SyntaxKind.ModuleBlock && isAmbientModule(node.parent.parent);
                    if (node.parent.kind !== SyntaxKind.SourceFile && !inAmbientExternalModule) {
                        error(node, Diagnostics.Export_declarations_are_not_permitted_in_a_namespace);
                    }
                }
                else {
                    // export * from "foo"
                    const moduleSymbol = resolveExternalModuleName(node, node.moduleSpecifier);
                    if (moduleSymbol && hasExportAssignmentSymbol(moduleSymbol)) {
                        error(node.moduleSpecifier, Diagnostics.Module_0_uses_export_and_cannot_be_used_with_export_Asterisk, symbolToString(moduleSymbol));
                    }
                }
            }
        }

        function checkGrammarModuleElementContext(node: Statement, errorMessage: DiagnosticMessage): boolean {
            if (node.parent.kind !== SyntaxKind.SourceFile && node.parent.kind !== SyntaxKind.ModuleBlock && node.parent.kind !== SyntaxKind.ModuleDeclaration) {
                return grammarErrorOnFirstToken(node, errorMessage);
            }
        }

        function checkExportSpecifier(node: ExportSpecifier) {
            checkAliasSymbol(node);
            if (!(<ExportDeclaration>node.parent.parent).moduleSpecifier) {
                const exportedName = node.propertyName || node.name;
                // find immediate value referenced by exported name (SymbolFlags.Alias is set so we don't chase down aliases)
                const symbol = resolveName(exportedName, exportedName.text, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias,
                    /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined);
                if (symbol && (symbol === undefinedSymbol || isGlobalSourceFile(getDeclarationContainer(symbol.declarations[0])))) {
                    error(exportedName, Diagnostics.Cannot_export_0_Only_local_declarations_can_be_exported_from_a_module, exportedName.text);
                }
                else {
                    markExportAsReferenced(node);
                }
            }
        }

        function checkExportAssignment(node: ExportAssignment) {
            if (checkGrammarModuleElementContext(node, Diagnostics.An_export_assignment_can_only_be_used_in_a_module)) {
                // If we hit an export assignment in an illegal context, just bail out to avoid cascading errors.
                return;
            }

            const container = node.parent.kind === SyntaxKind.SourceFile ? <SourceFile>node.parent : <ModuleDeclaration>node.parent.parent;
            if (container.kind === SyntaxKind.ModuleDeclaration && !isAmbientModule(container)) {
                error(node, Diagnostics.An_export_assignment_cannot_be_used_in_a_namespace);
                return;
            }
            // Grammar checking
            if (!checkGrammarDecorators(node) && !checkGrammarModifiers(node) && (node.flags & NodeFlags.Modifier)) {
                grammarErrorOnFirstToken(node, Diagnostics.An_export_assignment_cannot_have_modifiers);
            }
            if (node.expression.kind === SyntaxKind.Identifier) {
                markExportAsReferenced(node);
            }
            else {
                checkExpressionCached(node.expression);
            }

            checkExternalModuleExports(<SourceFile | ModuleDeclaration>container);

            if (node.isExportEquals && !isInAmbientContext(node)) {
                if (modulekind === ModuleKind.ES6) {
                    // export assignment is not supported in es6 modules
                    grammarErrorOnNode(node, Diagnostics.Export_assignment_cannot_be_used_when_targeting_ECMAScript_2015_modules_Consider_using_export_default_or_another_module_format_instead);
                }
                else if (modulekind === ModuleKind.System) {
                    // system modules does not support export assignment
                    grammarErrorOnNode(node, Diagnostics.Export_assignment_is_not_supported_when_module_flag_is_system);
                }
            }
        }

        function hasExportedMembers(moduleSymbol: Symbol) {
            for (const id in moduleSymbol.exports) {
                if (id !== "export=") {
                    return true;
                }
            }
            return false;
        }

        function checkExternalModuleExports(node: SourceFile | ModuleDeclaration) {
            const moduleSymbol = getSymbolOfNode(node);
            const links = getSymbolLinks(moduleSymbol);
            if (!links.exportsChecked) {
                const exportEqualsSymbol = moduleSymbol.exports["export="];
                if (exportEqualsSymbol && hasExportedMembers(moduleSymbol)) {
                    const declaration = getDeclarationOfAliasSymbol(exportEqualsSymbol) || exportEqualsSymbol.valueDeclaration;
                    if (!isTopLevelInExternalModuleAugmentation(declaration)) {
                        error(declaration, Diagnostics.An_export_assignment_cannot_be_used_in_a_module_with_other_exported_elements);
                    }
                }
                // Checks for export * conflicts
                const exports = getExportsOfModule(moduleSymbol);
                for (const id in exports) {
                    if (id === "__export") {
                        continue;
                    }
                    const { declarations, flags } = exports[id];
                    // ECMA262: 15.2.1.1 It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
                    // (TS Exceptions: namespaces, function overloads, enums, and interfaces)
                    if (flags & (SymbolFlags.Namespace | SymbolFlags.Interface | SymbolFlags.Enum)) {
                        continue;
                    }
                    const exportedDeclarationsCount = countWhere(declarations, isNotOverload);
                    if (flags & SymbolFlags.TypeAlias && exportedDeclarationsCount <= 2) {
                        // it is legal to merge type alias with other values
                        // so count should be either 1 (just type alias) or 2 (type alias + merged value)
                        continue;
                    }
                    if (exportedDeclarationsCount > 1) {
                        for (const declaration of declarations) {
                            if (isNotOverload(declaration)) {
                                diagnostics.add(createDiagnosticForNode(declaration, Diagnostics.Cannot_redeclare_exported_variable_0, id));
                            }
                        }
                    }
                }
                links.exportsChecked = true;
            }

            function isNotOverload(declaration: Declaration): boolean {
                return declaration.kind !== SyntaxKind.FunctionDeclaration || !!(declaration as FunctionDeclaration).body;
            }
        }

        function checkSourceElement(node: Node): void {
            if (!node) {
                return;
            }

            const kind = node.kind;
            if (cancellationToken) {
                // Only bother checking on a few construct kinds.  We don't want to be excessively
                // hitting the cancellation token on every node we check.
                switch (kind) {
                    case SyntaxKind.ModuleDeclaration:
                    case SyntaxKind.ClassDeclaration:
                    case SyntaxKind.InterfaceDeclaration:
                    case SyntaxKind.FunctionDeclaration:
                        cancellationToken.throwIfCancellationRequested();
                }
            }

            switch (kind) {
                case SyntaxKind.TypeParameter:
                    return checkTypeParameter(<TypeParameterDeclaration>node);
                case SyntaxKind.Parameter:
                    return checkParameter(<ParameterDeclaration>node);
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                    return checkPropertyDeclaration(<PropertyDeclaration>node);
                case SyntaxKind.FunctionType:
                case SyntaxKind.ConstructorType:
                case SyntaxKind.CallSignature:
                case SyntaxKind.ConstructSignature:
                    return checkSignatureDeclaration(<SignatureDeclaration>node);
                case SyntaxKind.IndexSignature:
                    return checkSignatureDeclaration(<SignatureDeclaration>node);
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                    return checkMethodDeclaration(<MethodDeclaration>node);
                case SyntaxKind.Constructor:
                    return checkConstructorDeclaration(<ConstructorDeclaration>node);
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                    return checkAccessorDeclaration(<AccessorDeclaration>node);
                case SyntaxKind.TypeReference:
                    return checkTypeReferenceNode(<TypeReferenceNode>node);
                case SyntaxKind.TypePredicate:
                    return checkTypePredicate(<TypePredicateNode>node);
                case SyntaxKind.TypeQuery:
                    return checkTypeQuery(<TypeQueryNode>node);
                case SyntaxKind.TypeLiteral:
                    return checkTypeLiteral(<TypeLiteralNode>node);
                case SyntaxKind.ArrayType:
                    return checkArrayType(<ArrayTypeNode>node);
                case SyntaxKind.TupleType:
                    return checkTupleType(<TupleTypeNode>node);
                case SyntaxKind.UnionType:
                case SyntaxKind.IntersectionType:
                    return checkUnionOrIntersectionType(<UnionOrIntersectionTypeNode>node);
                case SyntaxKind.ParenthesizedType:
                    return checkSourceElement((<ParenthesizedTypeNode>node).type);
                case SyntaxKind.FunctionDeclaration:
                    return checkFunctionDeclaration(<FunctionDeclaration>node);
                case SyntaxKind.Block:
                case SyntaxKind.ModuleBlock:
                    return checkBlock(<Block>node);
                case SyntaxKind.VariableStatement:
                    return checkVariableStatement(<VariableStatement>node);
                case SyntaxKind.ExpressionStatement:
                    return checkExpressionStatement(<ExpressionStatement>node);
                case SyntaxKind.IfStatement:
                    return checkIfStatement(<IfStatement>node);
                case SyntaxKind.DoStatement:
                    return checkDoStatement(<DoStatement>node);
                case SyntaxKind.WhileStatement:
                    return checkWhileStatement(<WhileStatement>node);
                case SyntaxKind.ForStatement:
                    return checkForStatement(<ForStatement>node);
                case SyntaxKind.ForInStatement:
                    return checkForInStatement(<ForInStatement>node);
                case SyntaxKind.ForOfStatement:
                    return checkForOfStatement(<ForOfStatement>node);
                case SyntaxKind.ContinueStatement:
                case SyntaxKind.BreakStatement:
                    return checkBreakOrContinueStatement(<BreakOrContinueStatement>node);
                case SyntaxKind.ReturnStatement:
                    return checkReturnStatement(<ReturnStatement>node);
                case SyntaxKind.WithStatement:
                    return checkWithStatement(<WithStatement>node);
                case SyntaxKind.SwitchStatement:
                    return checkSwitchStatement(<SwitchStatement>node);
                case SyntaxKind.LabeledStatement:
                    return checkLabeledStatement(<LabeledStatement>node);
                case SyntaxKind.ThrowStatement:
                    return checkThrowStatement(<ThrowStatement>node);
                case SyntaxKind.TryStatement:
                    return checkTryStatement(<TryStatement>node);
                case SyntaxKind.VariableDeclaration:
                    return checkVariableDeclaration(<VariableDeclaration>node);
                case SyntaxKind.BindingElement:
                    return checkBindingElement(<BindingElement>node);
                case SyntaxKind.ClassDeclaration:
                    return checkClassDeclaration(<ClassDeclaration>node);
                case SyntaxKind.InterfaceDeclaration:
                    return checkInterfaceDeclaration(<InterfaceDeclaration>node);
                case SyntaxKind.TypeAliasDeclaration:
                    return checkTypeAliasDeclaration(<TypeAliasDeclaration>node);
                case SyntaxKind.EnumDeclaration:
                    return checkEnumDeclaration(<EnumDeclaration>node);
                case SyntaxKind.ModuleDeclaration:
                    return checkModuleDeclaration(<ModuleDeclaration>node);
                case SyntaxKind.ImportDeclaration:
                    return checkImportDeclaration(<ImportDeclaration>node);
                case SyntaxKind.ImportEqualsDeclaration:
                    return checkImportEqualsDeclaration(<ImportEqualsDeclaration>node);
                case SyntaxKind.ExportDeclaration:
                    return checkExportDeclaration(<ExportDeclaration>node);
                case SyntaxKind.ExportAssignment:
                    return checkExportAssignment(<ExportAssignment>node);
                case SyntaxKind.EmptyStatement:
                    checkGrammarStatementInAmbientContext(node);
                    return;
                case SyntaxKind.DebuggerStatement:
                    checkGrammarStatementInAmbientContext(node);
                    return;
                case SyntaxKind.MissingDeclaration:
                    return checkMissingDeclaration(node);
            }
        }

        // Function and class expression bodies are checked after all statements in the enclosing body. This is
        // to ensure constructs like the following are permitted:
        //     const foo = function () {
        //        const s = foo();
        //        return "hello";
        //     }
        // Here, performing a full type check of the body of the function expression whilst in the process of
        // determining the type of foo would cause foo to be given type any because of the recursive reference.
        // Delaying the type check of the body ensures foo has been assigned a type.
        function checkNodeDeferred(node: Node) {
            if (deferredNodes) {
                deferredNodes.push(node);
            }
        }

        function checkDeferredNodes() {
            for (const node of deferredNodes) {
                switch (node.kind) {
                    case SyntaxKind.FunctionExpression:
                    case SyntaxKind.ArrowFunction:
                    case SyntaxKind.MethodDeclaration:
                    case SyntaxKind.MethodSignature:
                        checkFunctionExpressionOrObjectLiteralMethodDeferred(<FunctionExpression>node);
                        break;
                    case SyntaxKind.GetAccessor:
                    case SyntaxKind.SetAccessor:
                        checkAccessorDeferred(<AccessorDeclaration>node);
                        break;
                    case SyntaxKind.ClassExpression:
                        checkClassExpressionDeferred(<ClassExpression>node);
                        break;
                }
            }
        }

        function checkSourceFile(node: SourceFile) {
            performance.mark("beforeCheck");
            checkSourceFileWorker(node);
            performance.mark("afterCheck");
            performance.measure("Check", "beforeCheck", "afterCheck");
        }

        // Fully type check a source file and collect the relevant diagnostics.
        function checkSourceFileWorker(node: SourceFile) {
            const links = getNodeLinks(node);
            if (!(links.flags & NodeCheckFlags.TypeChecked)) {
                // If skipLibCheck is enabled, skip type checking if file is a declaration file.
                // If skipDefaultLibCheck is enabled, skip type checking if file contains a
                // '/// <reference no-default-lib="true"/>' directive.
                if (compilerOptions.skipLibCheck && node.isDeclarationFile || compilerOptions.skipDefaultLibCheck && node.hasNoDefaultLib) {
                    return;
                }

                // Grammar checking
                checkGrammarSourceFile(node);

                potentialThisCollisions.length = 0;

                deferredNodes = [];
                deferredUnusedIdentifierNodes = produceDiagnostics && noUnusedIdentifiers ? [] : undefined;

                forEach(node.statements, checkSourceElement);

                checkDeferredNodes();

                if (isExternalModule(node)) {
                    registerForUnusedIdentifiersCheck(node);
                }

                if (!node.isDeclarationFile) {
                    checkUnusedIdentifiers();
                }

                deferredNodes = undefined;
                deferredUnusedIdentifierNodes = undefined;

                if (isExternalOrCommonJsModule(node)) {
                    checkExternalModuleExports(node);
                }

                if (potentialThisCollisions.length) {
                    forEach(potentialThisCollisions, checkIfThisIsCapturedInEnclosingScope);
                    potentialThisCollisions.length = 0;
                }

                links.flags |= NodeCheckFlags.TypeChecked;
            }
        }

        function getDiagnostics(sourceFile: SourceFile, ct: CancellationToken): Diagnostic[] {
            try {
                // Record the cancellation token so it can be checked later on during checkSourceElement.
                // Do this in a finally block so we can ensure that it gets reset back to nothing after
                // this call is done.
                cancellationToken = ct;
                return getDiagnosticsWorker(sourceFile);
            }
            finally {
                cancellationToken = undefined;
            }
        }

        function getDiagnosticsWorker(sourceFile: SourceFile): Diagnostic[] {
            throwIfNonDiagnosticsProducing();
            if (sourceFile) {
                checkSourceFile(sourceFile);
                return diagnostics.getDiagnostics(sourceFile.fileName);
            }
            forEach(host.getSourceFiles(), checkSourceFile);
            return diagnostics.getDiagnostics();
        }

        function getGlobalDiagnostics(): Diagnostic[] {
            throwIfNonDiagnosticsProducing();
            return diagnostics.getGlobalDiagnostics();
        }

        function throwIfNonDiagnosticsProducing() {
            if (!produceDiagnostics) {
                throw new Error("Trying to get diagnostics from a type checker that does not produce them.");
            }
        }

        // Language service support

        function isInsideWithStatementBody(node: Node): boolean {
            if (node) {
                while (node.parent) {
                    if (node.parent.kind === SyntaxKind.WithStatement && (<WithStatement>node.parent).statement === node) {
                        return true;
                    }
                    node = node.parent;
                }
            }

            return false;
        }

        function getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[] {
            const symbols = createMap<Symbol>();
            let memberFlags: NodeFlags = 0;

            if (isInsideWithStatementBody(location)) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return [];
            }

            populateSymbols();

            return symbolsToArray(symbols);

            function populateSymbols() {
                while (location) {
                    if (location.locals && !isGlobalSourceFile(location)) {
                        copySymbols(location.locals, meaning);
                    }

                    switch (location.kind) {
                        case SyntaxKind.SourceFile:
                            if (!isExternalOrCommonJsModule(<SourceFile>location)) {
                                break;
                            }
                        case SyntaxKind.ModuleDeclaration:
                            copySymbols(getSymbolOfNode(location).exports, meaning & SymbolFlags.ModuleMember);
                            break;
                        case SyntaxKind.EnumDeclaration:
                            copySymbols(getSymbolOfNode(location).exports, meaning & SymbolFlags.EnumMember);
                            break;
                        case SyntaxKind.ClassExpression:
                            const className = (<ClassExpression>location).name;
                            if (className) {
                                copySymbol(location.symbol, meaning);
                            }
                        // fall through; this fall-through is necessary because we would like to handle
                        // type parameter inside class expression similar to how we handle it in classDeclaration and interface Declaration
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.InterfaceDeclaration:
                            // If we didn't come from static member of class or interface,
                            // add the type parameters into the symbol table
                            // (type parameters of classDeclaration/classExpression and interface are in member property of the symbol.
                            // Note: that the memberFlags come from previous iteration.
                            if (!(memberFlags & NodeFlags.Static)) {
                                copySymbols(getSymbolOfNode(location).members, meaning & SymbolFlags.Type);
                            }
                            break;
                        case SyntaxKind.FunctionExpression:
                            const funcName = (<FunctionExpression>location).name;
                            if (funcName) {
                                copySymbol(location.symbol, meaning);
                            }
                            break;
                    }

                    if (introducesArgumentsExoticObject(location)) {
                        copySymbol(argumentsSymbol, meaning);
                    }

                    memberFlags = location.flags;
                    location = location.parent;
                }

                copySymbols(globals, meaning);
            }

            /**
             * Copy the given symbol into symbol tables if the symbol has the given meaning
             * and it doesn't already existed in the symbol table
             * @param key a key for storing in symbol table; if undefined, use symbol.name
             * @param symbol the symbol to be added into symbol table
             * @param meaning meaning of symbol to filter by before adding to symbol table
             */
            function copySymbol(symbol: Symbol, meaning: SymbolFlags): void {
                if (symbol.flags & meaning) {
                    const id = symbol.name;
                    // We will copy all symbol regardless of its reserved name because
                    // symbolsToArray will check whether the key is a reserved name and
                    // it will not copy symbol with reserved name to the array
                    if (!symbols[id]) {
                        symbols[id] = symbol;
                    }
                }
            }

            function copySymbols(source: SymbolTable, meaning: SymbolFlags): void {
                if (meaning) {
                    for (const id in source) {
                        const symbol = source[id];
                        copySymbol(symbol, meaning);
                    }
                }
            }
        }

        function isTypeDeclarationName(name: Node): boolean {
            return name.kind === SyntaxKind.Identifier &&
                isTypeDeclaration(name.parent) &&
                (<Declaration>name.parent).name === name;
        }

        function isTypeDeclaration(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.TypeParameter:
                case SyntaxKind.ClassDeclaration:
                case SyntaxKind.InterfaceDeclaration:
                case SyntaxKind.TypeAliasDeclaration:
                case SyntaxKind.EnumDeclaration:
                    return true;
            }
        }

        // True if the given identifier is part of a type reference
        function isTypeReferenceIdentifier(entityName: EntityName): boolean {
            let node: Node = entityName;
            while (node.parent && node.parent.kind === SyntaxKind.QualifiedName) {
                node = node.parent;
            }

            return node.parent && (node.parent.kind === SyntaxKind.TypeReference || node.parent.kind === SyntaxKind.JSDocTypeReference) ;
        }

        function isHeritageClauseElementIdentifier(entityName: Node): boolean {
            let node = entityName;
            while (node.parent && node.parent.kind === SyntaxKind.PropertyAccessExpression) {
                node = node.parent;
            }

            return node.parent && node.parent.kind === SyntaxKind.ExpressionWithTypeArguments;
        }

        function forEachEnclosingClass<T>(node: Node, callback: (node: Node) => T): T {
            let result: T;

            while (true) {
                node = getContainingClass(node);
                if (!node) break;
                if (result = callback(node)) break;
            }

            return result;
        }

        function isNodeWithinClass(node: Node, classDeclaration: ClassLikeDeclaration) {
            return !!forEachEnclosingClass(node, n => n === classDeclaration);
        }

        function getLeftSideOfImportEqualsOrExportAssignment(nodeOnRightSide: EntityName): ImportEqualsDeclaration | ExportAssignment {
            while (nodeOnRightSide.parent.kind === SyntaxKind.QualifiedName) {
                nodeOnRightSide = <QualifiedName>nodeOnRightSide.parent;
            }

            if (nodeOnRightSide.parent.kind === SyntaxKind.ImportEqualsDeclaration) {
                return (<ImportEqualsDeclaration>nodeOnRightSide.parent).moduleReference === nodeOnRightSide && <ImportEqualsDeclaration>nodeOnRightSide.parent;
            }

            if (nodeOnRightSide.parent.kind === SyntaxKind.ExportAssignment) {
                return (<ExportAssignment>nodeOnRightSide.parent).expression === <Node>nodeOnRightSide && <ExportAssignment>nodeOnRightSide.parent;
            }

            return undefined;
        }

        function isInRightSideOfImportOrExportAssignment(node: EntityName) {
            return getLeftSideOfImportEqualsOrExportAssignment(node) !== undefined;
        }

        function getSymbolOfEntityNameOrPropertyAccessExpression(entityName: EntityName | PropertyAccessExpression): Symbol | undefined {
            if (isDeclarationName(entityName)) {
                return getSymbolOfNode(entityName.parent);
            }

            if (isInJavaScriptFile(entityName) && entityName.parent.kind === SyntaxKind.PropertyAccessExpression) {
                const specialPropertyAssignmentKind = getSpecialPropertyAssignmentKind(entityName.parent.parent);
                switch (specialPropertyAssignmentKind) {
                    case SpecialPropertyAssignmentKind.ExportsProperty:
                    case SpecialPropertyAssignmentKind.PrototypeProperty:
                        return getSymbolOfNode(entityName.parent);
                    case SpecialPropertyAssignmentKind.ThisProperty:
                    case SpecialPropertyAssignmentKind.ModuleExports:
                        return getSymbolOfNode(entityName.parent.parent);
                    default:
                        // Fall through if it is not a special property assignment
                }
            }

            if (entityName.parent.kind === SyntaxKind.ExportAssignment && isEntityNameExpression(<Identifier | PropertyAccessExpression>entityName)) {
                return resolveEntityName(<EntityNameExpression>entityName,
                    /*all meanings*/ SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
            }

            if (entityName.kind !== SyntaxKind.PropertyAccessExpression && isInRightSideOfImportOrExportAssignment(<EntityName>entityName)) {
                // Since we already checked for ExportAssignment, this really could only be an Import
                const importEqualsDeclaration = <ImportEqualsDeclaration>getAncestor(entityName, SyntaxKind.ImportEqualsDeclaration);
                Debug.assert(importEqualsDeclaration !== undefined);
                return getSymbolOfPartOfRightHandSideOfImportEquals(<EntityName>entityName, importEqualsDeclaration, /*dontResolveAlias*/ true);
            }

            if (isRightSideOfQualifiedNameOrPropertyAccess(entityName)) {
                entityName = <QualifiedName | PropertyAccessEntityNameExpression>entityName.parent;
            }

            if (isHeritageClauseElementIdentifier(<EntityName>entityName)) {
                let meaning = SymbolFlags.None;

                // In an interface or class, we're definitely interested in a type.
                if (entityName.parent.kind === SyntaxKind.ExpressionWithTypeArguments) {
                    meaning = SymbolFlags.Type;

                    // In a class 'extends' clause we are also looking for a value.
                    if (isExpressionWithTypeArgumentsInClassExtendsClause(entityName.parent)) {
                        meaning |= SymbolFlags.Value;
                    }
                }
                else {
                    meaning = SymbolFlags.Namespace;
                }

                meaning |= SymbolFlags.Alias;
                return resolveEntityName(<EntityName>entityName, meaning);
            }
            else if (isExpression(entityName)) {
                if (nodeIsMissing(entityName)) {
                    // Missing entity name.
                    return undefined;
                }

                if (entityName.kind === SyntaxKind.Identifier) {
                    if (isJSXTagName(entityName) && isJsxIntrinsicIdentifier(<Identifier>entityName)) {
                        return getIntrinsicTagSymbol(<JsxOpeningLikeElement>entityName.parent);
                    }

                    return resolveEntityName(<Identifier>entityName, SymbolFlags.Value, /*ignoreErrors*/ false, /*dontResolveAlias*/ true);
                }
                else if (entityName.kind === SyntaxKind.PropertyAccessExpression) {
                    const symbol = getNodeLinks(entityName).resolvedSymbol;
                    if (!symbol) {
                        checkPropertyAccessExpression(<PropertyAccessExpression>entityName);
                    }
                    return getNodeLinks(entityName).resolvedSymbol;
                }
                else if (entityName.kind === SyntaxKind.QualifiedName) {
                    const symbol = getNodeLinks(entityName).resolvedSymbol;
                    if (!symbol) {
                        checkQualifiedName(<QualifiedName>entityName);
                    }
                    return getNodeLinks(entityName).resolvedSymbol;
                }
            }
            else if (isTypeReferenceIdentifier(<EntityName>entityName)) {
                const meaning = (entityName.parent.kind === SyntaxKind.TypeReference || entityName.parent.kind === SyntaxKind.JSDocTypeReference) ? SymbolFlags.Type : SymbolFlags.Namespace;
                return resolveEntityName(<EntityName>entityName, meaning, /*ignoreErrors*/ false, /*dontResolveAlias*/true);
            }
            else if (entityName.parent.kind === SyntaxKind.JsxAttribute) {
                return getJsxAttributePropertySymbol(<JsxAttribute>entityName.parent);
            }

            if (entityName.parent.kind === SyntaxKind.TypePredicate) {
                return resolveEntityName(<Identifier>entityName, /*meaning*/ SymbolFlags.FunctionScopedVariable);
            }

            // Do we want to return undefined here?
            return undefined;
        }

        function getSymbolAtLocation(node: Node) {
            if (node.kind === SyntaxKind.SourceFile) {
                return isExternalModule(<SourceFile>node) ? getMergedSymbol(node.symbol) : undefined;
            }
            if (isInsideWithStatementBody(node)) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return undefined;
            }

            if (isDeclarationName(node)) {
                // This is a declaration, call getSymbolOfNode
                return getSymbolOfNode(node.parent);
            }
            else if (isLiteralComputedPropertyDeclarationName(node)) {
                return getSymbolOfNode(node.parent.parent);
            }

            if (node.kind === SyntaxKind.Identifier) {
                if (isInRightSideOfImportOrExportAssignment(<Identifier>node)) {
                    return getSymbolOfEntityNameOrPropertyAccessExpression(<Identifier>node);
                }
                else if (node.parent.kind === SyntaxKind.BindingElement &&
                    node.parent.parent.kind === SyntaxKind.ObjectBindingPattern &&
                    node === (<BindingElement>node.parent).propertyName) {
                    const typeOfPattern = getTypeOfNode(node.parent.parent);
                    const propertyDeclaration = typeOfPattern && getPropertyOfType(typeOfPattern, (<Identifier>node).text);

                    if (propertyDeclaration) {
                        return propertyDeclaration;
                    }
                }
            }

            switch (node.kind) {
                case SyntaxKind.Identifier:
                case SyntaxKind.PropertyAccessExpression:
                case SyntaxKind.QualifiedName:
                    return getSymbolOfEntityNameOrPropertyAccessExpression(<EntityName | PropertyAccessExpression>node);

                case SyntaxKind.ThisKeyword:
                    const container = getThisContainer(node, /*includeArrowFunctions*/ false);
                    if (isFunctionLike(container)) {
                        const sig = getSignatureFromDeclaration(container);
                        if (sig.thisParameter) {
                            return sig.thisParameter;
                        }
                    }
                    // fallthrough

                case SyntaxKind.SuperKeyword:
                    const type = isExpression(node) ? checkExpression(<Expression>node) : getTypeFromTypeNode(<TypeNode>node);
                    return type.symbol;

                case SyntaxKind.ThisType:
                    return getTypeFromTypeNode(<TypeNode>node).symbol;

                case SyntaxKind.ConstructorKeyword:
                    // constructor keyword for an overload, should take us to the definition if it exist
                    const constructorDeclaration = node.parent;
                    if (constructorDeclaration && constructorDeclaration.kind === SyntaxKind.Constructor) {
                        return (<ClassDeclaration>constructorDeclaration.parent).symbol;
                    }
                    return undefined;

                case SyntaxKind.StringLiteral:
                    // External module name in an import declaration
                    if ((isExternalModuleImportEqualsDeclaration(node.parent.parent) &&
                        getExternalModuleImportEqualsDeclarationExpression(node.parent.parent) === node) ||
                        ((node.parent.kind === SyntaxKind.ImportDeclaration || node.parent.kind === SyntaxKind.ExportDeclaration) &&
                            (<ImportDeclaration>node.parent).moduleSpecifier === node)) {
                        return resolveExternalModuleName(node, <LiteralExpression>node);
                    }
                // Fall through

                case SyntaxKind.NumericLiteral:
                    // index access
                    if (node.parent.kind === SyntaxKind.ElementAccessExpression && (<ElementAccessExpression>node.parent).argumentExpression === node) {
                        const objectType = checkExpression((<ElementAccessExpression>node.parent).expression);
                        if (objectType === unknownType) return undefined;
                        const apparentType = getApparentType(objectType);
                        if (apparentType === unknownType) return undefined;
                        return getPropertyOfType(apparentType, (<LiteralExpression>node).text);
                    }
                    break;
            }
            return undefined;
        }

        function getShorthandAssignmentValueSymbol(location: Node): Symbol {
            // The function returns a value symbol of an identifier in the short-hand property assignment.
            // This is necessary as an identifier in short-hand property assignment can contains two meaning:
            // property name and property value.
            if (location && location.kind === SyntaxKind.ShorthandPropertyAssignment) {
                return resolveEntityName((<ShorthandPropertyAssignment>location).name, SymbolFlags.Value | SymbolFlags.Alias);
            }
            return undefined;
        }

        /** Returns the target of an export specifier without following aliases */
        function getExportSpecifierLocalTargetSymbol(node: ExportSpecifier): Symbol {
            return (<ExportDeclaration>node.parent.parent).moduleSpecifier ?
                getExternalModuleMember(<ExportDeclaration>node.parent.parent, node) :
                resolveEntityName(node.propertyName || node.name, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
        }

        function getTypeOfNode(node: Node): Type {
            if (isInsideWithStatementBody(node)) {
                // We cannot answer semantic questions within a with block, do not proceed any further
                return unknownType;
            }

            if (isTypeNode(node)) {
                return getTypeFromTypeNode(<TypeNode>node);
            }

            if (isExpression(node)) {
                return getTypeOfExpression(<Expression>node);
            }

            if (isExpressionWithTypeArgumentsInClassExtendsClause(node)) {
                // A SyntaxKind.ExpressionWithTypeArguments is considered a type node, except when it occurs in the
                // extends clause of a class. We handle that case here.
                return getBaseTypes(<InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(node.parent.parent)))[0];
            }

            if (isTypeDeclaration(node)) {
                // In this case, we call getSymbolOfNode instead of getSymbolAtLocation because it is a declaration
                const symbol = getSymbolOfNode(node);
                return getDeclaredTypeOfSymbol(symbol);
            }

            if (isTypeDeclarationName(node)) {
                const symbol = getSymbolAtLocation(node);
                return symbol && getDeclaredTypeOfSymbol(symbol);
            }

            if (isDeclaration(node)) {
                // In this case, we call getSymbolOfNode instead of getSymbolAtLocation because it is a declaration
                const symbol = getSymbolOfNode(node);
                return getTypeOfSymbol(symbol);
            }

            if (isDeclarationName(node)) {
                const symbol = getSymbolAtLocation(node);
                return symbol && getTypeOfSymbol(symbol);
            }

            if (isBindingPattern(node)) {
                return getTypeForVariableLikeDeclaration(<VariableLikeDeclaration>node.parent, /*includeOptionality*/ true);
            }

            if (isInRightSideOfImportOrExportAssignment(<Identifier>node)) {
                const symbol = getSymbolAtLocation(node);
                const declaredType = symbol && getDeclaredTypeOfSymbol(symbol);
                return declaredType !== unknownType ? declaredType : getTypeOfSymbol(symbol);
            }

            return unknownType;
        }

        // Gets the type of object literal or array literal of destructuring assignment.
        // { a } from
        //     for ( { a } of elems) {
        //     }
        // [ a ] from
        //     [a] = [ some array ...]
        function getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(expr: Expression): Type {
            Debug.assert(expr.kind === SyntaxKind.ObjectLiteralExpression || expr.kind === SyntaxKind.ArrayLiteralExpression);
            // If this is from "for of"
            //     for ( { a } of elems) {
            //     }
            if (expr.parent.kind === SyntaxKind.ForOfStatement) {
                const iteratedType = checkRightHandSideOfForOf((<ForOfStatement>expr.parent).expression);
                return checkDestructuringAssignment(expr, iteratedType || unknownType);
            }
            // If this is from "for" initializer
            //     for ({a } = elems[0];.....) { }
            if (expr.parent.kind === SyntaxKind.BinaryExpression) {
                const iteratedType = checkExpression((<BinaryExpression>expr.parent).right);
                return checkDestructuringAssignment(expr, iteratedType || unknownType);
            }
            // If this is from nested object binding pattern
            //     for ({ skills: { primary, secondary } } = multiRobot, i = 0; i < 1; i++) {
            if (expr.parent.kind === SyntaxKind.PropertyAssignment) {
                const typeOfParentObjectLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>expr.parent.parent);
                return checkObjectLiteralDestructuringPropertyAssignment(typeOfParentObjectLiteral || unknownType, <ObjectLiteralElement>expr.parent);
            }
            // Array literal assignment - array destructuring pattern
            Debug.assert(expr.parent.kind === SyntaxKind.ArrayLiteralExpression);
            //    [{ property1: p1, property2 }] = elems;
            const typeOfArrayLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>expr.parent);
            const elementType = checkIteratedTypeOrElementType(typeOfArrayLiteral || unknownType, expr.parent, /*allowStringInput*/ false) || unknownType;
            return checkArrayLiteralDestructuringElementAssignment(<ArrayLiteralExpression>expr.parent, typeOfArrayLiteral,
                indexOf((<ArrayLiteralExpression>expr.parent).elements, expr), elementType || unknownType);
        }

        // Gets the property symbol corresponding to the property in destructuring assignment
        // 'property1' from
        //     for ( { property1: a } of elems) {
        //     }
        // 'property1' at location 'a' from:
        //     [a] = [ property1, property2 ]
        function getPropertySymbolOfDestructuringAssignment(location: Identifier) {
            // Get the type of the object or array literal and then look for property of given name in the type
            const typeOfObjectLiteral = getTypeOfArrayLiteralOrObjectLiteralDestructuringAssignment(<Expression>location.parent.parent);
            return typeOfObjectLiteral && getPropertyOfType(typeOfObjectLiteral, location.text);
        }

        function getTypeOfExpression(expr: Expression): Type {
            if (isRightSideOfQualifiedNameOrPropertyAccess(expr)) {
                expr = <Expression>expr.parent;
            }
            return checkExpression(expr);
        }

        /**
          * Gets either the static or instance type of a class element, based on
          * whether the element is declared as "static".
          */
        function getParentTypeOfClassElement(node: ClassElement) {
            const classSymbol = getSymbolOfNode(node.parent);
            return node.flags & NodeFlags.Static
                ? getTypeOfSymbol(classSymbol)
                : getDeclaredTypeOfSymbol(classSymbol);
        }

        // Return the list of properties of the given type, augmented with properties from Function
        // if the type has call or construct signatures
        function getAugmentedPropertiesOfType(type: Type): Symbol[] {
            type = getApparentType(type);
            const propsByName = createSymbolTable(getPropertiesOfType(type));
            if (getSignaturesOfType(type, SignatureKind.Call).length || getSignaturesOfType(type, SignatureKind.Construct).length) {
                forEach(getPropertiesOfType(globalFunctionType), p => {
                    if (!propsByName[p.name]) {
                        propsByName[p.name] = p;
                    }
                });
            }
            return getNamedMembers(propsByName);
        }

        function getRootSymbols(symbol: Symbol): Symbol[] {
            if (symbol.flags & SymbolFlags.SyntheticProperty) {
                const symbols: Symbol[] = [];
                const name = symbol.name;
                forEach(getSymbolLinks(symbol).containingType.types, t => {
                    const symbol = getPropertyOfType(t, name);
                    if (symbol) {
                        symbols.push(symbol);
                    }
                });
                return symbols;
            }
            else if (symbol.flags & SymbolFlags.Transient) {
                let target: Symbol;
                let next = symbol;
                while (next = getSymbolLinks(next).target) {
                   target = next;
                }
                if (target) {
                    return [target];
                }
            }
            return [symbol];
        }

        // Emitter support

        function isArgumentsLocalBinding(node: Identifier): boolean {
            return getReferencedValueSymbol(node) === argumentsSymbol;
        }

        function moduleExportsSomeValue(moduleReferenceExpression: Expression): boolean {
            let moduleSymbol = resolveExternalModuleName(moduleReferenceExpression.parent, moduleReferenceExpression);
            if (!moduleSymbol) {
                // module not found - be conservative
                return true;
            }

            const hasExportAssignment = hasExportAssignmentSymbol(moduleSymbol);
            // if module has export assignment then 'resolveExternalModuleSymbol' will return resolved symbol for export assignment
            // otherwise it will return moduleSymbol itself
            moduleSymbol = resolveExternalModuleSymbol(moduleSymbol);

            const symbolLinks = getSymbolLinks(moduleSymbol);
            if (symbolLinks.exportsSomeValue === undefined) {
                // for export assignments - check if resolved symbol for RHS is itself a value
                // otherwise - check if at least one export is value
                symbolLinks.exportsSomeValue = hasExportAssignment
                    ? !!(moduleSymbol.flags & SymbolFlags.Value)
                    : forEachProperty(getExportsOfModule(moduleSymbol), isValue);
            }

            return symbolLinks.exportsSomeValue;

            function isValue(s: Symbol): boolean {
                s = resolveSymbol(s);
                return s && !!(s.flags & SymbolFlags.Value);
            }
        }

        // When resolved as an expression identifier, if the given node references an exported entity, return the declaration
        // node of the exported entity's container. Otherwise, return undefined.
        function getReferencedExportContainer(node: Identifier): SourceFile | ModuleDeclaration | EnumDeclaration {
            let symbol = getReferencedValueSymbol(node);
            if (symbol) {
                if (symbol.flags & SymbolFlags.ExportValue) {
                    // If we reference an exported entity within the same module declaration, then whether
                    // we prefix depends on the kind of entity. SymbolFlags.ExportHasLocal encompasses all the
                    // kinds that we do NOT prefix.
                    const exportSymbol = getMergedSymbol(symbol.exportSymbol);
                    if (exportSymbol.flags & SymbolFlags.ExportHasLocal) {
                        return undefined;
                    }
                    symbol = exportSymbol;
                }
                const parentSymbol = getParentOfSymbol(symbol);
                if (parentSymbol) {
                    if (parentSymbol.flags & SymbolFlags.ValueModule && parentSymbol.valueDeclaration.kind === SyntaxKind.SourceFile) {
                        // If `node` accesses an export and that export isn't in the same file, then symbol is a namespace export, so return undefined.
                        if (parentSymbol.valueDeclaration === getSourceFileOfNode(node)) {
                            return <SourceFile>parentSymbol.valueDeclaration;
                        }
                    }
                    for (let n = node.parent; n; n = n.parent) {
                        if ((n.kind === SyntaxKind.ModuleDeclaration || n.kind === SyntaxKind.EnumDeclaration) && getSymbolOfNode(n) === parentSymbol) {
                            return <ModuleDeclaration | EnumDeclaration>n;
                        }
                    }
                }
            }
        }

        // When resolved as an expression identifier, if the given node references an import, return the declaration of
        // that import. Otherwise, return undefined.
        function getReferencedImportDeclaration(node: Identifier): Declaration {
            const symbol = getReferencedValueSymbol(node);
            return symbol && symbol.flags & SymbolFlags.Alias ? getDeclarationOfAliasSymbol(symbol) : undefined;
        }

        function isSymbolOfDeclarationWithCollidingName(symbol: Symbol): boolean {
            if (symbol.flags & SymbolFlags.BlockScoped) {
                const links = getSymbolLinks(symbol);
                if (links.isDeclarationWithCollidingName === undefined) {
                    const container = getEnclosingBlockScopeContainer(symbol.valueDeclaration);
                    if (isStatementWithLocals(container)) {
                        const nodeLinks = getNodeLinks(symbol.valueDeclaration);
                        if (!!resolveName(container.parent, symbol.name, SymbolFlags.Value, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined)) {
                            // redeclaration - always should be renamed
                            links.isDeclarationWithCollidingName = true;
                        }
                        else if (nodeLinks.flags & NodeCheckFlags.CapturedBlockScopedBinding) {
                            // binding is captured in the function
                            // should be renamed if:
                            // - binding is not top level - top level bindings never collide with anything
                            // AND
                            //   - binding is not declared in loop, should be renamed to avoid name reuse across siblings
                            //     let a, b
                            //     { let x = 1; a = () => x;  }
                            //     { let x = 100; b = () => x; }
                            //     console.log(a()); // should print '1'
                            //     console.log(b()); // should print '100'
                            //     OR
                            //   - binding is declared inside loop but not in inside initializer of iteration statement or directly inside loop body
                            //     * variables from initializer are passed to rewritten loop body as parameters so they are not captured directly
                            //     * variables that are declared immediately in loop body will become top level variable after loop is rewritten and thus
                            //       they will not collide with anything
                            const isDeclaredInLoop = nodeLinks.flags & NodeCheckFlags.BlockScopedBindingInLoop;
                            const inLoopInitializer = isIterationStatement(container, /*lookInLabeledStatements*/ false);
                            const inLoopBodyBlock = container.kind === SyntaxKind.Block && isIterationStatement(container.parent, /*lookInLabeledStatements*/ false);

                            links.isDeclarationWithCollidingName = !isBlockScopedContainerTopLevel(container) && (!isDeclaredInLoop || (!inLoopInitializer && !inLoopBodyBlock));
                        }
                        else {
                            links.isDeclarationWithCollidingName = false;
                        }
                    }
                }
                return links.isDeclarationWithCollidingName;
            }
            return false;
        }

        // When resolved as an expression identifier, if the given node references a nested block scoped entity with
        // a name that either hides an existing name or might hide it when compiled downlevel,
        // return the declaration of that entity. Otherwise, return undefined.
        function getReferencedDeclarationWithCollidingName(node: Identifier): Declaration {
            const symbol = getReferencedValueSymbol(node);
            return symbol && isSymbolOfDeclarationWithCollidingName(symbol) ? symbol.valueDeclaration : undefined;
        }

        // Return true if the given node is a declaration of a nested block scoped entity with a name that either hides an
        // existing name or might hide a name when compiled downlevel
        function isDeclarationWithCollidingName(node: Declaration): boolean {
            return isSymbolOfDeclarationWithCollidingName(getSymbolOfNode(node));
        }

        function isValueAliasDeclaration(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ImportClause:
                case SyntaxKind.NamespaceImport:
                case SyntaxKind.ImportSpecifier:
                case SyntaxKind.ExportSpecifier:
                    return isAliasResolvedToValue(getSymbolOfNode(node));
                case SyntaxKind.ExportDeclaration:
                    const exportClause = (<ExportDeclaration>node).exportClause;
                    return exportClause && forEach(exportClause.elements, isValueAliasDeclaration);
                case SyntaxKind.ExportAssignment:
                    return (<ExportAssignment>node).expression && (<ExportAssignment>node).expression.kind === SyntaxKind.Identifier ? isAliasResolvedToValue(getSymbolOfNode(node)) : true;
            }
            return false;
        }

        function isTopLevelValueImportEqualsWithEntityName(node: ImportEqualsDeclaration): boolean {
            if (node.parent.kind !== SyntaxKind.SourceFile || !isInternalModuleImportEqualsDeclaration(node)) {
                // parent is not source file or it is not reference to internal module
                return false;
            }

            const isValue = isAliasResolvedToValue(getSymbolOfNode(node));
            return isValue && node.moduleReference && !nodeIsMissing(node.moduleReference);
        }

        function isAliasResolvedToValue(symbol: Symbol): boolean {
            const target = resolveAlias(symbol);
            if (target === unknownSymbol) {
                return true;
            }
            // const enums and modules that contain only const enums are not considered values from the emit perspective
            // unless 'preserveConstEnums' option is set to true
            return target.flags & SymbolFlags.Value &&
                (compilerOptions.preserveConstEnums || !isConstEnumOrConstEnumOnlyModule(target));
        }

        function isConstEnumOrConstEnumOnlyModule(s: Symbol): boolean {
            return isConstEnumSymbol(s) || s.constEnumOnlyModule;
        }

        function isReferencedAliasDeclaration(node: Node, checkChildren?: boolean): boolean {
            if (isAliasSymbolDeclaration(node)) {
                const symbol = getSymbolOfNode(node);
                if (getSymbolLinks(symbol).referenced) {
                    return true;
                }
            }

            if (checkChildren) {
                return forEachChild(node, node => isReferencedAliasDeclaration(node, checkChildren));
            }
            return false;
        }

        function isImplementationOfOverload(node: FunctionLikeDeclaration) {
            if (nodeIsPresent(node.body)) {
                const symbol = getSymbolOfNode(node);
                const signaturesOfSymbol = getSignaturesOfSymbol(symbol);
                // If this function body corresponds to function with multiple signature, it is implementation of overload
                // e.g.: function foo(a: string): string;
                //       function foo(a: number): number;
                //       function foo(a: any) { // This is implementation of the overloads
                //           return a;
                //       }
                return signaturesOfSymbol.length > 1 ||
                    // If there is single signature for the symbol, it is overload if that signature isn't coming from the node
                    // e.g.: function foo(a: string): string;
                    //       function foo(a: any) { // This is implementation of the overloads
                    //           return a;
                    //       }
                    (signaturesOfSymbol.length === 1 && signaturesOfSymbol[0].declaration !== node);
            }
            return false;
        }

        function getNodeCheckFlags(node: Node): NodeCheckFlags {
            return getNodeLinks(node).flags;
        }

        function getEnumMemberValue(node: EnumMember): number {
            computeEnumMemberValues(<EnumDeclaration>node.parent);
            return getNodeLinks(node).enumMemberValue;
        }

        function getConstantValue(node: EnumMember | PropertyAccessExpression | ElementAccessExpression): number {
            if (node.kind === SyntaxKind.EnumMember) {
                return getEnumMemberValue(<EnumMember>node);
            }

            const symbol = getNodeLinks(node).resolvedSymbol;
            if (symbol && (symbol.flags & SymbolFlags.EnumMember)) {
                // inline property\index accesses only for const enums
                if (isConstEnumDeclaration(symbol.valueDeclaration.parent)) {
                    return getEnumMemberValue(<EnumMember>symbol.valueDeclaration);
                }
            }

            return undefined;
        }

        function isFunctionType(type: Type): boolean {
            return type.flags & TypeFlags.ObjectType && getSignaturesOfType(type, SignatureKind.Call).length > 0;
        }

        function getTypeReferenceSerializationKind(typeName: EntityName): TypeReferenceSerializationKind {
            // Resolve the symbol as a value to ensure the type can be reached at runtime during emit.
            const valueSymbol = resolveEntityName(typeName, SymbolFlags.Value, /*ignoreErrors*/ true);
            const constructorType = valueSymbol ? getTypeOfSymbol(valueSymbol) : undefined;
            if (constructorType && isConstructorType(constructorType)) {
                return TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue;
            }

            // Resolve the symbol as a type so that we can provide a more useful hint for the type serializer.
            const typeSymbol = resolveEntityName(typeName, SymbolFlags.Type, /*ignoreErrors*/ true);
            // We might not be able to resolve type symbol so use unknown type in that case (eg error case)
            if (!typeSymbol) {
                return TypeReferenceSerializationKind.ObjectType;
            }
            const type = getDeclaredTypeOfSymbol(typeSymbol);
            if (type === unknownType) {
                return TypeReferenceSerializationKind.Unknown;
            }
            else if (type.flags & TypeFlags.Any) {
                return TypeReferenceSerializationKind.ObjectType;
            }
            else if (isTypeOfKind(type, TypeFlags.Void)) {
                return TypeReferenceSerializationKind.VoidType;
            }
            else if (isTypeOfKind(type, TypeFlags.BooleanLike)) {
                return TypeReferenceSerializationKind.BooleanType;
            }
            else if (isTypeOfKind(type, TypeFlags.NumberLike)) {
                return TypeReferenceSerializationKind.NumberLikeType;
            }
            else if (isTypeOfKind(type, TypeFlags.StringLike)) {
                return TypeReferenceSerializationKind.StringLikeType;
            }
            else if (isTypeOfKind(type, TypeFlags.Tuple)) {
                return TypeReferenceSerializationKind.ArrayLikeType;
            }
            else if (isTypeOfKind(type, TypeFlags.ESSymbol)) {
                return TypeReferenceSerializationKind.ESSymbolType;
            }
            else if (isFunctionType(type)) {
                return TypeReferenceSerializationKind.TypeWithCallSignature;
            }
            else if (isArrayType(type)) {
                return TypeReferenceSerializationKind.ArrayLikeType;
            }
            else {
                return TypeReferenceSerializationKind.ObjectType;
            }
        }

        function writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter) {
            // Get type of the symbol if this is the valid symbol otherwise get type at location
            const symbol = getSymbolOfNode(declaration);
            const type = symbol && !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.Signature))
                ? getTypeOfSymbol(symbol)
                : unknownType;

            getSymbolDisplayBuilder().buildTypeDisplay(type, writer, enclosingDeclaration, flags);
        }

        function writeReturnTypeOfSignatureDeclaration(signatureDeclaration: SignatureDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter) {
            const signature = getSignatureFromDeclaration(signatureDeclaration);
            getSymbolDisplayBuilder().buildTypeDisplay(getReturnTypeOfSignature(signature), writer, enclosingDeclaration, flags);
        }

        function writeTypeOfExpression(expr: Expression, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter) {
            const type = getWidenedType(getTypeOfExpression(expr));
            getSymbolDisplayBuilder().buildTypeDisplay(type, writer, enclosingDeclaration, flags);
        }

        function writeBaseConstructorTypeOfClass(node: ClassLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter) {
            const classType = <InterfaceType>getDeclaredTypeOfSymbol(getSymbolOfNode(node));
            resolveBaseTypesOfClass(classType);
            const baseType = classType.resolvedBaseTypes.length ? classType.resolvedBaseTypes[0] : unknownType;
            getSymbolDisplayBuilder().buildTypeDisplay(baseType, writer, enclosingDeclaration, flags);
        }

        function hasGlobalName(name: string): boolean {
            return !!globals[name];
        }

        function getReferencedValueSymbol(reference: Identifier): Symbol {
            return getNodeLinks(reference).resolvedSymbol ||
                resolveName(reference, reference.text, SymbolFlags.Value | SymbolFlags.ExportValue | SymbolFlags.Alias,
                    /*nodeNotFoundMessage*/ undefined, /*nameArg*/ undefined);
        }

        function getReferencedValueDeclaration(reference: Identifier): Declaration {
            Debug.assert(!nodeIsSynthesized(reference));
            const symbol = getReferencedValueSymbol(reference);
            return symbol && getExportSymbolOfValueSymbolIfExported(symbol).valueDeclaration;
        }

        function createResolver(): EmitResolver {
            // this variable and functions that use it are deliberately moved here from the outer scope
            // to avoid scope pollution
            const resolvedTypeReferenceDirectives = host.getResolvedTypeReferenceDirectives();
            let fileToDirective: FileMap<string>;
            if (resolvedTypeReferenceDirectives) {
                // populate reverse mapping: file path -> type reference directive that was resolved to this file
                fileToDirective = createFileMap<string>();
                for (const key in resolvedTypeReferenceDirectives) {
                    const resolvedDirective = resolvedTypeReferenceDirectives[key];
                    if (!resolvedDirective) {
                        continue;
                    }
                    const file = host.getSourceFile(resolvedDirective.resolvedFileName);
                    fileToDirective.set(file.path, key);
                }
            }
            return {
                getReferencedExportContainer,
                getReferencedImportDeclaration,
                getReferencedDeclarationWithCollidingName,
                isDeclarationWithCollidingName,
                isValueAliasDeclaration,
                hasGlobalName,
                isReferencedAliasDeclaration,
                getNodeCheckFlags,
                isTopLevelValueImportEqualsWithEntityName,
                isDeclarationVisible,
                isImplementationOfOverload,
                writeTypeOfDeclaration,
                writeReturnTypeOfSignatureDeclaration,
                writeTypeOfExpression,
                writeBaseConstructorTypeOfClass,
                isSymbolAccessible,
                isEntityNameVisible,
                getConstantValue,
                collectLinkedAliases,
                getReferencedValueDeclaration,
                getTypeReferenceSerializationKind,
                isOptionalParameter,
                moduleExportsSomeValue,
                isArgumentsLocalBinding,
                getExternalModuleFileFromDeclaration,
                getTypeReferenceDirectivesForEntityName,
                getTypeReferenceDirectivesForSymbol
            };

            // defined here to avoid outer scope pollution
            function getTypeReferenceDirectivesForEntityName(node: EntityNameOrEntityNameExpression): string[] {
                // program does not have any files with type reference directives - bail out
                if (!fileToDirective) {
                    return undefined;
                }
                // property access can only be used as values
                // qualified names can only be used as types\namespaces
                // identifiers are treated as values only if they appear in type queries
                const meaning = (node.kind === SyntaxKind.PropertyAccessExpression) || (node.kind === SyntaxKind.Identifier && isInTypeQuery(node))
                    ? SymbolFlags.Value | SymbolFlags.ExportValue
                    : SymbolFlags.Type | SymbolFlags.Namespace;

                const symbol = resolveEntityName(node, meaning, /*ignoreErrors*/true);
                return symbol && symbol !== unknownSymbol ? getTypeReferenceDirectivesForSymbol(symbol, meaning) : undefined;
            }

            // defined here to avoid outer scope pollution
            function getTypeReferenceDirectivesForSymbol(symbol: Symbol, meaning?: SymbolFlags): string[] {
                // program does not have any files with type reference directives - bail out
                if (!fileToDirective) {
                    return undefined;
                }
                if (!isSymbolFromTypeDeclarationFile(symbol)) {
                    return undefined;
                }
                // check what declarations in the symbol can contribute to the target meaning
                let typeReferenceDirectives: string[];
                for (const decl of symbol.declarations) {
                    // check meaning of the local symbol to see if declaration needs to be analyzed further
                    if (decl.symbol && decl.symbol.flags & meaning) {
                        const file = getSourceFileOfNode(decl);
                        const typeReferenceDirective = fileToDirective.get(file.path);
                        if (typeReferenceDirective) {
                            (typeReferenceDirectives || (typeReferenceDirectives = [])).push(typeReferenceDirective);
                        }
                    }
                }
                return typeReferenceDirectives;
            }

            function isSymbolFromTypeDeclarationFile(symbol: Symbol): boolean {
                // bail out if symbol does not have associated declarations (i.e. this is transient symbol created for property in binding pattern)
                if (!symbol.declarations) {
                    return false;
                }

                // walk the parent chain for symbols to make sure that top level parent symbol is in the global scope
                // external modules cannot define or contribute to type declaration files
                let current =  symbol;
                while (true) {
                    const parent = getParentOfSymbol(current);
                    if (parent) {
                        current = parent;
                    }
                    else {
                        break;
                    }
                }

                if (current.valueDeclaration && current.valueDeclaration.kind === SyntaxKind.SourceFile && current.flags & SymbolFlags.ValueModule) {
                    return false;
                }

                // check that at least one declaration of top level symbol originates from type declaration file
                for (const decl of symbol.declarations) {
                    const file = getSourceFileOfNode(decl);
                    if (fileToDirective.contains(file.path)) {
                        return true;
                    }
                }
                return false;
            }
        }

        function getExternalModuleFileFromDeclaration(declaration: ImportEqualsDeclaration | ImportDeclaration | ExportDeclaration | ModuleDeclaration): SourceFile {
            const specifier = getExternalModuleName(declaration);
            const moduleSymbol = resolveExternalModuleNameWorker(specifier, specifier, /*moduleNotFoundError*/ undefined);
            if (!moduleSymbol) {
                return undefined;
            }
            return getDeclarationOfKind(moduleSymbol, SyntaxKind.SourceFile) as SourceFile;
        }

        function initializeTypeChecker() {
            // Bind all source files and propagate errors
            forEach(host.getSourceFiles(), file => {
                bindSourceFile(file, compilerOptions);
            });

            let augmentations: LiteralExpression[][];
            // Initialize global symbol table
            forEach(host.getSourceFiles(), file => {
                if (!isExternalOrCommonJsModule(file)) {
                    mergeSymbolTable(globals, file.locals);
                }
                if (file.patternAmbientModules && file.patternAmbientModules.length) {
                    patternAmbientModules = concatenate(patternAmbientModules, file.patternAmbientModules);
                }

                if (file.moduleAugmentations.length) {
                    (augmentations || (augmentations = [])).push(file.moduleAugmentations);
                }
                if (file.symbol && file.symbol.globalExports) {
                    mergeSymbolTable(globals, file.symbol.globalExports);
                }
            });

            if (augmentations) {
                // merge module augmentations.
                // this needs to be done after global symbol table is initialized to make sure that all ambient modules are indexed
                for (const list of augmentations) {
                    for (const augmentation of list) {
                        mergeModuleAugmentation(augmentation);
                    }
                }
            }

            // Setup global builtins
            addToSymbolTable(globals, builtinGlobals, Diagnostics.Declaration_name_conflicts_with_built_in_global_identifier_0);

            getSymbolLinks(undefinedSymbol).type = undefinedWideningType;
            getSymbolLinks(argumentsSymbol).type = getGlobalType("IArguments");
            getSymbolLinks(unknownSymbol).type = unknownType;

            // Initialize special types
            globalArrayType = <GenericType>getGlobalType("Array", /*arity*/ 1);
            globalObjectType = getGlobalType("Object");
            globalFunctionType = getGlobalType("Function");
            globalStringType = getGlobalType("String");
            globalNumberType = getGlobalType("Number");
            globalBooleanType = getGlobalType("Boolean");
            globalRegExpType = getGlobalType("RegExp");

            jsxElementType = getExportedTypeFromNamespace("JSX", JsxNames.Element);
            getGlobalClassDecoratorType = memoize(() => getGlobalType("ClassDecorator"));
            getGlobalPropertyDecoratorType = memoize(() => getGlobalType("PropertyDecorator"));
            getGlobalMethodDecoratorType = memoize(() => getGlobalType("MethodDecorator"));
            getGlobalParameterDecoratorType = memoize(() => getGlobalType("ParameterDecorator"));
            getGlobalTypedPropertyDescriptorType = memoize(() => getGlobalType("TypedPropertyDescriptor", /*arity*/ 1));
            getGlobalESSymbolConstructorSymbol = memoize(() => getGlobalValueSymbol("Symbol"));
            getGlobalPromiseType = memoize(() => getGlobalType("Promise", /*arity*/ 1));
            tryGetGlobalPromiseType = memoize(() => getGlobalSymbol("Promise", SymbolFlags.Type, /*diagnostic*/ undefined) && getGlobalPromiseType());
            getGlobalPromiseLikeType = memoize(() => getGlobalType("PromiseLike", /*arity*/ 1));
            getInstantiatedGlobalPromiseLikeType = memoize(createInstantiatedPromiseLikeType);
            getGlobalPromiseConstructorSymbol = memoize(() => getGlobalValueSymbol("Promise"));
            getGlobalPromiseConstructorLikeType = memoize(() => getGlobalType("PromiseConstructorLike"));
            getGlobalThenableType = memoize(createThenableType);

            getGlobalTemplateStringsArrayType = memoize(() => getGlobalType("TemplateStringsArray"));

            if (languageVersion >= ScriptTarget.ES6) {
                getGlobalESSymbolType = memoize(() => getGlobalType("Symbol"));
                getGlobalIterableType = memoize(() => <GenericType>getGlobalType("Iterable", /*arity*/ 1));
                getGlobalIteratorType = memoize(() => <GenericType>getGlobalType("Iterator", /*arity*/ 1));
                getGlobalIterableIteratorType = memoize(() => <GenericType>getGlobalType("IterableIterator", /*arity*/ 1));
            }
            else {
                getGlobalESSymbolType = memoize(() => emptyObjectType);
                getGlobalIterableType = memoize(() => emptyGenericType);
                getGlobalIteratorType = memoize(() => emptyGenericType);
                getGlobalIterableIteratorType = memoize(() => emptyGenericType);
            }

            anyArrayType = createArrayType(anyType);

            const symbol = getGlobalSymbol("ReadonlyArray", SymbolFlags.Type, /*diagnostic*/ undefined);
            globalReadonlyArrayType = symbol && <GenericType>getTypeOfGlobalSymbol(symbol, /*arity*/ 1);
            anyReadonlyArrayType = globalReadonlyArrayType ? createTypeFromGenericGlobalType(globalReadonlyArrayType, [anyType]) : anyArrayType;
        }

        function createInstantiatedPromiseLikeType(): ObjectType {
            const promiseLikeType = getGlobalPromiseLikeType();
            if (promiseLikeType !== emptyGenericType) {
                return createTypeReference(<GenericType>promiseLikeType, [anyType]);
            }

            return emptyObjectType;
        }

        function createThenableType() {
            // build the thenable type that is used to verify against a non-promise "thenable" operand to `await`.
            const thenPropertySymbol = createSymbol(SymbolFlags.Transient | SymbolFlags.Property, "then");
            getSymbolLinks(thenPropertySymbol).type = globalFunctionType;

            const thenableType = <ResolvedType>createObjectType(TypeFlags.Anonymous);
            thenableType.properties = [thenPropertySymbol];
            thenableType.members = createSymbolTable(thenableType.properties);
            thenableType.callSignatures = [];
            thenableType.constructSignatures = [];
            return thenableType;
        }

        // GRAMMAR CHECKING
        function checkGrammarDecorators(node: Node): boolean {
            if (!node.decorators) {
                return false;
            }
            if (!nodeCanBeDecorated(node)) {
                if (node.kind === SyntaxKind.MethodDeclaration && !ts.nodeIsPresent((<MethodDeclaration>node).body)) {
                    return grammarErrorOnFirstToken(node, Diagnostics.A_decorator_can_only_decorate_a_method_implementation_not_an_overload);
                }
                else {
                    return grammarErrorOnFirstToken(node, Diagnostics.Decorators_are_not_valid_here);
                }
            }
            else if (node.kind === SyntaxKind.GetAccessor || node.kind === SyntaxKind.SetAccessor) {
                const accessors = getAllAccessorDeclarations((<ClassDeclaration>node.parent).members, <AccessorDeclaration>node);
                if (accessors.firstAccessor.decorators && node === accessors.secondAccessor) {
                    return grammarErrorOnFirstToken(node, Diagnostics.Decorators_cannot_be_applied_to_multiple_get_Slashset_accessors_of_the_same_name);
                }
            }
            return false;
        }

        function checkGrammarModifiers(node: Node): boolean {
            const quickResult = reportObviousModifierErrors(node);
            if (quickResult !== undefined) {
                return quickResult;
            }

            let lastStatic: Node, lastPrivate: Node, lastProtected: Node, lastDeclare: Node, lastAsync: Node, lastReadonly: Node;
            let flags = 0;
            for (const modifier of node.modifiers) {
                if (modifier.kind !== SyntaxKind.ReadonlyKeyword) {
                    if (node.kind === SyntaxKind.PropertySignature || node.kind === SyntaxKind.MethodSignature) {
                        return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_type_member, tokenToString(modifier.kind));
                    }
                    if (node.kind === SyntaxKind.IndexSignature) {
                        return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_an_index_signature, tokenToString(modifier.kind));
                    }
                }
                switch (modifier.kind) {
                    case SyntaxKind.ConstKeyword:
                        if (node.kind !== SyntaxKind.EnumDeclaration && node.parent.kind === SyntaxKind.ClassDeclaration) {
                            return grammarErrorOnNode(node, Diagnostics.A_class_member_cannot_have_the_0_keyword, tokenToString(SyntaxKind.ConstKeyword));
                        }
                        break;
                    case SyntaxKind.PublicKeyword:
                    case SyntaxKind.ProtectedKeyword:
                    case SyntaxKind.PrivateKeyword:
                        let text = visibilityToString(modifierToFlag(modifier.kind));

                        if (modifier.kind === SyntaxKind.ProtectedKeyword) {
                            lastProtected = modifier;
                        }
                        else if (modifier.kind === SyntaxKind.PrivateKeyword) {
                            lastPrivate = modifier;
                        }

                        if (flags & NodeFlags.AccessibilityModifier) {
                            return grammarErrorOnNode(modifier, Diagnostics.Accessibility_modifier_already_seen);
                        }
                        else if (flags & NodeFlags.Static) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "static");
                        }
                        else if (flags & NodeFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "readonly");
                        }
                        else if (flags & NodeFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_module_or_namespace_element, text);
                        }
                        else if (flags & NodeFlags.Abstract) {
                            if (modifier.kind === SyntaxKind.PrivateKeyword) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, text, "abstract");
                            }
                            else {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, text, "abstract");
                            }
                        }
                        flags |= modifierToFlag(modifier.kind);
                        break;

                    case SyntaxKind.StaticKeyword:
                        if (flags & NodeFlags.Static) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "static");
                        }
                        else if (flags & NodeFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "readonly");
                        }
                        else if (flags & NodeFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "static", "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_module_or_namespace_element, "static");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "static");
                        }
                        else if (flags & NodeFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "abstract");
                        }
                        flags |= NodeFlags.Static;
                        lastStatic = modifier;
                        break;

                    case SyntaxKind.ReadonlyKeyword:
                        if (flags & NodeFlags.Readonly) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "readonly");
                        }
                        else if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature && node.kind !== SyntaxKind.IndexSignature && node.kind !== SyntaxKind.Parameter) {
                            // If node.kind === SyntaxKind.Parameter, checkParameter report an error if it's not a parameter property.
                            return grammarErrorOnNode(modifier, Diagnostics.readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature);
                        }
                        flags |= NodeFlags.Readonly;
                        lastReadonly = modifier;
                        break;

                    case SyntaxKind.ExportKeyword:
                        if (flags & NodeFlags.Export) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "export");
                        }
                        else if (flags & NodeFlags.Ambient) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "declare");
                        }
                        else if (flags & NodeFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "abstract");
                        }
                        else if (flags & NodeFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_must_precede_1_modifier, "export", "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ClassDeclaration) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "export");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "export");
                        }
                        flags |= NodeFlags.Export;
                        break;

                    case SyntaxKind.DeclareKeyword:
                        if (flags & NodeFlags.Ambient) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "declare");
                        }
                        else if (flags & NodeFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                        }
                        else if (node.parent.kind === SyntaxKind.ClassDeclaration) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_class_element, "declare");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "declare");
                        }
                        else if (isInAmbientContext(node.parent) && node.parent.kind === SyntaxKind.ModuleBlock) {
                            return grammarErrorOnNode(modifier, Diagnostics.A_declare_modifier_cannot_be_used_in_an_already_ambient_context);
                        }
                        flags |= NodeFlags.Ambient;
                        lastDeclare = modifier;
                        break;

                    case SyntaxKind.AbstractKeyword:
                        if (flags & NodeFlags.Abstract) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "abstract");
                        }
                        if (node.kind !== SyntaxKind.ClassDeclaration) {
                            if (node.kind !== SyntaxKind.MethodDeclaration &&
                                node.kind !== SyntaxKind.PropertyDeclaration &&
                                node.kind !== SyntaxKind.GetAccessor &&
                                node.kind !== SyntaxKind.SetAccessor) {
                                return grammarErrorOnNode(modifier, Diagnostics.abstract_modifier_can_only_appear_on_a_class_method_or_property_declaration);
                            }
                            if (!(node.parent.kind === SyntaxKind.ClassDeclaration && node.parent.flags & NodeFlags.Abstract)) {
                                return grammarErrorOnNode(modifier, Diagnostics.Abstract_methods_can_only_appear_within_an_abstract_class);
                            }
                            if (flags & NodeFlags.Static) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "static", "abstract");
                            }
                            if (flags & NodeFlags.Private) {
                                return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_with_1_modifier, "private", "abstract");
                            }
                        }

                        flags |= NodeFlags.Abstract;
                        break;

                    case SyntaxKind.AsyncKeyword:
                        if (flags & NodeFlags.Async) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "async");
                        }
                        else if (flags & NodeFlags.Ambient || isInAmbientContext(node.parent)) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                        }
                        else if (node.kind === SyntaxKind.Parameter) {
                            return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "async");
                        }
                        flags |= NodeFlags.Async;
                        lastAsync = modifier;
                        break;
                }
            }

            if (node.kind === SyntaxKind.Constructor) {
                if (flags & NodeFlags.Static) {
                    return grammarErrorOnNode(lastStatic, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "static");
                }
                if (flags & NodeFlags.Abstract) {
                    return grammarErrorOnNode(lastStatic, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "abstract");
                }
                else if (flags & NodeFlags.Async) {
                    return grammarErrorOnNode(lastAsync, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "async");
                }
                else if (flags & NodeFlags.Readonly) {
                    return grammarErrorOnNode(lastReadonly, Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration, "readonly");
                }
                return;
            }
            else if ((node.kind === SyntaxKind.ImportDeclaration || node.kind === SyntaxKind.ImportEqualsDeclaration) && flags & NodeFlags.Ambient) {
                return grammarErrorOnNode(lastDeclare, Diagnostics.A_0_modifier_cannot_be_used_with_an_import_declaration, "declare");
            }
            else if (node.kind === SyntaxKind.Parameter && (flags & NodeFlags.ParameterPropertyModifier) && isBindingPattern((<ParameterDeclaration>node).name)) {
                return grammarErrorOnNode(node, Diagnostics.A_parameter_property_may_not_be_declared_using_a_binding_pattern);
            }
            else if (node.kind === SyntaxKind.Parameter && (flags & NodeFlags.ParameterPropertyModifier) && (<ParameterDeclaration>node).dotDotDotToken) {
                return grammarErrorOnNode(node, Diagnostics.A_parameter_property_cannot_be_declared_using_a_rest_parameter);
            }
            if (flags & NodeFlags.Async) {
                return checkGrammarAsyncModifier(node, lastAsync);
            }
        }

        /**
         * true | false: Early return this value from checkGrammarModifiers.
         * undefined: Need to do full checking on the modifiers.
         */
        function reportObviousModifierErrors(node: Node): boolean | undefined {
            return !node.modifiers
                ? false
                : shouldReportBadModifier(node)
                    ? grammarErrorOnFirstToken(node, Diagnostics.Modifiers_cannot_appear_here)
                    : undefined;
        }
        function shouldReportBadModifier(node: Node): boolean {
            switch (node.kind) {
                case SyntaxKind.GetAccessor:
                case SyntaxKind.SetAccessor:
                case SyntaxKind.Constructor:
                case SyntaxKind.PropertyDeclaration:
                case SyntaxKind.PropertySignature:
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.MethodSignature:
                case SyntaxKind.IndexSignature:
                case SyntaxKind.ModuleDeclaration:
                case SyntaxKind.ImportDeclaration:
                case SyntaxKind.ImportEqualsDeclaration:
                case SyntaxKind.ExportDeclaration:
                case SyntaxKind.ExportAssignment:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                case SyntaxKind.Parameter:
                    return false;
                default:
                    if (node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                        return false;
                    }
                    switch (node.kind) {
                        case SyntaxKind.FunctionDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.AsyncKeyword);
                        case SyntaxKind.ClassDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.AbstractKeyword);
                        case SyntaxKind.InterfaceDeclaration:
                        case SyntaxKind.VariableStatement:
                        case SyntaxKind.TypeAliasDeclaration:
                            return true;
                        case SyntaxKind.EnumDeclaration:
                            return nodeHasAnyModifiersExcept(node, SyntaxKind.ConstKeyword);
                        default:
                            Debug.fail();
                            return false;
                    }
            }
        }
        function nodeHasAnyModifiersExcept(node: Node, allowedModifier: SyntaxKind): boolean {
            return node.modifiers.length > 1 || node.modifiers[0].kind !== allowedModifier;
        }

        function checkGrammarAsyncModifier(node: Node, asyncModifier: Node): boolean {
            if (languageVersion < ScriptTarget.ES6) {
                return grammarErrorOnNode(asyncModifier, Diagnostics.Async_functions_are_only_available_when_targeting_ECMAScript_2015_or_higher);
            }

            switch (node.kind) {
                case SyntaxKind.MethodDeclaration:
                case SyntaxKind.FunctionDeclaration:
                case SyntaxKind.FunctionExpression:
                case SyntaxKind.ArrowFunction:
                    if (!(<FunctionLikeDeclaration>node).asteriskToken) {
                        return false;
                    }
                    break;
            }

            return grammarErrorOnNode(asyncModifier, Diagnostics._0_modifier_cannot_be_used_here, "async");
        }

        function checkGrammarForDisallowedTrailingComma(list: NodeArray<Node>): boolean {
            if (list && list.hasTrailingComma) {
                const start = list.end - ",".length;
                const end = list.end;
                const sourceFile = getSourceFileOfNode(list[0]);
                return grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.Trailing_comma_not_allowed);
            }
        }

        function checkGrammarTypeParameterList(node: FunctionLikeDeclaration, typeParameters: NodeArray<TypeParameterDeclaration>, file: SourceFile): boolean {
            if (checkGrammarForDisallowedTrailingComma(typeParameters)) {
                return true;
            }

            if (typeParameters && typeParameters.length === 0) {
                const start = typeParameters.pos - "<".length;
                const end = skipTrivia(file.text, typeParameters.end) + ">".length;
                return grammarErrorAtPos(file, start, end - start, Diagnostics.Type_parameter_list_cannot_be_empty);
            }
        }

        function checkGrammarParameterList(parameters: NodeArray<ParameterDeclaration>) {
            let seenOptionalParameter = false;
            const parameterCount = parameters.length;

            for (let i = 0; i < parameterCount; i++) {
                const parameter = parameters[i];
                if (parameter.dotDotDotToken) {
                    if (i !== (parameterCount - 1)) {
                        return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.A_rest_parameter_must_be_last_in_a_parameter_list);
                    }

                    if (isBindingPattern(parameter.name)) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_element_cannot_contain_a_binding_pattern);
                    }

                    if (parameter.questionToken) {
                        return grammarErrorOnNode(parameter.questionToken, Diagnostics.A_rest_parameter_cannot_be_optional);
                    }

                    if (parameter.initializer) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_parameter_cannot_have_an_initializer);
                    }
                }
                else if (parameter.questionToken) {
                    seenOptionalParameter = true;

                    if (parameter.initializer) {
                        return grammarErrorOnNode(parameter.name, Diagnostics.Parameter_cannot_have_question_mark_and_initializer);
                    }
                }
                else if (seenOptionalParameter && !parameter.initializer) {
                    return grammarErrorOnNode(parameter.name, Diagnostics.A_required_parameter_cannot_follow_an_optional_parameter);
                }
            }
        }

        function checkGrammarFunctionLikeDeclaration(node: FunctionLikeDeclaration): boolean {
            // Prevent cascading error by short-circuit
            const file = getSourceFileOfNode(node);
            return checkGrammarDecorators(node) || checkGrammarModifiers(node) || checkGrammarTypeParameterList(node, node.typeParameters, file) ||
                checkGrammarParameterList(node.parameters) || checkGrammarArrowFunction(node, file);
        }

        function checkGrammarArrowFunction(node: FunctionLikeDeclaration, file: SourceFile): boolean {
            if (node.kind === SyntaxKind.ArrowFunction) {
                const arrowFunction = <ArrowFunction>node;
                const startLine = getLineAndCharacterOfPosition(file, arrowFunction.equalsGreaterThanToken.pos).line;
                const endLine = getLineAndCharacterOfPosition(file, arrowFunction.equalsGreaterThanToken.end).line;
                if (startLine !== endLine) {
                    return grammarErrorOnNode(arrowFunction.equalsGreaterThanToken, Diagnostics.Line_terminator_not_permitted_before_arrow);
                }
            }
            return false;
        }

        function checkGrammarIndexSignatureParameters(node: SignatureDeclaration): boolean {
            const parameter = node.parameters[0];
            if (node.parameters.length !== 1) {
                if (parameter) {
                    return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_must_have_exactly_one_parameter);
                }
                else {
                    return grammarErrorOnNode(node, Diagnostics.An_index_signature_must_have_exactly_one_parameter);
                }
            }
            if (parameter.dotDotDotToken) {
                return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.An_index_signature_cannot_have_a_rest_parameter);
            }
            if (parameter.flags & NodeFlags.Modifier) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_cannot_have_an_accessibility_modifier);
            }
            if (parameter.questionToken) {
                return grammarErrorOnNode(parameter.questionToken, Diagnostics.An_index_signature_parameter_cannot_have_a_question_mark);
            }
            if (parameter.initializer) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_cannot_have_an_initializer);
            }
            if (!parameter.type) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_must_have_a_type_annotation);
            }
            if (parameter.type.kind !== SyntaxKind.StringKeyword && parameter.type.kind !== SyntaxKind.NumberKeyword) {
                return grammarErrorOnNode(parameter.name, Diagnostics.An_index_signature_parameter_type_must_be_string_or_number);
            }
            if (!node.type) {
                return grammarErrorOnNode(node, Diagnostics.An_index_signature_must_have_a_type_annotation);
            }
        }

        function checkGrammarIndexSignature(node: SignatureDeclaration) {
            // Prevent cascading error by short-circuit
            return checkGrammarDecorators(node) || checkGrammarModifiers(node) || checkGrammarIndexSignatureParameters(node);
        }

        function checkGrammarForAtLeastOneTypeArgument(node: Node, typeArguments: NodeArray<TypeNode>): boolean {
            if (typeArguments && typeArguments.length === 0) {
                const sourceFile = getSourceFileOfNode(node);
                const start = typeArguments.pos - "<".length;
                const end = skipTrivia(sourceFile.text, typeArguments.end) + ">".length;
                return grammarErrorAtPos(sourceFile, start, end - start, Diagnostics.Type_argument_list_cannot_be_empty);
            }
        }

        function checkGrammarTypeArguments(node: Node, typeArguments: NodeArray<TypeNode>): boolean {
            return checkGrammarForDisallowedTrailingComma(typeArguments) ||
                checkGrammarForAtLeastOneTypeArgument(node, typeArguments);
        }

        function checkGrammarForOmittedArgument(node: CallExpression, args: NodeArray<Expression>): boolean {
            if (args) {
                const sourceFile = getSourceFileOfNode(node);
                for (const arg of args) {
                    if (arg.kind === SyntaxKind.OmittedExpression) {
                        return grammarErrorAtPos(sourceFile, arg.pos, 0, Diagnostics.Argument_expression_expected);
                    }
                }
            }
        }

        function checkGrammarArguments(node: CallExpression, args: NodeArray<Expression>): boolean {
            return checkGrammarForOmittedArgument(node, args);
        }

        function checkGrammarHeritageClause(node: HeritageClause): boolean {
            const types = node.types;
            if (checkGrammarForDisallowedTrailingComma(types)) {
                return true;
            }
            if (types && types.length === 0) {
                const listType = tokenToString(node.token);
                const sourceFile = getSourceFileOfNode(node);
                return grammarErrorAtPos(sourceFile, types.pos, 0, Diagnostics._0_list_cannot_be_empty, listType);
            }
        }

        function checkGrammarClassDeclarationHeritageClauses(node: ClassLikeDeclaration) {
            let seenExtendsClause = false;
            let seenImplementsClause = false;

            if (!checkGrammarDecorators(node) && !checkGrammarModifiers(node) && node.heritageClauses) {
                for (const heritageClause of node.heritageClauses) {
                    if (heritageClause.token === SyntaxKind.ExtendsKeyword) {
                        if (seenExtendsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_already_seen);
                        }

                        if (seenImplementsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_must_precede_implements_clause);
                        }

                        if (heritageClause.types.length > 1) {
                            return grammarErrorOnFirstToken(heritageClause.types[1], Diagnostics.Classes_can_only_extend_a_single_class);
                        }

                        seenExtendsClause = true;
                    }
                    else {
                        Debug.assert(heritageClause.token === SyntaxKind.ImplementsKeyword);
                        if (seenImplementsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.implements_clause_already_seen);
                        }

                        seenImplementsClause = true;
                    }

                    // Grammar checking heritageClause inside class declaration
                    checkGrammarHeritageClause(heritageClause);
                }
            }
        }

        function checkGrammarInterfaceDeclaration(node: InterfaceDeclaration) {
            let seenExtendsClause = false;

            if (node.heritageClauses) {
                for (const heritageClause of node.heritageClauses) {
                    if (heritageClause.token === SyntaxKind.ExtendsKeyword) {
                        if (seenExtendsClause) {
                            return grammarErrorOnFirstToken(heritageClause, Diagnostics.extends_clause_already_seen);
                        }

                        seenExtendsClause = true;
                    }
                    else {
                        Debug.assert(heritageClause.token === SyntaxKind.ImplementsKeyword);
                        return grammarErrorOnFirstToken(heritageClause, Diagnostics.Interface_declaration_cannot_have_implements_clause);
                    }

                    // Grammar checking heritageClause inside class declaration
                    checkGrammarHeritageClause(heritageClause);
                }
            }

            return false;
        }

        function checkGrammarComputedPropertyName(node: Node): boolean {
            // If node is not a computedPropertyName, just skip the grammar checking
            if (node.kind !== SyntaxKind.ComputedPropertyName) {
                return false;
            }

            const computedPropertyName = <ComputedPropertyName>node;
            if (computedPropertyName.expression.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>computedPropertyName.expression).operatorToken.kind === SyntaxKind.CommaToken) {
                return grammarErrorOnNode(computedPropertyName.expression, Diagnostics.A_comma_expression_is_not_allowed_in_a_computed_property_name);
            }
        }

        function checkGrammarForGenerator(node: FunctionLikeDeclaration) {
            if (node.asteriskToken) {
                Debug.assert(
                    node.kind === SyntaxKind.FunctionDeclaration ||
                    node.kind === SyntaxKind.FunctionExpression ||
                    node.kind === SyntaxKind.MethodDeclaration);
                if (isInAmbientContext(node)) {
                    return grammarErrorOnNode(node.asteriskToken, Diagnostics.Generators_are_not_allowed_in_an_ambient_context);
                }
                if (!node.body) {
                    return grammarErrorOnNode(node.asteriskToken, Diagnostics.An_overload_signature_cannot_be_declared_as_a_generator);
                }
                if (languageVersion < ScriptTarget.ES6) {
                    return grammarErrorOnNode(node.asteriskToken, Diagnostics.Generators_are_only_available_when_targeting_ECMAScript_2015_or_higher);
                }
            }
        }

        function checkGrammarForInvalidQuestionMark(node: Declaration, questionToken: Node, message: DiagnosticMessage): boolean {
            if (questionToken) {
                return grammarErrorOnNode(questionToken, message);
            }
        }

        function checkGrammarObjectLiteralExpression(node: ObjectLiteralExpression, inDestructuring: boolean) {
            const seen = createMap<SymbolFlags>();
            const Property = 1;
            const GetAccessor = 2;
            const SetAccessor = 4;
            const GetOrSetAccessor = GetAccessor | SetAccessor;

            for (const prop of node.properties) {
                const name = prop.name;
                if (prop.kind === SyntaxKind.OmittedExpression ||
                    name.kind === SyntaxKind.ComputedPropertyName) {
                    // If the name is not a ComputedPropertyName, the grammar checking will skip it
                    checkGrammarComputedPropertyName(<ComputedPropertyName>name);
                }

                if (prop.kind === SyntaxKind.ShorthandPropertyAssignment && !inDestructuring && (<ShorthandPropertyAssignment>prop).objectAssignmentInitializer) {
                    // having objectAssignmentInitializer is only valid in ObjectAssignmentPattern
                    // outside of destructuring it is a syntax error
                    return grammarErrorOnNode((<ShorthandPropertyAssignment>prop).equalsToken, Diagnostics.can_only_be_used_in_an_object_literal_property_inside_a_destructuring_assignment);
                }

                // Modifiers are never allowed on properties except for 'async' on a method declaration
                forEach(prop.modifiers, mod => {
                    if (mod.kind !== SyntaxKind.AsyncKeyword || prop.kind !== SyntaxKind.MethodDeclaration) {
                        grammarErrorOnNode(mod, Diagnostics._0_modifier_cannot_be_used_here, getTextOfNode(mod));
                    }
                });

                // ECMA-262 11.1.5 Object Initializer
                // If previous is not undefined then throw a SyntaxError exception if any of the following conditions are true
                // a.This production is contained in strict code and IsDataDescriptor(previous) is true and
                // IsDataDescriptor(propId.descriptor) is true.
                //    b.IsDataDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true.
                //    c.IsAccessorDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
                //    d.IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true
                // and either both previous and propId.descriptor have[[Get]] fields or both previous and propId.descriptor have[[Set]] fields
                let currentKind: number;
                if (prop.kind === SyntaxKind.PropertyAssignment || prop.kind === SyntaxKind.ShorthandPropertyAssignment) {
                    // Grammar checking for computedPropertyName and shorthandPropertyAssignment
                    checkGrammarForInvalidQuestionMark(prop, (<PropertyAssignment>prop).questionToken, Diagnostics.An_object_member_cannot_be_declared_optional);
                    if (name.kind === SyntaxKind.NumericLiteral) {
                        checkGrammarNumericLiteral(<LiteralExpression>name);
                    }
                    currentKind = Property;
                }
                else if (prop.kind === SyntaxKind.MethodDeclaration) {
                    currentKind = Property;
                }
                else if (prop.kind === SyntaxKind.GetAccessor) {
                    currentKind = GetAccessor;
                }
                else if (prop.kind === SyntaxKind.SetAccessor) {
                    currentKind = SetAccessor;
                }
                else {
                    Debug.fail("Unexpected syntax kind:" + prop.kind);
                }

                const effectiveName = getPropertyNameForPropertyNameNode(name);
                if (effectiveName === undefined) {
                    continue;
                }

                if (!seen[effectiveName]) {
                    seen[effectiveName] = currentKind;
                }
                else {
                    const existingKind = seen[effectiveName];
                    if (currentKind === Property && existingKind === Property) {
                        grammarErrorOnNode(name, Diagnostics.Duplicate_identifier_0, getTextOfNode(name));
                    }
                    else if ((currentKind & GetOrSetAccessor) && (existingKind & GetOrSetAccessor)) {
                        if (existingKind !== GetOrSetAccessor && currentKind !== existingKind) {
                            seen[effectiveName] = currentKind | existingKind;
                        }
                        else {
                            return grammarErrorOnNode(name, Diagnostics.An_object_literal_cannot_have_multiple_get_Slashset_accessors_with_the_same_name);
                        }
                    }
                    else {
                        return grammarErrorOnNode(name, Diagnostics.An_object_literal_cannot_have_property_and_accessor_with_the_same_name);
                    }
                }
            }
        }

        function checkGrammarJsxElement(node: JsxOpeningLikeElement) {
            const seen = createMap<boolean>();
            for (const attr of node.attributes) {
                if (attr.kind === SyntaxKind.JsxSpreadAttribute) {
                    continue;
                }

                const jsxAttr = (<JsxAttribute>attr);
                const name = jsxAttr.name;
                if (!seen[name.text]) {
                    seen[name.text] = true;
                }
                else {
                    return grammarErrorOnNode(name, Diagnostics.JSX_elements_cannot_have_multiple_attributes_with_the_same_name);
                }

                const initializer = jsxAttr.initializer;
                if (initializer && initializer.kind === SyntaxKind.JsxExpression && !(<JsxExpression>initializer).expression) {
                    return grammarErrorOnNode(jsxAttr.initializer, Diagnostics.JSX_attributes_must_only_be_assigned_a_non_empty_expression);
                }
            }
        }

        function checkGrammarForInOrForOfStatement(forInOrOfStatement: ForInStatement | ForOfStatement): boolean {
            if (checkGrammarStatementInAmbientContext(forInOrOfStatement)) {
                return true;
            }

            if (forInOrOfStatement.initializer.kind === SyntaxKind.VariableDeclarationList) {
                const variableList = <VariableDeclarationList>forInOrOfStatement.initializer;
                if (!checkGrammarVariableDeclarationList(variableList)) {
                    const declarations = variableList.declarations;

                    // declarations.length can be zero if there is an error in variable declaration in for-of or for-in
                    // See http://www.ecma-international.org/ecma-262/6.0/#sec-for-in-and-for-of-statements for details
                    // For example:
                    //      var let = 10;
                    //      for (let of [1,2,3]) {} // this is invalid ES6 syntax
                    //      for (let in [1,2,3]) {} // this is invalid ES6 syntax
                    // We will then want to skip on grammar checking on variableList declaration
                    if (!declarations.length) {
                        return false;
                    }

                    if (declarations.length > 1) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_in_statement
                            : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
                        return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
                    }
                    const firstDeclaration = declarations[0];

                    if (firstDeclaration.initializer) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
                            : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
                        return grammarErrorOnNode(firstDeclaration.name, diagnostic);
                    }
                    if (firstDeclaration.type) {
                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
                            ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
                            : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
                        return grammarErrorOnNode(firstDeclaration, diagnostic);
                    }
                }
            }

            return false;
        }

        function checkGrammarAccessor(accessor: AccessorDeclaration): boolean {
            const kind = accessor.kind;
            if (languageVersion < ScriptTarget.ES5) {
                return grammarErrorOnNode(accessor.name, Diagnostics.Accessors_are_only_available_when_targeting_ECMAScript_5_and_higher);
            }
            else if (isInAmbientContext(accessor)) {
                return grammarErrorOnNode(accessor.name, Diagnostics.An_accessor_cannot_be_declared_in_an_ambient_context);
            }
            else if (accessor.body === undefined && !(accessor.flags & NodeFlags.Abstract)) {
                return grammarErrorAtPos(getSourceFileOfNode(accessor), accessor.end - 1, ";".length, Diagnostics._0_expected, "{");
            }
            else if (accessor.typeParameters) {
                return grammarErrorOnNode(accessor.name, Diagnostics.An_accessor_cannot_have_type_parameters);
            }
            else if (!doesAccessorHaveCorrectParameterCount(accessor)) {
                return grammarErrorOnNode(accessor.name,
                                          kind === SyntaxKind.GetAccessor ?
                                          Diagnostics.A_get_accessor_cannot_have_parameters :
                                          Diagnostics.A_set_accessor_must_have_exactly_one_parameter);
            }
            else if (kind === SyntaxKind.SetAccessor) {
                if (accessor.type) {
                    return grammarErrorOnNode(accessor.name, Diagnostics.A_set_accessor_cannot_have_a_return_type_annotation);
                }
                else {
                    const parameter = accessor.parameters[0];
                    if (parameter.dotDotDotToken) {
                        return grammarErrorOnNode(parameter.dotDotDotToken, Diagnostics.A_set_accessor_cannot_have_rest_parameter);
                    }
                    else if (parameter.questionToken) {
                        return grammarErrorOnNode(parameter.questionToken, Diagnostics.A_set_accessor_cannot_have_an_optional_parameter);
                    }
                    else if (parameter.initializer) {
                        return grammarErrorOnNode(accessor.name, Diagnostics.A_set_accessor_parameter_cannot_have_an_initializer);
                    }
                }
            }
        }

        /** Does the accessor have the right number of parameters?

            A get accessor has no parameters or a single `this` parameter.
            A set accessor has one parameter or a `this` parameter and one more parameter */
        function doesAccessorHaveCorrectParameterCount(accessor: AccessorDeclaration) {
            return getAccessorThisParameter(accessor) || accessor.parameters.length === (accessor.kind === SyntaxKind.GetAccessor ? 0 : 1);
        }

        function getAccessorThisParameter(accessor: AccessorDeclaration): ParameterDeclaration {
            if (accessor.parameters.length === (accessor.kind === SyntaxKind.GetAccessor ? 1 : 2) &&
                accessor.parameters[0].name.kind === SyntaxKind.Identifier &&
                (<Identifier>accessor.parameters[0].name).originalKeywordKind === SyntaxKind.ThisKeyword) {
                return accessor.parameters[0];
            }
        }

        function getFunctionLikeThisParameter(func: FunctionLikeDeclaration) {
            if (func.parameters.length &&
                func.parameters[0].name.kind === SyntaxKind.Identifier &&
                (<Identifier>func.parameters[0].name).originalKeywordKind === SyntaxKind.ThisKeyword) {
                return func.parameters[0];
            }
        }

        function checkGrammarForNonSymbolComputedProperty(node: DeclarationName, message: DiagnosticMessage) {
            if (isDynamicName(node)) {
                return grammarErrorOnNode(node, message);
            }
        }

        function checkGrammarMethod(node: MethodDeclaration) {
            if (checkGrammarDisallowedModifiersOnObjectLiteralExpressionMethod(node) ||
                checkGrammarFunctionLikeDeclaration(node) ||
                checkGrammarForGenerator(node)) {
                return true;
            }

            if (node.parent.kind === SyntaxKind.ObjectLiteralExpression) {
                if (checkGrammarForInvalidQuestionMark(node, node.questionToken, Diagnostics.An_object_member_cannot_be_declared_optional)) {
                    return true;
                }
                else if (node.body === undefined) {
                    return grammarErrorAtPos(getSourceFileOfNode(node), node.end - 1, ";".length, Diagnostics._0_expected, "{");
                }
            }

            if (isClassLike(node.parent)) {
                // Technically, computed properties in ambient contexts is disallowed
                // for property declarations and accessors too, not just methods.
                // However, property declarations disallow computed names in general,
                // and accessors are not allowed in ambient contexts in general,
                // so this error only really matters for methods.
                if (isInAmbientContext(node)) {
                    return checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_an_ambient_context_must_directly_refer_to_a_built_in_symbol);
                }
                else if (!node.body) {
                    return checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_method_overload_must_directly_refer_to_a_built_in_symbol);
                }
            }
            else if (node.parent.kind === SyntaxKind.InterfaceDeclaration) {
                return checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_an_interface_must_directly_refer_to_a_built_in_symbol);
            }
            else if (node.parent.kind === SyntaxKind.TypeLiteral) {
                return checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_type_literal_must_directly_refer_to_a_built_in_symbol);
            }
        }

        function checkGrammarBreakOrContinueStatement(node: BreakOrContinueStatement): boolean {
            let current: Node = node;
            while (current) {
                if (isFunctionLike(current)) {
                    return grammarErrorOnNode(node, Diagnostics.Jump_target_cannot_cross_function_boundary);
                }

                switch (current.kind) {
                    case SyntaxKind.LabeledStatement:
                        if (node.label && (<LabeledStatement>current).label.text === node.label.text) {
                            // found matching label - verify that label usage is correct
                            // continue can only target labels that are on iteration statements
                            const isMisplacedContinueLabel = node.kind === SyntaxKind.ContinueStatement
                                && !isIterationStatement((<LabeledStatement>current).statement, /*lookInLabeledStatement*/ true);

                            if (isMisplacedContinueLabel) {
                                return grammarErrorOnNode(node, Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement);
                            }

                            return false;
                        }
                        break;
                    case SyntaxKind.SwitchStatement:
                        if (node.kind === SyntaxKind.BreakStatement && !node.label) {
                            // unlabeled break within switch statement - ok
                            return false;
                        }
                        break;
                    default:
                        if (isIterationStatement(current, /*lookInLabeledStatement*/ false) && !node.label) {
                            // unlabeled break or continue within iteration statement - ok
                            return false;
                        }
                        break;
                }

                current = current.parent;
            }

            if (node.label) {
                const message = node.kind === SyntaxKind.BreakStatement
                    ? Diagnostics.A_break_statement_can_only_jump_to_a_label_of_an_enclosing_statement
                    : Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement;

                return grammarErrorOnNode(node, message);
            }
            else {
                const message = node.kind === SyntaxKind.BreakStatement
                    ? Diagnostics.A_break_statement_can_only_be_used_within_an_enclosing_iteration_or_switch_statement
                    : Diagnostics.A_continue_statement_can_only_be_used_within_an_enclosing_iteration_statement;
                return grammarErrorOnNode(node, message);
            }
        }

        function checkGrammarBindingElement(node: BindingElement) {
            if (node.dotDotDotToken) {
                const elements = (<BindingPattern>node.parent).elements;
                if (node !== lastOrUndefined(elements)) {
                    return grammarErrorOnNode(node, Diagnostics.A_rest_element_must_be_last_in_an_array_destructuring_pattern);
                }

                if (node.name.kind === SyntaxKind.ArrayBindingPattern || node.name.kind === SyntaxKind.ObjectBindingPattern) {
                    return grammarErrorOnNode(node.name, Diagnostics.A_rest_element_cannot_contain_a_binding_pattern);
                }

                if (node.initializer) {
                    // Error on equals token which immediate precedes the initializer
                    return grammarErrorAtPos(getSourceFileOfNode(node), node.initializer.pos - 1, 1, Diagnostics.A_rest_element_cannot_have_an_initializer);
                }
            }
        }

        function checkGrammarVariableDeclaration(node: VariableDeclaration) {
            if (node.parent.parent.kind !== SyntaxKind.ForInStatement && node.parent.parent.kind !== SyntaxKind.ForOfStatement) {
                if (isInAmbientContext(node)) {
                    if (node.initializer) {
                        // Error on equals token which immediate precedes the initializer
                        const equalsTokenLength = "=".length;
                        return grammarErrorAtPos(getSourceFileOfNode(node), node.initializer.pos - equalsTokenLength,
                            equalsTokenLength, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
                    }
                }
                else if (!node.initializer) {
                    if (isBindingPattern(node.name) && !isBindingPattern(node.parent)) {
                        return grammarErrorOnNode(node, Diagnostics.A_destructuring_declaration_must_have_an_initializer);
                    }
                    if (isConst(node)) {
                        return grammarErrorOnNode(node, Diagnostics.const_declarations_must_be_initialized);
                    }
                }
            }

            const checkLetConstNames = (isLet(node) || isConst(node));

            // 1. LexicalDeclaration : LetOrConst BindingList ;
            // It is a Syntax Error if the BoundNames of BindingList contains "let".
            // 2. ForDeclaration: ForDeclaration : LetOrConst ForBinding
            // It is a Syntax Error if the BoundNames of ForDeclaration contains "let".

            // It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code
            // and its Identifier is eval or arguments
            return checkLetConstNames && checkGrammarNameInLetOrConstDeclarations(node.name);
        }

        function checkGrammarNameInLetOrConstDeclarations(name: Identifier | BindingPattern): boolean {
            if (name.kind === SyntaxKind.Identifier) {
                if ((<Identifier>name).originalKeywordKind === SyntaxKind.LetKeyword) {
                    return grammarErrorOnNode(name, Diagnostics.let_is_not_allowed_to_be_used_as_a_name_in_let_or_const_declarations);
                }
            }
            else {
                const elements = (<BindingPattern>name).elements;
                for (const element of elements) {
                    if (element.kind !== SyntaxKind.OmittedExpression) {
                        checkGrammarNameInLetOrConstDeclarations(element.name);
                    }
                }
            }
        }

        function checkGrammarVariableDeclarationList(declarationList: VariableDeclarationList): boolean {
            const declarations = declarationList.declarations;
            if (checkGrammarForDisallowedTrailingComma(declarationList.declarations)) {
                return true;
            }

            if (!declarationList.declarations.length) {
                return grammarErrorAtPos(getSourceFileOfNode(declarationList), declarations.pos, declarations.end - declarations.pos, Diagnostics.Variable_declaration_list_cannot_be_empty);
            }
        }

        function allowLetAndConstDeclarations(parent: Node): boolean {
            switch (parent.kind) {
                case SyntaxKind.IfStatement:
                case SyntaxKind.DoStatement:
                case SyntaxKind.WhileStatement:
                case SyntaxKind.WithStatement:
                case SyntaxKind.ForStatement:
                case SyntaxKind.ForInStatement:
                case SyntaxKind.ForOfStatement:
                    return false;
                case SyntaxKind.LabeledStatement:
                    return allowLetAndConstDeclarations(parent.parent);
            }

            return true;
        }

        function checkGrammarForDisallowedLetOrConstStatement(node: VariableStatement) {
            if (!allowLetAndConstDeclarations(node.parent)) {
                if (isLet(node.declarationList)) {
                    return grammarErrorOnNode(node, Diagnostics.let_declarations_can_only_be_declared_inside_a_block);
                }
                else if (isConst(node.declarationList)) {
                    return grammarErrorOnNode(node, Diagnostics.const_declarations_can_only_be_declared_inside_a_block);
                }
            }
        }

        function hasParseDiagnostics(sourceFile: SourceFile): boolean {
            return sourceFile.parseDiagnostics.length > 0;
        }

        function grammarErrorOnFirstToken(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
                diagnostics.add(createFileDiagnostic(sourceFile, span.start, span.length, message, arg0, arg1, arg2));
                return true;
            }
        }

        function grammarErrorAtPos(sourceFile: SourceFile, start: number, length: number, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            if (!hasParseDiagnostics(sourceFile)) {
                diagnostics.add(createFileDiagnostic(sourceFile, start, length, message, arg0, arg1, arg2));
                return true;
            }
        }

        function grammarErrorOnNode(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                diagnostics.add(createDiagnosticForNode(node, message, arg0, arg1, arg2));
                return true;
            }
        }

        function checkGrammarConstructorTypeParameters(node: ConstructorDeclaration) {
            if (node.typeParameters) {
                return grammarErrorAtPos(getSourceFileOfNode(node), node.typeParameters.pos, node.typeParameters.end - node.typeParameters.pos, Diagnostics.Type_parameters_cannot_appear_on_a_constructor_declaration);
            }
        }

        function checkGrammarConstructorTypeAnnotation(node: ConstructorDeclaration) {
            if (node.type) {
                return grammarErrorOnNode(node.type, Diagnostics.Type_annotation_cannot_appear_on_a_constructor_declaration);
            }
        }

        function checkGrammarProperty(node: PropertyDeclaration) {
            if (isClassLike(node.parent)) {
                if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_class_property_declaration_must_directly_refer_to_a_built_in_symbol)) {
                    return true;
                }
            }
            else if (node.parent.kind === SyntaxKind.InterfaceDeclaration) {
                if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_an_interface_must_directly_refer_to_a_built_in_symbol)) {
                    return true;
                }
                if (node.initializer) {
                    return grammarErrorOnNode(node.initializer, Diagnostics.An_interface_property_cannot_have_an_initializer);
                }
            }
            else if (node.parent.kind === SyntaxKind.TypeLiteral) {
                if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_type_literal_must_directly_refer_to_a_built_in_symbol)) {
                    return true;
                }
                if (node.initializer) {
                    return grammarErrorOnNode(node.initializer, Diagnostics.A_type_literal_property_cannot_have_an_initializer);
                }
            }

            if (isInAmbientContext(node) && node.initializer) {
                return grammarErrorOnFirstToken(node.initializer, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
            }
        }

        function checkGrammarTopLevelElementForRequiredDeclareModifier(node: Node): boolean {
            // A declare modifier is required for any top level .d.ts declaration except export=, export default, export as namespace
            // interfaces and imports categories:
            //
            //  DeclarationElement:
            //     ExportAssignment
            //     export_opt   InterfaceDeclaration
            //     export_opt   TypeAliasDeclaration
            //     export_opt   ImportDeclaration
            //     export_opt   ExternalImportDeclaration
            //     export_opt   AmbientDeclaration
            //
            // TODO: The spec needs to be amended to reflect this grammar.
            if (node.kind === SyntaxKind.InterfaceDeclaration ||
                node.kind === SyntaxKind.TypeAliasDeclaration ||
                node.kind === SyntaxKind.ImportDeclaration ||
                node.kind === SyntaxKind.ImportEqualsDeclaration ||
                node.kind === SyntaxKind.ExportDeclaration ||
                node.kind === SyntaxKind.ExportAssignment ||
                node.kind === SyntaxKind.NamespaceExportDeclaration ||
                (node.flags & NodeFlags.Ambient) ||
                (node.flags & (NodeFlags.Export | NodeFlags.Default))) {

                return false;
            }

            return grammarErrorOnFirstToken(node, Diagnostics.A_declare_modifier_is_required_for_a_top_level_declaration_in_a_d_ts_file);
        }

        function checkGrammarTopLevelElementsForRequiredDeclareModifier(file: SourceFile): boolean {
            for (const decl of file.statements) {
                if (isDeclaration(decl) || decl.kind === SyntaxKind.VariableStatement) {
                    if (checkGrammarTopLevelElementForRequiredDeclareModifier(decl)) {
                        return true;
                    }
                }
            }
        }

        function checkGrammarSourceFile(node: SourceFile): boolean {
            return isInAmbientContext(node) && checkGrammarTopLevelElementsForRequiredDeclareModifier(node);
        }

        function checkGrammarStatementInAmbientContext(node: Node): boolean {
            if (isInAmbientContext(node)) {
                // An accessors is already reported about the ambient context
                if (isAccessor(node.parent.kind)) {
                    return getNodeLinks(node).hasReportedStatementInAmbientContext = true;
                }

                // Find containing block which is either Block, ModuleBlock, SourceFile
                const links = getNodeLinks(node);
                if (!links.hasReportedStatementInAmbientContext && isFunctionLike(node.parent)) {
                    return getNodeLinks(node).hasReportedStatementInAmbientContext = grammarErrorOnFirstToken(node, Diagnostics.An_implementation_cannot_be_declared_in_ambient_contexts);
                }

                // We are either parented by another statement, or some sort of block.
                // If we're in a block, we only want to really report an error once
                // to prevent noisiness.  So use a bit on the block to indicate if
                // this has already been reported, and don't report if it has.
                //
                if (node.parent.kind === SyntaxKind.Block || node.parent.kind === SyntaxKind.ModuleBlock || node.parent.kind === SyntaxKind.SourceFile) {
                    const links = getNodeLinks(node.parent);
                    // Check if the containing block ever report this error
                    if (!links.hasReportedStatementInAmbientContext) {
                        return links.hasReportedStatementInAmbientContext = grammarErrorOnFirstToken(node, Diagnostics.Statements_are_not_allowed_in_ambient_contexts);
                    }
                }
                else {
                    // We must be parented by a statement.  If so, there's no need
                    // to report the error as our parent will have already done it.
                    // Debug.assert(isStatement(node.parent));
                }
            }
        }

        function checkGrammarNumericLiteral(node: LiteralExpression): boolean {
            // Grammar checking
            if (node.isOctalLiteral && languageVersion >= ScriptTarget.ES5) {
                return grammarErrorOnNode(node, Diagnostics.Octal_literals_are_not_available_when_targeting_ECMAScript_5_and_higher);
            }
        }

        function grammarErrorAfterFirstToken(node: Node, message: DiagnosticMessage, arg0?: any, arg1?: any, arg2?: any): boolean {
            const sourceFile = getSourceFileOfNode(node);
            if (!hasParseDiagnostics(sourceFile)) {
                const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
                diagnostics.add(createFileDiagnostic(sourceFile, textSpanEnd(span), /*length*/ 0, message, arg0, arg1, arg2));
                return true;
            }
        }
    }
}