diff --git a/Jakefile.js b/Jakefile.js
index d327b5bb47b..17346e8bc1f 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -250,6 +250,7 @@ var harnessSources = harnessCoreSources.concat([
     "convertToBase64.ts",
     "transpile.ts",
     "reuseProgramStructure.ts",
+    "textStorage.ts",
     "cachingInServerLSHost.ts",
     "moduleResolution.ts",
     "tsconfigParsing.ts",
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 67c4db715e2..6bbbab8822e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3484,20 +3484,7 @@ namespace ts {
                 }
 
                 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));
-                        }
-                    }
+                    type = reportCircularityError(symbol);
                 }
                 links.type = type;
             }
@@ -3631,11 +3618,33 @@ namespace ts {
         function getTypeOfInstantiatedSymbol(symbol: Symbol): Type {
             const links = getSymbolLinks(symbol);
             if (!links.type) {
-                links.type = instantiateType(getTypeOfSymbol(links.target), links.mapper);
+                if (!pushTypeResolution(symbol, TypeSystemPropertyName.Type)) {
+                    return unknownType;
+                }
+                let type = instantiateType(getTypeOfSymbol(links.target), links.mapper);
+                if (!popTypeResolution()) {
+                    type = reportCircularityError(symbol);
+                }
+                links.type = type;
             }
             return links.type;
         }
 
+        function reportCircularityError(symbol: Symbol) {
+            // Check if variable has type annotation that circularly references the variable itself
+            if ((<VariableLikeDeclaration>symbol.valueDeclaration).type) {
+                error(symbol.valueDeclaration, Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_type_annotation,
+                    symbolToString(symbol));
+                return unknownType;
+            }
+            // Otherwise variable has initializer that circularly references the variable itself
+            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));
+            }
+            return anyType;
+        }
+
         function getTypeOfSymbol(symbol: Symbol): Type {
             if (symbol.flags & SymbolFlags.Instantiated) {
                 return getTypeOfInstantiatedSymbol(symbol);
@@ -4524,12 +4533,11 @@ namespace ts {
             // Resolve upfront such that recursive references see an empty object type.
             setStructuredTypeMembers(type, emptySymbols, emptyArray, emptyArray, undefined, undefined);
             // In { [P in K]: T }, we refer to P as the type parameter type, K as the constraint type,
-            // and T as the template type. If K is of the form 'keyof S', the mapped type and S are
-            // homomorphic and we copy property modifiers from corresponding properties in S.
+            // and T as the template type.
             const typeParameter = getTypeParameterFromMappedType(type);
             const constraintType = getConstraintTypeFromMappedType(type);
-            const homomorphicType = getHomomorphicTypeFromMappedType(type);
             const templateType = getTemplateTypeFromMappedType(type);
+            const modifiersType = getModifiersTypeFromMappedType(type);
             const templateReadonly = !!type.declaration.readonlyToken;
             const templateOptional = !!type.declaration.questionToken;
             // First, if the constraint type is a type parameter, obtain the base constraint. Then,
@@ -4548,11 +4556,11 @@ namespace ts {
                 // Otherwise, for type string create a string index signature.
                 if (t.flags & TypeFlags.StringLiteral) {
                     const propName = (<LiteralType>t).text;
-                    const homomorphicProp = homomorphicType && getPropertyOfType(homomorphicType, propName);
-                    const isOptional = templateOptional || !!(homomorphicProp && homomorphicProp.flags & SymbolFlags.Optional);
+                    const modifiersProp = getPropertyOfType(modifiersType, propName);
+                    const isOptional = templateOptional || !!(modifiersProp && modifiersProp.flags & SymbolFlags.Optional);
                     const prop = <TransientSymbol>createSymbol(SymbolFlags.Property | SymbolFlags.Transient | (isOptional ? SymbolFlags.Optional : 0), propName);
                     prop.type = propType;
-                    prop.isReadonly = templateReadonly || homomorphicProp && isReadonlySymbol(homomorphicProp);
+                    prop.isReadonly = templateReadonly || modifiersProp && isReadonlySymbol(modifiersProp);
                     members.set(propName, prop);
                 }
                 else if (t.flags & TypeFlags.String) {
@@ -4579,9 +4587,16 @@ namespace ts {
                     unknownType);
         }
 
-        function getHomomorphicTypeFromMappedType(type: MappedType) {
-            const constraint = getConstraintDeclaration(getTypeParameterFromMappedType(type));
-            return constraint.kind === SyntaxKind.TypeOperator ? instantiateType(getTypeFromTypeNode((<TypeOperatorNode>constraint).type), type.mapper || identityMapper) : undefined;
+        function getModifiersTypeFromMappedType(type: MappedType) {
+            if (!type.modifiersType) {
+                // If the mapped type was declared as { [P in keyof T]: X } or as { [P in K]: X }, where
+                // K is constrained to 'K extends keyof T', then we will copy property modifiers from T.
+                const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
+                const constraint = getConstraintTypeFromMappedType(declaredType);
+                const extendedConstraint = constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
+                type.modifiersType = extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
+            }
+            return type.modifiersType;
         }
 
         function getErasedTemplateTypeFromMappedType(type: MappedType) {
@@ -4678,33 +4693,24 @@ namespace ts {
          * 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) {
+        function getApparentTypeOfTypeVariable(type: TypeVariable) {
             if (!type.resolvedApparentType) {
-                let constraintType = getConstraintOfTypeParameter(type);
+                let constraintType = getConstraintOfTypeVariable(type);
                 while (constraintType && constraintType.flags & TypeFlags.TypeParameter) {
-                    constraintType = getConstraintOfTypeParameter(<TypeParameter>constraintType);
+                    constraintType = getConstraintOfTypeVariable(<TypeVariable>constraintType);
                 }
                 type.resolvedApparentType = getTypeWithThisArgument(constraintType || emptyObjectType, type);
             }
             return type.resolvedApparentType;
         }
 
-        /**
-         * The apparent type of an indexed access T[K] is the type of T's string index signature, if any.
-         */
-        function getApparentTypeOfIndexedAccess(type: IndexedAccessType) {
-            return getIndexTypeOfType(getApparentType(type.objectType), IndexKind.String) || type;
-        }
-
         /**
          * 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 {
-            const t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) :
-                type.flags & TypeFlags.IndexedAccess ? getApparentTypeOfIndexedAccess(<IndexedAccessType>type) :
-                type;
+            const t = type.flags & TypeFlags.TypeVariable ? getApparentTypeOfTypeVariable(<TypeVariable>type) : type;
             return t.flags & TypeFlags.StringLike ? globalStringType :
                 t.flags & TypeFlags.NumberLike ? globalNumberType :
                 t.flags & TypeFlags.BooleanLike ? globalBooleanType :
@@ -5290,6 +5296,12 @@ namespace ts {
             return typeParameter.constraint === noConstraintType ? undefined : typeParameter.constraint;
         }
 
+        function getConstraintOfTypeVariable(type: TypeVariable): Type {
+            return type.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>type) :
+                type.flags & TypeFlags.IndexedAccess ? (<IndexedAccessType>type).constraint :
+                undefined;
+        }
+
         function getParentSymbolOfTypeParameter(typeParameter: TypeParameter): Symbol {
             return getSymbolOfNode(getDeclarationOfKind(typeParameter.symbol, SyntaxKind.TypeParameter).parent);
         }
@@ -5965,6 +5977,24 @@ namespace ts {
             const type = <IndexedAccessType>createType(TypeFlags.IndexedAccess);
             type.objectType = objectType;
             type.indexType = indexType;
+            // We eagerly compute the constraint of the indexed access type such that circularity
+            // errors are immediately caught and reported. For example, class C { x: this["x"] }
+            // becomes an error only when the constraint is eagerly computed.
+            if (type.objectType.flags & TypeFlags.StructuredType) {
+                // The constraint of T[K], where T is an object, union, or intersection type,
+                // is the type of the string index signature of T, if any.
+                type.constraint = getIndexTypeOfType(type.objectType, IndexKind.String);
+            }
+            else if (type.objectType.flags & TypeFlags.TypeVariable) {
+                // The constraint of T[K], where T is a type variable, is A[K], where A is the
+                // apparent type of T.
+                const apparentType = getApparentTypeOfTypeVariable(<TypeVariable>type.objectType);
+                if (apparentType !== emptyObjectType) {
+                    type.constraint = isTypeOfKind((<IndexedAccessType>type).indexType, TypeFlags.StringLike) ?
+                        getIndexedAccessType(apparentType, (<IndexedAccessType>type).indexType) :
+                        getIndexTypeOfType(apparentType, IndexKind.String);
+                }
+            }
             return type;
         }
 
@@ -6043,14 +6073,19 @@ namespace ts {
         }
 
         function getIndexedAccessType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode) {
-            if (maybeTypeOfKind(indexType, TypeFlags.TypeVariable | TypeFlags.Index) || isGenericMappedType(objectType)) {
+            // If the index type is generic, if the object type is generic and doesn't originate in an expression,
+            // or if the object type is a mapped type with a generic constraint, we are performing a higher-order
+            // index access where we cannot meaningfully access the properties of the object type. Note that for a
+            // generic T and a non-generic K, we eagerly resolve T[K] if it originates in an expression. This is to
+            // preserve backwards compatibility. For example, an element access 'this["foo"]' has always been resolved
+            // eagerly using the constraint type of 'this' at the given location.
+            if (maybeTypeOfKind(indexType, TypeFlags.TypeVariable | TypeFlags.Index) ||
+                maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && !(accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression) ||
+                isGenericMappedType(objectType)) {
                 if (objectType.flags & TypeFlags.Any) {
                     return objectType;
                 }
-                // If the index type is generic or if the object type is a mapped type with a generic constraint,
-                // we are performing a higher-order index access where we cannot meaningfully access the properties
-                // of the object type. In those cases, we first check that the index type is assignable to 'keyof T'
-                // for the object type.
+                // We first check that the index type is assignable to 'keyof T' for the object type.
                 if (accessNode) {
                     if (!isTypeAssignableTo(indexType, getIndexType(objectType))) {
                         error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
@@ -6067,6 +6102,7 @@ namespace ts {
                 const id = objectType.id + "," + indexType.id;
                 return indexedAccessTypes.get(id) || set(indexedAccessTypes, id, createIndexedAccessType(objectType, indexType));
             }
+            // In the following we resolve T[K] to the type of the property in T selected by K.
             const apparentObjectType = getApparentType(objectType);
             if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Primitive)) {
                 const propTypes: Type[] = [];
@@ -7255,8 +7291,7 @@ namespace ts {
                         return result;
                     }
                 }
-
-                if (target.flags & TypeFlags.TypeParameter) {
+                else if (target.flags & TypeFlags.TypeParameter) {
                     // A source type { [P in keyof T]: X } is related to a target type T if X is related to T[P].
                     if (getObjectFlags(source) & ObjectFlags.Mapped && getConstraintTypeFromMappedType(<MappedType>source) === getIndexType(target)) {
                         if (!(<MappedType>source).declaration.questionToken) {
@@ -7285,10 +7320,10 @@ namespace ts {
                             return result;
                         }
                     }
-                    // Given a type parameter T with a constraint C, a type S is assignable to
+                    // Given a type variable T with a constraint C, a type S is assignable to
                     // keyof T if S is assignable to keyof C.
-                    if ((<IndexType>target).type.flags & TypeFlags.TypeParameter) {
-                        const constraint = getConstraintOfTypeParameter(<TypeParameter>(<IndexType>target).type);
+                    if ((<IndexType>target).type.flags & TypeFlags.TypeVariable) {
+                        const constraint = getConstraintOfTypeVariable(<TypeVariable>(<IndexType>target).type);
                         if (constraint) {
                             if (result = isRelatedTo(source, getIndexType(constraint), reportErrors)) {
                                 return result;
@@ -7304,6 +7339,14 @@ namespace ts {
                             return result;
                         }
                     }
+                    // A type S is related to a type T[K] if S is related to A[K], where K is string-like and
+                    // A is the apparent type of S.
+                    if ((<IndexedAccessType>target).constraint) {
+                        if (result = isRelatedTo(source, (<IndexedAccessType>target).constraint, reportErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
+                    }
                 }
 
                 if (source.flags & TypeFlags.TypeParameter) {
@@ -7312,6 +7355,7 @@ namespace ts {
                         const indexedAccessType = getIndexedAccessType(source, getTypeParameterFromMappedType(<MappedType>target));
                         const templateType = getTemplateTypeFromMappedType(<MappedType>target);
                         if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
+                            errorInfo = saveErrorInfo;
                             return result;
                         }
                     }
@@ -7333,6 +7377,16 @@ namespace ts {
                         }
                     }
                 }
+                else if (source.flags & TypeFlags.IndexedAccess) {
+                    // A type S[K] is related to a type T if A[K] is related to T, where K is string-like and
+                    // A is the apparent type of S.
+                    if ((<IndexedAccessType>source).constraint) {
+                        if (result = isRelatedTo((<IndexedAccessType>source).constraint, target, reportErrors)) {
+                            errorInfo = saveErrorInfo;
+                            return result;
+                        }
+                    }
+                }
                 else {
                     if (getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (<TypeReference>source).target === (<TypeReference>target).target) {
                         // We have type references to same target type, see if relationship holds for all type arguments
@@ -13488,13 +13542,14 @@ namespace ts {
                 const containingClass = getContainingClass(node);
                 if (containingClass) {
                     const containingType = getTypeOfNode(containingClass);
-                    const baseTypes = getBaseTypes(<InterfaceType>containingType);
-                    if (baseTypes.length) {
+                    let baseTypes = getBaseTypes(containingType as InterfaceType);
+                    while (baseTypes.length) {
                         const baseType = baseTypes[0];
                         if (modifiers & ModifierFlags.Protected &&
                             baseType.symbol === declaration.parent.symbol) {
                             return true;
                         }
+                        baseTypes = getBaseTypes(baseType as InterfaceType);
                     }
                 }
                 if (modifiers & ModifierFlags.Private) {
@@ -15002,8 +15057,8 @@ namespace ts {
 
         function isLiteralContextualType(contextualType: Type) {
             if (contextualType) {
-                if (contextualType.flags & TypeFlags.TypeParameter) {
-                    const apparentType = getApparentTypeOfTypeParameter(<TypeParameter>contextualType);
+                if (contextualType.flags & TypeFlags.TypeVariable) {
+                    const apparentType = getApparentTypeOfTypeVariable(<TypeVariable>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.
@@ -15838,7 +15893,7 @@ namespace ts {
             checkSourceElement(node.type);
             const type = <MappedType>getTypeFromMappedTypeNode(node);
             const constraintType = getConstraintTypeFromMappedType(type);
-            const keyType = constraintType.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>constraintType) : constraintType;
+            const keyType = constraintType.flags & TypeFlags.TypeVariable ? getApparentTypeOfTypeVariable(<TypeVariable>constraintType) : constraintType;
             checkTypeAssignableTo(keyType, stringType, node.typeParameter.constraint);
         }
 
@@ -16713,6 +16768,14 @@ namespace ts {
             }
         }
 
+        function isRemovedPropertyFromObjectSpread(node: Node) {
+            if (isBindingElement(node) && isObjectBindingPattern(node.parent)) {
+                const lastElement = lastOrUndefined(node.parent.elements);
+                return lastElement !== node && !!lastElement.dotDotDotToken;
+            }
+            return false;
+        }
+
         function errorUnusedLocal(node: Node, name: string) {
             if (isIdentifierThatStartsWithUnderScore(node)) {
                 const declaration = getRootDeclaration(node.parent);
@@ -16722,7 +16785,10 @@ namespace ts {
                     return;
                 }
             }
-            error(node, Diagnostics._0_is_declared_but_never_used, name);
+
+            if (!isRemovedPropertyFromObjectSpread(node.kind === SyntaxKind.Identifier ? node.parent : node)) {
+                error(node, Diagnostics._0_is_declared_but_never_used, name);
+            }
         }
 
         function parameterNameStartsWithUnderscore(parameterName: DeclarationName) {
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 7571804b5f4..b3c538afd0a 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -840,7 +840,7 @@ namespace ts {
       * @param basePath A root directory to resolve relative path entries in the config
       *    file to. e.g. outDir
       */
-    export function parseJsonConfigFileContent(json: any, host: ParseConfigHost, basePath: string, existingOptions: CompilerOptions = {}, configFileName?: string, resolutionStack: Path[] = []): ParsedCommandLine {
+    export function parseJsonConfigFileContent(json: any, host: ParseConfigHost, basePath: string, existingOptions: CompilerOptions = {}, configFileName?: string, resolutionStack: Path[] = [], extraFileExtensions: FileExtensionInfo[] = []): ParsedCommandLine {
         const errors: Diagnostic[] = [];
         const getCanonicalFileName = createGetCanonicalFileName(host.useCaseSensitiveFileNames);
         const resolvedPath = toPath(configFileName || "", basePath, getCanonicalFileName);
@@ -980,7 +980,7 @@ namespace ts {
                 includeSpecs = ["**/*"];
             }
 
-            const result = matchFileNames(fileNames, includeSpecs, excludeSpecs, basePath, options, host, errors);
+            const result = matchFileNames(fileNames, includeSpecs, excludeSpecs, basePath, options, host, errors, extraFileExtensions);
 
             if (result.fileNames.length === 0 && !hasProperty(json, "files") && resolutionStack.length === 0) {
                 errors.push(
@@ -1185,7 +1185,7 @@ namespace ts {
      * @param host The host used to resolve files and directories.
      * @param errors An array for diagnostic reporting.
      */
-    function matchFileNames(fileNames: string[], include: string[], exclude: string[], basePath: string, options: CompilerOptions, host: ParseConfigHost, errors: Diagnostic[]): ExpandResult {
+    function matchFileNames(fileNames: string[], include: string[], exclude: string[], basePath: string, options: CompilerOptions, host: ParseConfigHost, errors: Diagnostic[], extraFileExtensions: FileExtensionInfo[]): ExpandResult {
         basePath = normalizePath(basePath);
 
         // The exclude spec list is converted into a regular expression, which allows us to quickly
@@ -1219,7 +1219,7 @@ namespace ts {
 
         // Rather than requery this for each file and filespec, we query the supported extensions
         // once and store it on the expansion context.
-        const supportedExtensions = getSupportedExtensions(options);
+        const supportedExtensions = getSupportedExtensions(options, extraFileExtensions);
 
         // Literal files are always included verbatim. An "include" or "exclude" specification cannot
         // remove a literal file.
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 3c370f7b154..fd9eedeeece 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -2032,8 +2032,18 @@ namespace ts {
     export const supportedJavascriptExtensions = [".js", ".jsx"];
     const allSupportedExtensions = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
 
-    export function getSupportedExtensions(options?: CompilerOptions): string[] {
-        return options && options.allowJs ? allSupportedExtensions : supportedTypeScriptExtensions;
+    export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: FileExtensionInfo[]): string[] {
+        const needAllExtensions = options && options.allowJs;
+        if (!extraFileExtensions || extraFileExtensions.length === 0) {
+            return needAllExtensions ? allSupportedExtensions : supportedTypeScriptExtensions;
+        }
+        const extensions = (needAllExtensions ? allSupportedExtensions : supportedTypeScriptExtensions).slice(0);
+        for (const extInfo of extraFileExtensions) {
+            if (needAllExtensions || extInfo.scriptKind === ScriptKind.TS) {
+                extensions.push(extInfo.extension);
+            }
+        }
+        return extensions;
     }
 
     export function hasJavaScriptFileExtension(fileName: string) {
@@ -2044,10 +2054,10 @@ namespace ts {
         return forEach(supportedTypeScriptExtensions, extension => fileExtensionIs(fileName, extension));
     }
 
-    export function isSupportedSourceFileName(fileName: string, compilerOptions?: CompilerOptions) {
+    export function isSupportedSourceFileName(fileName: string, compilerOptions?: CompilerOptions, extraFileExtensions?: FileExtensionInfo[]) {
         if (!fileName) { return false; }
 
-        for (const extension of getSupportedExtensions(compilerOptions)) {
+        for (const extension of getSupportedExtensions(compilerOptions, extraFileExtensions)) {
             if (fileExtensionIs(fileName, extension)) {
                 return true;
             }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index ca2295e5ba2..b98a5a493ba 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2948,6 +2948,7 @@ namespace ts {
         typeParameter?: TypeParameter;
         constraintType?: Type;
         templateType?: Type;
+        modifiersType?: Type;
         mapper?: TypeMapper;  // Instantiation mapper
     }
 
@@ -2983,6 +2984,8 @@ namespace ts {
     }
 
     export interface TypeVariable extends Type {
+        /* @internal */
+        resolvedApparentType: Type;
         /* @internal */
         resolvedIndexType: IndexType;
     }
@@ -2995,8 +2998,6 @@ namespace ts {
         /* @internal */
         mapper?: TypeMapper;     // Instantiation mapper
         /* @internal */
-        resolvedApparentType: Type;
-        /* @internal */
         isThisType?: boolean;
     }
 
@@ -3005,6 +3006,7 @@ namespace ts {
     export interface IndexedAccessType extends TypeVariable {
         objectType: Type;
         indexType: Type;
+        constraint?: Type;
     }
 
     // keyof T types (TypeFlags.Index)
@@ -3101,6 +3103,12 @@ namespace ts {
         ThisProperty
     }
 
+    export interface FileExtensionInfo {
+        extension: string;
+        scriptKind: ScriptKind;
+        isMixedContent: boolean;
+    }
+
     export interface DiagnosticMessage {
         key: string;
         category: DiagnosticCategory;
diff --git a/src/harness/unittests/textStorage.ts b/src/harness/unittests/textStorage.ts
new file mode 100644
index 00000000000..b4287f2610c
--- /dev/null
+++ b/src/harness/unittests/textStorage.ts
@@ -0,0 +1,70 @@
+/// <reference path="../harness.ts" />
+/// <reference path="../../server/scriptVersionCache.ts"/>
+/// <reference path="./tsserverProjectSystem.ts" />
+
+namespace ts.textStorage {
+    describe("Text storage", () => {
+        const f = {
+            path: "/a/app.ts",
+            content: `
+                let x = 1;
+                let y = 2;
+                function bar(a: number) {
+                    return a + 1;
+                }`
+        };
+
+        it("text based storage should be have exactly the same as script version cache", () => {
+
+            const host = ts.projectSystem.createServerHost([f]);
+
+            const ts1 = new server.TextStorage(host, server.asNormalizedPath(f.path));
+            const ts2 = new server.TextStorage(host, server.asNormalizedPath(f.path));
+
+            ts1.useScriptVersionCache();
+            ts2.useText();
+
+            const lineMap = computeLineStarts(f.content);
+
+            for (let line = 0; line < lineMap.length; line++) {
+                const start = lineMap[line];
+                const end = line === lineMap.length - 1 ? f.path.length : lineMap[line + 1];
+
+                for (let offset = 0; offset < end - start; offset++) {
+                    const pos1 = ts1.lineOffsetToPosition(line + 1, offset + 1);
+                    const pos2 = ts2.lineOffsetToPosition(line + 1, offset + 1);
+                    assert.isTrue(pos1 === pos2, `lineOffsetToPosition ${line + 1}-${offset + 1}: expected ${pos1} to equal ${pos2}`);
+                }
+
+                const {start: start1, length: length1 } = ts1.lineToTextSpan(line);
+                const {start: start2, length: length2 } = ts2.lineToTextSpan(line);
+                assert.isTrue(start1 === start2, `lineToTextSpan ${line}::start:: expected ${start1} to equal ${start2}`);
+                assert.isTrue(length1 === length2, `lineToTextSpan ${line}::length:: expected ${length1} to equal ${length2}`);
+            }
+
+            for (let pos = 0; pos < f.content.length; pos++) {
+                const { line: line1, offset: offset1 } = ts1.positionToLineOffset(pos);
+                const { line: line2, offset: offset2 } = ts2.positionToLineOffset(pos);
+                assert.isTrue(line1 === line2, `positionToLineOffset ${pos}::line:: expected ${line1} to equal ${line2}`);
+                assert.isTrue(offset1 === offset2, `positionToLineOffset ${pos}::offset:: expected ${offset1} to equal ${offset2}`);
+            }
+        });
+
+        it("should switch to script version cache if necessary", () => {
+            const host = ts.projectSystem.createServerHost([f]);
+            const ts1 = new server.TextStorage(host, server.asNormalizedPath(f.path));
+
+            ts1.getSnapshot();
+            assert.isTrue(!ts1.hasScriptVersionCache(), "should not have script version cache - 1");
+
+            ts1.edit(0, 5, "   ");
+            assert.isTrue(ts1.hasScriptVersionCache(), "have script version cache - 1");
+
+            ts1.useText();
+            assert.isTrue(!ts1.hasScriptVersionCache(), "should not have script version cache - 2");
+
+            ts1.getLineInfo(0);
+            assert.isTrue(ts1.hasScriptVersionCache(), "have script version cache - 2");
+        })
+    });
+}
\ No newline at end of file
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index 9beefe480df..5b4ba7d7223 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -140,7 +140,6 @@ namespace ts.projectSystem {
     export interface TestServerHostCreationParameters {
         useCaseSensitiveFileNames?: boolean;
         executingFilePath?: string;
-        libFile?: FileOrFolder;
         currentDirectory?: string;
     }
 
@@ -1137,6 +1136,69 @@ namespace ts.projectSystem {
             checkNumberOfProjects(projectService, {});
         });
 
+        it("reload regular file after closing", () => {
+            const f1 = {
+                path: "/a/b/app.ts",
+                content: "x."
+            };
+            const f2 = {
+                path: "/a/b/lib.ts",
+                content: "let x: number;"
+            };
+
+            const host = createServerHost([f1, f2, libFile]);
+            const service = createProjectService(host);
+            service.openExternalProject({ projectFileName: "/a/b/project", rootFiles: toExternalFiles([f1.path, f2.path]), options: {} })
+
+            service.openClientFile(f1.path);
+            service.openClientFile(f2.path, "let x: string");
+
+            service.checkNumberOfProjects({ externalProjects: 1 });
+            checkProjectActualFiles(service.externalProjects[0], [f1.path, f2.path, libFile.path]);
+
+            const completions1 = service.externalProjects[0].getLanguageService().getCompletionsAtPosition(f1.path, 2);
+            // should contain completions for string
+            assert.isTrue(completions1.entries.some(e => e.name === "charAt"), "should contain 'charAt'");
+            assert.isFalse(completions1.entries.some(e => e.name === "toExponential"), "should not contain 'toExponential'");
+
+            service.closeClientFile(f2.path);
+            const completions2 = service.externalProjects[0].getLanguageService().getCompletionsAtPosition(f1.path, 2);
+            // should contain completions for string
+            assert.isFalse(completions2.entries.some(e => e.name === "charAt"), "should not contain 'charAt'");
+            assert.isTrue(completions2.entries.some(e => e.name === "toExponential"), "should contain 'toExponential'");
+        });
+
+        it("clear mixed content file after closing", () => {
+            const f1 = {
+                path: "/a/b/app.ts",
+                content: " "
+            };
+            const f2 = {
+                path: "/a/b/lib.html",
+                content: "<html/>"
+            };
+
+            const host = createServerHost([f1, f2, libFile]);
+            const service = createProjectService(host);
+            service.openExternalProject({ projectFileName: "/a/b/project", rootFiles: [{ fileName: f1.path }, { fileName: f2.path, hasMixedContent: true }], options: {} })
+
+            service.openClientFile(f1.path);
+            service.openClientFile(f2.path, "let somelongname: string");
+
+            service.checkNumberOfProjects({ externalProjects: 1 });
+            checkProjectActualFiles(service.externalProjects[0], [f1.path, f2.path, libFile.path]);
+
+            const completions1 = service.externalProjects[0].getLanguageService().getCompletionsAtPosition(f1.path, 0);
+            assert.isTrue(completions1.entries.some(e => e.name === "somelongname"), "should contain 'somelongname'");
+
+            service.closeClientFile(f2.path);
+            const completions2 = service.externalProjects[0].getLanguageService().getCompletionsAtPosition(f1.path, 0);
+            assert.isFalse(completions2.entries.some(e => e.name === "somelongname"), "should not contain 'somelongname'");
+            const sf2 = service.externalProjects[0].getLanguageService().getProgram().getSourceFile(f2.path);
+            assert.equal(sf2.text, "");
+        });
+
+
         it("external project with included config file opened after configured project", () => {
             const file1 = {
                 path: "/a/b/f1.ts",
@@ -1521,6 +1583,67 @@ namespace ts.projectSystem {
             checkProjectActualFiles(projectService.inferredProjects[1], [file2.path]);
         });
 
+        it("tsconfig script block support", () => {
+            const file1 = {
+                path: "/a/b/f1.ts",
+                content: ` `
+            };
+            const file2 = {
+                path: "/a/b/f2.html",
+                content: `var hello = "hello";`
+            };
+            const config = {
+                path: "/a/b/tsconfig.json",
+                content: JSON.stringify({ compilerOptions: { allowJs: true } })
+            };
+            const host = createServerHost([file1, file2, config]);
+            const session = createSession(host);
+            openFilesForSession([file1], session);
+            const projectService = session.getProjectService();
+
+            // HTML file will not be included in any projects yet
+            checkNumberOfProjects(projectService, { configuredProjects: 1 });
+            checkProjectActualFiles(projectService.configuredProjects[0], [file1.path]);
+
+            // Specify .html extension as mixed content
+            const extraFileExtensions = [{ extension: ".html", scriptKind: ScriptKind.JS, isMixedContent: true }];
+            const configureHostRequest = makeSessionRequest<protocol.ConfigureRequestArguments>(CommandNames.Configure, { extraFileExtensions });
+            session.executeCommand(configureHostRequest).response;
+
+             // HTML file still not included in the project as it is closed
+            checkNumberOfProjects(projectService, { configuredProjects: 1 });
+            checkProjectActualFiles(projectService.configuredProjects[0], [file1.path]);
+
+            // Open HTML file
+            projectService.applyChangesInOpenFiles(
+                /*openFiles*/[{ fileName: file2.path, hasMixedContent: true, scriptKind: ScriptKind.JS, content: `var hello = "hello";` }],
+                /*changedFiles*/undefined,
+                /*closedFiles*/undefined);
+
+            // Now HTML file is included in the project
+            checkNumberOfProjects(projectService, { configuredProjects: 1 });
+            checkProjectActualFiles(projectService.configuredProjects[0], [file1.path, file2.path]);
+
+            // Check identifiers defined in HTML content are available in .ts file
+            const project = projectService.configuredProjects[0];
+            let completions = project.getLanguageService().getCompletionsAtPosition(file1.path, 1);
+            assert(completions && completions.entries[0].name === "hello", `expected entry hello to be in completion list`);
+
+            // Close HTML file
+            projectService.applyChangesInOpenFiles(
+                /*openFiles*/undefined,
+                /*changedFiles*/undefined,
+                /*closedFiles*/[file2.path]);
+
+            // HTML file is still included in project
+            checkNumberOfProjects(projectService, { configuredProjects: 1 });
+            checkProjectActualFiles(projectService.configuredProjects[0], [file1.path, file2.path]);
+
+            // Check identifiers defined in HTML content are not available in .ts file
+            completions = project.getLanguageService().getCompletionsAtPosition(file1.path, 5);
+            assert(completions && completions.entries[0].name !== "hello", `unexpected hello entry in completion list`);
+        });
+
         it("project structure update is deferred if files are not added\removed", () => {
             const file1 = {
                 path: "/a/b/f1.ts",
@@ -1566,7 +1689,7 @@ namespace ts.projectSystem {
             const project = projectService.externalProjects[0];
 
             const scriptInfo = project.getScriptInfo(file1.path);
-            const snap = scriptInfo.snap();
+            const snap = scriptInfo.getSnapshot();
             const actualText = snap.getText(0, snap.getLength());
             assert.equal(actualText, "", `expected content to be empty string, got "${actualText}"`);
 
@@ -1579,7 +1702,7 @@ namespace ts.projectSystem {
             projectService.closeClientFile(file1.path);
 
             const scriptInfo2 = project.getScriptInfo(file1.path);
-            const snap2 = scriptInfo2.snap();
+            const snap2 = scriptInfo2.getSnapshot();
             const actualText2 = snap2.getText(0, snap.getLength());
             assert.equal(actualText2, "", `expected content to be empty string, got "${actualText2}"`);
         });
@@ -2277,13 +2400,13 @@ namespace ts.projectSystem {
             p.updateGraph();
 
             const scriptInfo = p.getScriptInfo(f.path);
-            checkSnapLength(scriptInfo.snap(), f.content.length);
+            checkSnapLength(scriptInfo.getSnapshot(), f.content.length);
 
             // open project and replace its content with empty string
             projectService.openClientFile(f.path, "");
-            checkSnapLength(scriptInfo.snap(), 0);
+            checkSnapLength(scriptInfo.getSnapshot(), 0);
         });
-        function checkSnapLength(snap: server.LineIndexSnapshot, expectedLength: number) {
+        function checkSnapLength(snap: IScriptSnapshot, expectedLength: number) {
             assert.equal(snap.getLength(), expectedLength, "Incorrect snapshot size");
         }
     });
@@ -2436,7 +2559,6 @@ namespace ts.projectSystem {
             const cwd = {
                 path: "/a/c"
             };
-            debugger;
             const host = createServerHost([f1, config, node, cwd], { currentDirectory: cwd.path });
             const projectService = createProjectService(host);
             projectService.openClientFile(f1.path);
@@ -2707,7 +2829,7 @@ namespace ts.projectSystem {
 
             // verify content
             const projectServiice = session.getProjectService();
-            const snap1 = projectServiice.getScriptInfo(f1.path).snap();
+            const snap1 = projectServiice.getScriptInfo(f1.path).getSnapshot();
             assert.equal(snap1.getText(0, snap1.getLength()), tmp.content, "content should be equal to the content of temp file");
 
             // reload from original file file
@@ -2719,7 +2841,7 @@ namespace ts.projectSystem {
             });
 
             // verify content
-            const snap2 = projectServiice.getScriptInfo(f1.path).snap();
+            const snap2 = projectServiice.getScriptInfo(f1.path).getSnapshot();
             assert.equal(snap2.getText(0, snap2.getLength()), f1.content, "content should be equal to the content of original file");
 
         });
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index a358b0dab00..f558288f655 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -107,6 +107,7 @@ namespace ts.server {
     export interface HostConfiguration {
         formatCodeOptions: FormatCodeSettings;
         hostInfo: string;
+        extraFileExtensions?: FileExtensionInfo[];
     }
 
     interface ConfigFileConversionResult {
@@ -131,13 +132,16 @@ namespace ts.server {
     interface FilePropertyReader<T> {
         getFileName(f: T): string;
         getScriptKind(f: T): ScriptKind;
-        hasMixedContent(f: T): boolean;
+        hasMixedContent(f: T, extraFileExtensions: FileExtensionInfo[]): boolean;
     }
 
     const fileNamePropertyReader: FilePropertyReader<string> = {
         getFileName: x => x,
         getScriptKind: _ => undefined,
-        hasMixedContent: _ => false
+        hasMixedContent: (fileName, extraFileExtensions) => {
+            const mixedContentExtensions = ts.map(ts.filter(extraFileExtensions, item => item.isMixedContent), item => item.extension);
+            return forEach(mixedContentExtensions, extension => fileExtensionIs(fileName, extension))
+        }
     };
 
     const externalFilePropertyReader: FilePropertyReader<protocol.ExternalFile> = {
@@ -282,7 +286,8 @@ namespace ts.server {
 
             this.hostConfiguration = {
                 formatCodeOptions: getDefaultFormatCodeSettings(this.host),
-                hostInfo: "Unknown host"
+                hostInfo: "Unknown host",
+                extraFileExtensions: []
             };
 
             this.documentRegistry = createDocumentRegistry(host.useCaseSensitiveFileNames, host.getCurrentDirectory());
@@ -424,7 +429,7 @@ namespace ts.server {
                 this.handleDeletedFile(info);
             }
             else {
-                if (info && (!info.isOpen)) {
+                if (info && (!info.isScriptOpen())) {
                     // file has been changed which might affect the set of referenced files in projects that include
                     // this file and set of inferred projects
                     info.reloadFromFile();
@@ -440,7 +445,7 @@ namespace ts.server {
 
             // TODO: handle isOpen = true case
 
-            if (!info.isOpen) {
+            if (!info.isScriptOpen()) {
                 this.filenameToScriptInfo.remove(info.path);
                 this.lastDeletedFile = info;
 
@@ -486,7 +491,7 @@ namespace ts.server {
             // If a change was made inside "folder/file", node will trigger the callback twice:
             // one with the fileName being "folder/file", and the other one with "folder".
             // We don't respond to the second one.
-            if (fileName && !ts.isSupportedSourceFileName(fileName, project.getCompilerOptions())) {
+            if (fileName && !ts.isSupportedSourceFileName(fileName, project.getCompilerOptions(), this.hostConfiguration.extraFileExtensions)) {
                 return;
             }
 
@@ -634,15 +639,17 @@ namespace ts.server {
             // Closing file should trigger re-reading the file content from disk. This is
             // because the user may chose to discard the buffer content before saving
             // to the disk, and the server's version of the file can be out of sync.
-            info.reloadFromFile();
+            info.close();
 
             removeItemFromSet(this.openFiles, info);
-            info.isOpen = false;
 
             // collect all projects that should be removed
             let projectsToRemove: Project[];
             for (const p of info.containingProjects) {
                 if (p.projectKind === ProjectKind.Configured) {
+                    if (info.hasMixedContent) {
+                        info.registerFileUpdate();
+                    }
                     // last open file in configured project - close it
                     if ((<ConfiguredProject>p).deleteOpenRef() === 0) {
                         (projectsToRemove || (projectsToRemove = [])).push(p);
@@ -811,7 +818,9 @@ namespace ts.server {
                 this.host,
                 getDirectoryPath(configFilename),
                 /*existingOptions*/ {},
-                configFilename);
+                configFilename,
+                /*resolutionStack*/ [],
+                this.hostConfiguration.extraFileExtensions);
 
             if (parsedCommandLine.errors.length) {
                 errors = concatenate(errors, parsedCommandLine.errors);
@@ -915,7 +924,7 @@ namespace ts.server {
             for (const f of files) {
                 const rootFilename = propertyReader.getFileName(f);
                 const scriptKind = propertyReader.getScriptKind(f);
-                const hasMixedContent = propertyReader.hasMixedContent(f);
+                const hasMixedContent = propertyReader.hasMixedContent(f, this.hostConfiguration.extraFileExtensions);
                 if (this.host.fileExists(rootFilename)) {
                     const info = this.getOrCreateScriptInfoForNormalizedPath(toNormalizedPath(rootFilename), /*openedByClient*/ clientFileName == rootFilename, /*fileContent*/ undefined, scriptKind, hasMixedContent);
                     project.addRoot(info);
@@ -961,7 +970,7 @@ namespace ts.server {
                     rootFilesChanged = true;
                     if (!scriptInfo) {
                         const scriptKind = propertyReader.getScriptKind(f);
-                        const hasMixedContent = propertyReader.hasMixedContent(f);
+                        const hasMixedContent = propertyReader.hasMixedContent(f, this.hostConfiguration.extraFileExtensions);
                         scriptInfo = this.getOrCreateScriptInfoForNormalizedPath(normalizedPath, /*openedByClient*/ false, /*fileContent*/ undefined, scriptKind, hasMixedContent);
                     }
                 }
@@ -989,7 +998,7 @@ namespace ts.server {
                 }
                 if (toAdd) {
                     for (const f of toAdd) {
-                        if (f.isOpen && isRootFileInInferredProject(f)) {
+                        if (f.isScriptOpen() && isRootFileInInferredProject(f)) {
                             // if file is already root in some inferred project
                             // - remove the file from that project and delete the project if necessary
                             const inferredProject = f.containingProjects[0];
@@ -1089,32 +1098,34 @@ namespace ts.server {
         getOrCreateScriptInfoForNormalizedPath(fileName: NormalizedPath, openedByClient: boolean, fileContent?: string, scriptKind?: ScriptKind, hasMixedContent?: boolean) {
             let info = this.getScriptInfoForNormalizedPath(fileName);
             if (!info) {
-                let content: string;
-                if (this.host.fileExists(fileName)) {
-                    // by default pick whatever content was supplied as the argument
-                    // if argument was not given - then for mixed content files assume that its content is empty string
-                    content = fileContent || (hasMixedContent ? "" : this.host.readFile(fileName));
-                }
-                if (!content) {
-                    if (openedByClient) {
-                        content = "";
-                    }
-                }
-                if (content !== undefined) {
-                    info = new ScriptInfo(this.host, fileName, content, scriptKind, openedByClient, hasMixedContent);
-                    // do not watch files with mixed content - server doesn't know how to interpret it
+                if (openedByClient || this.host.fileExists(fileName)) {
+                    info = new ScriptInfo(this.host, fileName, scriptKind, hasMixedContent);
+
                     this.filenameToScriptInfo.set(info.path, info);
-                    if (!info.isOpen && !hasMixedContent) {
-                        info.setWatcher(this.host.watchFile(fileName, _ => this.onSourceFileChanged(fileName)));
+
+                    if (openedByClient) {
+                        if (fileContent === undefined) {
+                            // if file is opened by client and its content is not specified - use file text
+                            fileContent = this.host.readFile(fileName) || "";
+                        }
+                    }
+                    else {
+                        // do not watch files with mixed content - server doesn't know how to interpret it
+                        if (!hasMixedContent) {
+                            info.setWatcher(this.host.watchFile(fileName, _ => this.onSourceFileChanged(fileName)));
+                        }
                     }
                 }
             }
             if (info) {
-                if (fileContent !== undefined) {
-                    info.reload(fileContent);
+                if (openedByClient && !info.isScriptOpen()) {
+                    info.open(fileContent);
+                    if (hasMixedContent) {
+                        info.registerFileUpdate();
+                    }
                 }
-                if (openedByClient) {
-                    info.isOpen = true;
+                else if (fileContent !== undefined) {
+                    info.reload(fileContent);
                 }
             }
             return info;
@@ -1146,6 +1157,10 @@ namespace ts.server {
                     mergeMapLikes(this.hostConfiguration.formatCodeOptions, convertFormatOptions(args.formatOptions));
                     this.logger.info("Format host information updated");
                 }
+                if (args.extraFileExtensions) {
+                    this.hostConfiguration.extraFileExtensions = args.extraFileExtensions;
+                    this.logger.info("Host file extension mappings updated");
+                }
             }
         }
 
@@ -1230,7 +1245,6 @@ namespace ts.server {
             const info = this.getScriptInfoForNormalizedPath(toNormalizedPath(uncheckedFileName));
             if (info) {
                 this.closeOpenFile(info);
-                info.isOpen = false;
             }
             this.printProjects();
         }
@@ -1255,7 +1269,7 @@ namespace ts.server {
             if (openFiles) {
                 for (const file of openFiles) {
                     const scriptInfo = this.getScriptInfo(file.fileName);
-                    Debug.assert(!scriptInfo || !scriptInfo.isOpen);
+                    Debug.assert(!scriptInfo || !scriptInfo.isScriptOpen());
                     const normalizedPath = scriptInfo ? scriptInfo.fileName : toNormalizedPath(file.fileName);
                     this.openClientFileWithNormalizedPath(normalizedPath, file.content, tryConvertScriptKindName(file.scriptKind), file.hasMixedContent);
                 }
diff --git a/src/server/lsHost.ts b/src/server/lsHost.ts
index ffeeb57a0e8..1989b34c477 100644
--- a/src/server/lsHost.ts
+++ b/src/server/lsHost.ts
@@ -170,7 +170,7 @@ namespace ts.server {
         getScriptSnapshot(filename: string): ts.IScriptSnapshot {
             const scriptInfo = this.project.getScriptInfoLSHost(filename);
             if (scriptInfo) {
-                return scriptInfo.snap();
+                return scriptInfo.getSnapshot();
             }
         }
 
diff --git a/src/server/project.ts b/src/server/project.ts
index 14889dccd8a..4ff09003edc 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -1,4 +1,4 @@
-/// <reference path="..\services\services.ts" />
+/// <reference path="..\services\services.ts" />
 /// <reference path="utilities.ts"/>
 /// <reference path="scriptInfo.ts"/>
 /// <reference path="lsHost.ts"/>
@@ -187,6 +187,10 @@ namespace ts.server {
         public languageServiceEnabled = true;
 
         builder: Builder;
+        /**
+         * Set of files names that were updated since the last call to getChangesSinceVersion.
+         */
+        private updatedFileNames: Map<string>;
         /**
          * Set of files that was returned from the last call to getChangesSinceVersion.
          */
@@ -448,7 +452,7 @@ namespace ts.server {
 
         containsFile(filename: NormalizedPath, requireOpen?: boolean) {
             const info = this.projectService.getScriptInfoForNormalizedPath(filename);
-            if (info && (info.isOpen || !requireOpen)) {
+            if (info && (info.isScriptOpen() || !requireOpen)) {
                 return this.containsScriptInfo(info);
             }
         }
@@ -480,6 +484,10 @@ namespace ts.server {
             this.markAsDirty();
         }
 
+        registerFileUpdate(fileName: string) {
+            (this.updatedFileNames || (this.updatedFileNames = createMap<string>())).set(fileName, fileName);
+        }
+
         markAsDirty() {
             this.projectStateVersion++;
         }
@@ -667,10 +675,12 @@ namespace ts.server {
                 isInferred: this.projectKind === ProjectKind.Inferred,
                 options: this.getCompilerOptions()
             };
+            const updatedFileNames = this.updatedFileNames;
+            this.updatedFileNames = undefined;
             // check if requested version is the same that we have reported last time
             if (this.lastReportedFileNames && lastKnownVersion === this.lastReportedVersion) {
-                // if current structure version is the same - return info witout any changes
-                if (this.projectStructureVersion == this.lastReportedVersion) {
+                // if current structure version is the same - return info without any changes
+                if (this.projectStructureVersion == this.lastReportedVersion && !updatedFileNames) {
                     return { info, projectErrors: this.projectErrors };
                 }
                 // compute and return the difference
@@ -679,6 +689,8 @@ namespace ts.server {
 
                 const added: string[] = [];
                 const removed: string[] = [];
+                const updated: string[] = keysOfMap(updatedFileNames);
+
                 forEachKeyInMap(currentFiles, id => {
                     if (!lastReportedFileNames.has(id)) {
                         added.push(id);
@@ -691,7 +703,7 @@ namespace ts.server {
                 });
                 this.lastReportedFileNames = currentFiles;
                 this.lastReportedVersion = this.projectStructureVersion;
-                return { info, changes: { added, removed }, projectErrors: this.projectErrors };
+                return { info, changes: { added, removed, updated }, projectErrors: this.projectErrors };
             }
             else {
                 // unknown version - return everything
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index 1e29b029104..27faf417282 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -1,4 +1,4 @@
-/**
+/**
   * Declaration module describing the TypeScript Server protocol
   */
 namespace ts.server.protocol {
@@ -918,6 +918,10 @@ namespace ts.server.protocol {
          * List of removed files
          */
         removed: string[];
+        /**
+         * List of updated files
+         */
+        updated: string[];
     }
 
     /**
@@ -990,6 +994,11 @@ namespace ts.server.protocol {
          * The format options to use during formatting and other code editing features.
          */
         formatOptions?: FormatCodeSettings;
+
+        /**
+         * The host's additional supported file extensions
+         */
+        extraFileExtensions?: FileExtensionInfo[];
     }
 
     /**
diff --git a/src/server/scriptInfo.ts b/src/server/scriptInfo.ts
index fa9eac0e8e3..340510f6580 100644
--- a/src/server/scriptInfo.ts
+++ b/src/server/scriptInfo.ts
@@ -2,6 +2,161 @@
 
 namespace ts.server {
 
+    /* @internal */
+    export class TextStorage {
+        private svc: ScriptVersionCache | undefined;
+        private svcVersion = 0;
+
+        private text: string;
+        private lineMap: number[];
+        private textVersion = 0;
+
+        constructor(private readonly host: ServerHost, private readonly fileName: NormalizedPath) {
+        }
+
+        public getVersion() {
+            return this.svc
+                ? `SVC-${this.svcVersion}-${this.svc.getSnapshot().version}`
+                : `Text-${this.textVersion}`;
+        }
+
+        public hasScriptVersionCache() {
+            return this.svc !== undefined;
+        }
+
+        public useScriptVersionCache(newText?: string) {
+            this.switchToScriptVersionCache(newText);
+        }
+
+        public useText(newText?: string) {
+            this.svc = undefined;
+            this.setText(newText);
+        }
+
+        public edit(start: number, end: number, newText: string) {
+            this.switchToScriptVersionCache().edit(start, end - start, newText);
+        }
+
+        public reload(text: string) {
+            if (this.svc) {
+                this.svc.reload(text);
+            }
+            else {
+                this.setText(text);
+            }
+        }
+
+        public reloadFromFile(tempFileName?: string) {
+            if (this.svc || (tempFileName !== this.fileName)) {
+                this.reload(this.getFileText(tempFileName))
+            }
+            else {
+                this.setText(undefined);
+            }
+        }
+
+        public getSnapshot(): IScriptSnapshot {
+            return this.svc
+                ? this.svc.getSnapshot()
+                : ScriptSnapshot.fromString(this.getOrLoadText());
+        }
+
+        public getLineInfo(line: number) {
+            return this.switchToScriptVersionCache().getSnapshot().index.lineNumberToInfo(line);
+        }
+        /**
+         *  @param line 0 based index
+         */
+        lineToTextSpan(line: number) {
+            if (!this.svc) {
+                const lineMap = this.getLineMap();
+                const start = lineMap[line]; // -1 since line is 1-based
+                const end = line + 1 < lineMap.length ? lineMap[line + 1] : this.text.length;
+                return ts.createTextSpanFromBounds(start, end);
+            }
+            const index = this.svc.getSnapshot().index;
+            const lineInfo = index.lineNumberToInfo(line + 1);
+            let len: number;
+            if (lineInfo.leaf) {
+                len = lineInfo.leaf.text.length;
+            }
+            else {
+                const nextLineInfo = index.lineNumberToInfo(line + 2);
+                len = nextLineInfo.offset - lineInfo.offset;
+            }
+            return ts.createTextSpan(lineInfo.offset, len);
+        }
+
+        /**
+         * @param line 1 based index
+         * @param offset 1 based index
+         */
+        lineOffsetToPosition(line: number, offset: number): number {
+            if (!this.svc) {
+                return computePositionOfLineAndCharacter(this.getLineMap(), line - 1, offset - 1);
+            }
+            const index = this.svc.getSnapshot().index;
+
+            const lineInfo = index.lineNumberToInfo(line);
+            // TODO: assert this offset is actually on the line
+            return (lineInfo.offset + offset - 1);
+        }
+
+        /**
+         * @param line 1-based index
+         * @param offset 1-based index
+         */
+        positionToLineOffset(position: number): ILineInfo {
+            if (!this.svc) {
+                const { line, character } = computeLineAndCharacterOfPosition(this.getLineMap(), position);
+                return { line: line + 1, offset: character + 1 };
+            }
+            const index = this.svc.getSnapshot().index;
+            const lineOffset = index.charOffsetToLineNumberAndPos(position);
+            return { line: lineOffset.line, offset: lineOffset.offset + 1 };
+        }
+
+        private getFileText(tempFileName?: string) {
+            return this.host.readFile(tempFileName || this.fileName) || "";
+        }
+
+        private ensureNoScriptVersionCache() {
+            Debug.assert(!this.svc, "ScriptVersionCache should not be set");
+        }
+
+        private switchToScriptVersionCache(newText?: string): ScriptVersionCache {
+            if (!this.svc) {
+                this.svc = ScriptVersionCache.fromString(this.host, newText !== undefined ? newText : this.getOrLoadText());
+                this.svcVersion++;
+                this.text = undefined;
+            }
+            return this.svc;
+        }
+
+        private getOrLoadText() {
+            this.ensureNoScriptVersionCache();
+            if (this.text === undefined) {
+                this.setText(this.getFileText());
+            }
+            return this.text;
+        }
+
+        private getLineMap() {
+            this.ensureNoScriptVersionCache();
+            return this.lineMap || (this.lineMap = computeLineStarts(this.getOrLoadText()));
+        }
+
+        private setText(newText: string) {
+            this.ensureNoScriptVersionCache();
+            if (newText === undefined || this.text !== newText) {
+                this.text = newText;
+                this.lineMap = undefined;
+                this.textVersion++;
+            }
+        }
+    }
+
+
     export class ScriptInfo {
         /**
          * All projects that include this file
@@ -11,24 +166,46 @@ namespace ts.server {
         readonly path: Path;
 
         private fileWatcher: FileWatcher;
-        private svc: ScriptVersionCache;
+        private textStorage: TextStorage;
+
+        private isOpen: boolean;
 
-        // TODO: allow to update hasMixedContent from the outside
         constructor(
             private readonly host: ServerHost,
             readonly fileName: NormalizedPath,
-            content: string,
             readonly scriptKind: ScriptKind,
-            public isOpen = false,
             public hasMixedContent = false) {
 
             this.path = toPath(fileName, host.getCurrentDirectory(), createGetCanonicalFileName(host.useCaseSensitiveFileNames));
-            this.svc = ScriptVersionCache.fromString(host, content);
+            this.textStorage = new TextStorage(host, fileName);
+            if (hasMixedContent) {
+                this.textStorage.reload("");
+            }
             this.scriptKind = scriptKind
                 ? scriptKind
                 : getScriptKindFromFileName(fileName);
         }
 
+        public isScriptOpen() {
+            return this.isOpen;
+        }
+
+        public open(newText: string) {
+            this.isOpen = true;
+            this.textStorage.useScriptVersionCache(newText);
+            this.markContainingProjectsAsDirty();
+        }
+
+        public close() {
+            this.isOpen = false;
+            this.textStorage.useText(this.hasMixedContent ? "" : undefined);
+            this.markContainingProjectsAsDirty();
+        }
+
+        public getSnapshot() {
+            return this.textStorage.getSnapshot();
+        }
+
         getFormatCodeSettings() {
             return this.formatCodeSettings;
         }
@@ -90,6 +267,12 @@ namespace ts.server {
             return this.containingProjects[0];
         }
 
+        registerFileUpdate(): void {
+            for (const p of this.containingProjects) {
+                p.registerFileUpdate(this.path);
+            }
+        }
+
         setFormatOptions(formatSettings: FormatCodeSettings): void {
             if (formatSettings) {
                 if (!this.formatCodeSettings) {
@@ -112,16 +295,16 @@ namespace ts.server {
         }
 
         getLatestVersion() {
-            return this.svc.latestVersion().toString();
+            return this.textStorage.getVersion();
         }
 
         reload(script: string) {
-            this.svc.reload(script);
+            this.textStorage.reload(script);
             this.markContainingProjectsAsDirty();
         }
 
         saveTo(fileName: string) {
-            const snap = this.snap();
+            const snap = this.textStorage.getSnapshot();
             this.host.writeFile(fileName, snap.getText(0, snap.getLength()));
         }
 
@@ -130,22 +313,17 @@ namespace ts.server {
                 this.reload("");
             }
             else {
-                this.svc.reloadFromFile(tempFileName || this.fileName);
+                this.textStorage.reloadFromFile(tempFileName);
                 this.markContainingProjectsAsDirty();
             }
         }
 
-        snap() {
-            return this.svc.getSnapshot();
-        }
-
         getLineInfo(line: number) {
-            const snap = this.snap();
-            return snap.index.lineNumberToInfo(line);
+            return this.textStorage.getLineInfo(line);
         }
 
         editContent(start: number, end: number, newText: string): void {
-            this.svc.edit(start, end - start, newText);
+            this.textStorage.edit(start, end, newText);
             this.markContainingProjectsAsDirty();
         }
 
@@ -159,17 +337,7 @@ namespace ts.server {
          *  @param line 1 based index
          */
         lineToTextSpan(line: number) {
-            const index = this.snap().index;
-            const lineInfo = index.lineNumberToInfo(line + 1);
-            let len: number;
-            if (lineInfo.leaf) {
-                len = lineInfo.leaf.text.length;
-            }
-            else {
-                const nextLineInfo = index.lineNumberToInfo(line + 2);
-                len = nextLineInfo.offset - lineInfo.offset;
-            }
-            return ts.createTextSpan(lineInfo.offset, len);
+            return this.textStorage.lineToTextSpan(line);
         }
 
         /**
@@ -177,11 +345,7 @@ namespace ts.server {
          * @param offset 1 based index
          */
         lineOffsetToPosition(line: number, offset: number): number {
-            const index = this.snap().index;
-
-            const lineInfo = index.lineNumberToInfo(line);
-            // TODO: assert this offset is actually on the line
-            return (lineInfo.offset + offset - 1);
+            return this.textStorage.lineOffsetToPosition(line, offset);
         }
 
         /**
@@ -189,9 +353,7 @@ namespace ts.server {
          * @param offset 1-based index
          */
         positionToLineOffset(position: number): ILineInfo {
-            const index = this.snap().index;
-            const lineOffset = index.charOffsetToLineNumberAndPos(position);
-            return { line: lineOffset.line, offset: lineOffset.offset + 1 };
+            return this.textStorage.positionToLineOffset(position);
         }
     }
 }
\ No newline at end of file
diff --git a/src/server/scriptVersionCache.ts b/src/server/scriptVersionCache.ts
index f094a183610..aaf04d39a1f 100644
--- a/src/server/scriptVersionCache.ts
+++ b/src/server/scriptVersionCache.ts
@@ -438,8 +438,9 @@ namespace ts.server {
             }
         }
         getChangeRange(oldSnapshot: ts.IScriptSnapshot): ts.TextChangeRange {
-            const oldSnap = <LineIndexSnapshot>oldSnapshot;
-            return this.getTextChangeRangeSinceVersion(oldSnap.version);
+            if (oldSnapshot instanceof LineIndexSnapshot) {
+                return this.getTextChangeRangeSinceVersion(oldSnapshot.version);
+            }
         }
     }
 
diff --git a/src/server/session.ts b/src/server/session.ts
index 0c200b33b84..203a940e7cf 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -709,7 +709,7 @@ namespace ts.server {
                 const displayString = ts.displayPartsToString(nameInfo.displayParts);
                 const nameSpan = nameInfo.textSpan;
                 const nameColStart = scriptInfo.positionToLineOffset(nameSpan.start).offset;
-                const nameText = scriptInfo.snap().getText(nameSpan.start, ts.textSpanEnd(nameSpan));
+                const nameText = scriptInfo.getSnapshot().getText(nameSpan.start, ts.textSpanEnd(nameSpan));
                 const refs = combineProjectOutput<protocol.ReferencesResponseItem>(
                     projects,
                     (project: Project) => {
@@ -722,7 +722,7 @@ namespace ts.server {
                             const refScriptInfo = project.getScriptInfo(ref.fileName);
                             const start = refScriptInfo.positionToLineOffset(ref.textSpan.start);
                             const refLineSpan = refScriptInfo.lineToTextSpan(start.line - 1);
-                            const lineText = refScriptInfo.snap().getText(refLineSpan.start, ts.textSpanEnd(refLineSpan)).replace(/\r|\n/g, "");
+                            const lineText = refScriptInfo.getSnapshot().getText(refLineSpan.start, ts.textSpanEnd(refLineSpan)).replace(/\r|\n/g, "");
                             return {
                                 file: ref.fileName,
                                 start: start,
@@ -1326,7 +1326,7 @@ namespace ts.server {
                     highPriorityFiles.push(fileNameInProject);
                 else {
                     const info = this.projectService.getScriptInfo(fileNameInProject);
-                    if (!info.isOpen) {
+                    if (!info.isScriptOpen()) {
                         if (fileNameInProject.indexOf(".d.ts") > 0)
                             veryLowPriorityFiles.push(fileNameInProject);
                         else
diff --git a/src/server/utilities.ts b/src/server/utilities.ts
index eb7801e5fd9..4b1149d31d4 100644
--- a/src/server/utilities.ts
+++ b/src/server/utilities.ts
@@ -1,4 +1,4 @@
-/// <reference path="types.d.ts" />
+/// <reference path="types.d.ts" />
 /// <reference path="shared.ts" />
 
 namespace ts.server {
diff --git a/src/services/jsDoc.ts b/src/services/jsDoc.ts
index 3369ac23223..a937294567b 100644
--- a/src/services/jsDoc.ts
+++ b/src/services/jsDoc.ts
@@ -1,4 +1,4 @@
-/* @internal */
+/* @internal */
 namespace ts.JsDoc {
     const jsDocTagNames = [
         "augments",
@@ -166,6 +166,7 @@ namespace ts.JsDoc {
         const lineStart = sourceFile.getLineStarts()[posLineAndChar.line];
 
         const indentationStr = sourceFile.text.substr(lineStart, posLineAndChar.character);
+        const isJavaScriptFile = hasJavaScriptFileExtension(sourceFile.fileName);
 
         let docParams = "";
         for (let i = 0, numParams = parameters.length; i < numParams; i++) {
@@ -173,8 +174,12 @@ namespace ts.JsDoc {
             const paramName = currentName.kind === SyntaxKind.Identifier ?
                 (<Identifier>currentName).text :
                 "param" + i;
-
-            docParams += `${indentationStr} * @param ${paramName}${newLine}`;
+            if (isJavaScriptFile) {
+                docParams += `${indentationStr} * @param {any} ${paramName}${newLine}`;
+            }
+            else {
+                docParams += `${indentationStr} * @param ${paramName}${newLine}`;
+            }
         }
 
         // A doc comment consists of the following
diff --git a/src/services/services.ts b/src/services/services.ts
index 77a5e916067..87ae9fb8561 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1725,7 +1725,7 @@ namespace ts {
             const sourceFile = syntaxTreeCache.getCurrentSourceFile(fileName);
 
             // Check if in a context where we don't want to perform any insertion
-            if (isInString(sourceFile, position) || isInComment(sourceFile, position)) {
+            if (isInString(sourceFile, position)) {
                 return false;
             }
 
diff --git a/src/services/types.ts b/src/services/types.ts
index 6a0e6e886b5..3865fe7fac9 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -91,7 +91,9 @@ namespace ts {
             }
 
             public getText(start: number, end: number): string {
-                return this.text.substring(start, end);
+                return start === 0 && end === this.text.length
+                    ? this.text
+                    : this.text.substring(start, end);
             }
 
             public getLength(): number {
diff --git a/tests/baselines/reference/circularIndexedAccessErrors.errors.txt b/tests/baselines/reference/circularIndexedAccessErrors.errors.txt
new file mode 100644
index 00000000000..e3076e08b7e
--- /dev/null
+++ b/tests/baselines/reference/circularIndexedAccessErrors.errors.txt
@@ -0,0 +1,57 @@
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(3,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(7,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(15,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(19,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(23,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(27,5): error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(28,5): error TS2502: 'y' is referenced directly or indirectly in its own type annotation.
+tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts(29,5): error TS2502: 'z' is referenced directly or indirectly in its own type annotation.
+
+
+==== tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts (8 errors) ====
+    
+    type T1 = {
+        x: T1["x"];  // Error
+        ~~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+    };
+    
+    type T2<K extends "x" | "y"> = {
+        x: T2<K>[K];  // Error
+        ~~~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+        y: number;
+    }
+    
+    declare let x2: T2<"x">;
+    let x2x = x2.x;
+    
+    interface T3<T extends T3<T>> {
+        x: T["x"];  // Error
+        ~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+    }
+    
+    interface T4<T extends T4<T>> {
+        x: T4<T>["x"];  // Error
+        ~~~~~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+    }
+    
+    class C1 {
+        x: C1["x"];  // Error
+        ~~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+    }
+    
+    class C2 {
+        x: this["y"];  // Error
+        ~~~~~~~~~~~~~
+!!! error TS2502: 'x' is referenced directly or indirectly in its own type annotation.
+        y: this["z"];  // Error
+        ~~~~~~~~~~~~~
+!!! error TS2502: 'y' is referenced directly or indirectly in its own type annotation.
+        z: this["x"];  // Error
+        ~~~~~~~~~~~~~
+!!! error TS2502: 'z' is referenced directly or indirectly in its own type annotation.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/circularIndexedAccessErrors.js b/tests/baselines/reference/circularIndexedAccessErrors.js
new file mode 100644
index 00000000000..46784ae8d18
--- /dev/null
+++ b/tests/baselines/reference/circularIndexedAccessErrors.js
@@ -0,0 +1,70 @@
+//// [circularIndexedAccessErrors.ts]
+
+type T1 = {
+    x: T1["x"];  // Error
+};
+
+type T2<K extends "x" | "y"> = {
+    x: T2<K>[K];  // Error
+    y: number;
+}
+
+declare let x2: T2<"x">;
+let x2x = x2.x;
+
+interface T3<T extends T3<T>> {
+    x: T["x"];  // Error
+}
+
+interface T4<T extends T4<T>> {
+    x: T4<T>["x"];  // Error
+}
+
+class C1 {
+    x: C1["x"];  // Error
+}
+
+class C2 {
+    x: this["y"];  // Error
+    y: this["z"];  // Error
+    z: this["x"];  // Error
+}
+
+//// [circularIndexedAccessErrors.js]
+var x2x = x2.x;
+var C1 = (function () {
+    function C1() {
+    }
+    return C1;
+}());
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+}());
+
+
+//// [circularIndexedAccessErrors.d.ts]
+declare type T1 = {
+    x: T1["x"];
+};
+declare type T2<K extends "x" | "y"> = {
+    x: T2<K>[K];
+    y: number;
+};
+declare let x2: T2<"x">;
+declare let x2x: any;
+interface T3<T extends T3<T>> {
+    x: T["x"];
+}
+interface T4<T extends T4<T>> {
+    x: T4<T>["x"];
+}
+declare class C1 {
+    x: C1["x"];
+}
+declare class C2 {
+    x: this["y"];
+    y: this["z"];
+    z: this["x"];
+}
diff --git a/tests/baselines/reference/keyofAndIndexedAccess.js b/tests/baselines/reference/keyofAndIndexedAccess.js
index a9a5ee29993..4b93897fdc4 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess.js
+++ b/tests/baselines/reference/keyofAndIndexedAccess.js
@@ -249,6 +249,53 @@ function f74(func: <T, U, K extends keyof (T | U)>(x: T, y: U, k: K) => (T | U)[
     let b = func({ a: 1, b: "hello" }, { a: 2, b: true }, 'b');  // string | boolean
 }
 
+function f80<T extends { a: { x: any } }>(obj: T) {
+    let a1 = obj.a;  // { x: any }
+    let a2 = obj['a'];  // { x: any }
+    let a3 = obj['a'] as T['a'];  // T["a"]
+    let x1 = obj.a.x;  // any
+    let x2 = obj['a']['x'];  // any
+    let x3 = obj['a']['x'] as T['a']['x'];  // T["a"]["x"]
+}
+
+function f81<T extends { a: { x: any } }>(obj: T) {
+    return obj['a']['x'] as T['a']['x'];
+}
+
+function f82() {
+    let x1 = f81({ a: { x: "hello" } });  // string
+    let x2 = f81({ a: { x: 42 } });  // number
+}
+
+function f83<T extends { [x: string]: { x: any } }, K extends keyof T>(obj: T, key: K) {
+    return obj[key]['x'] as T[K]['x'];
+}
+
+function f84() {
+    let x1 = f83({ foo: { x: "hello" } }, "foo");  // string
+    let x2 = f83({ bar: { x: 42 } }, "bar");  // number
+}
+
+class C1 {
+    x: number;
+    get<K extends keyof this>(key: K) {
+        return this[key];
+    }
+    set<K extends keyof this>(key: K, value: this[K]) {
+        this[key] = value;
+    }
+    foo() {
+        let x1 = this.x;  // number
+        let x2 = this["x"];  // number
+        let x3 = this.get("x");  // this["x"]
+        let x4 = getProperty(this, "x"); // this["x"]
+        this.x = 42;
+        this["x"] = 42;
+        this.set("x", 42);
+        setProperty(this, "x", 42);
+    }
+}
+
 // Repros from #12011
 
 class Base {
@@ -364,7 +411,25 @@ interface R {
 function f<K extends keyof R>(p: K) {
     let a: any;
     a[p].add;  // any
-}
+}
+
+// Repro from #12651
+
+type MethodDescriptor = {
+	name: string;
+	args: any[];
+	returnValue: any;
+}
+
+declare function dispatchMethod<M extends MethodDescriptor>(name: M['name'], args: M['args']): M['returnValue'];
+
+type SomeMethodDescriptor = {
+	name: "someMethod";
+	args: [string, number];
+	returnValue: string[];
+}
+
+let result = dispatchMethod<SomeMethodDescriptor>("someMethod", ["hello", 35]);
 
 //// [keyofAndIndexedAccess.js]
 var __extends = (this && this.__extends) || function (d, b) {
@@ -536,6 +601,49 @@ function f74(func) {
     var a = func({ a: 1, b: "hello" }, { a: 2, b: true }, 'a'); // number
     var b = func({ a: 1, b: "hello" }, { a: 2, b: true }, 'b'); // string | boolean
 }
+function f80(obj) {
+    var a1 = obj.a; // { x: any }
+    var a2 = obj['a']; // { x: any }
+    var a3 = obj['a']; // T["a"]
+    var x1 = obj.a.x; // any
+    var x2 = obj['a']['x']; // any
+    var x3 = obj['a']['x']; // T["a"]["x"]
+}
+function f81(obj) {
+    return obj['a']['x'];
+}
+function f82() {
+    var x1 = f81({ a: { x: "hello" } }); // string
+    var x2 = f81({ a: { x: 42 } }); // number
+}
+function f83(obj, key) {
+    return obj[key]['x'];
+}
+function f84() {
+    var x1 = f83({ foo: { x: "hello" } }, "foo"); // string
+    var x2 = f83({ bar: { x: 42 } }, "bar"); // number
+}
+var C1 = (function () {
+    function C1() {
+    }
+    C1.prototype.get = function (key) {
+        return this[key];
+    };
+    C1.prototype.set = function (key, value) {
+        this[key] = value;
+    };
+    C1.prototype.foo = function () {
+        var x1 = this.x; // number
+        var x2 = this["x"]; // number
+        var x3 = this.get("x"); // this["x"]
+        var x4 = getProperty(this, "x"); // this["x"]
+        this.x = 42;
+        this["x"] = 42;
+        this.set("x", 42);
+        setProperty(this, "x", 42);
+    };
+    return C1;
+}());
 // Repros from #12011
 var Base = (function () {
     function Base() {
@@ -604,6 +712,7 @@ function f(p) {
     var a;
     a[p].add; // any
 }
+var result = dispatchMethod("someMethod", ["hello", 35]);
 
 
 //// [keyofAndIndexedAccess.d.ts]
@@ -716,6 +825,29 @@ declare function f71(func: <T, U>(x: T, y: U) => Partial<T & U>): void;
 declare function f72(func: <T, U, K extends keyof T | keyof U>(x: T, y: U, k: K) => (T & U)[K]): void;
 declare function f73(func: <T, U, K extends keyof (T & U)>(x: T, y: U, k: K) => (T & U)[K]): void;
 declare function f74(func: <T, U, K extends keyof (T | U)>(x: T, y: U, k: K) => (T | U)[K]): void;
+declare function f80<T extends {
+    a: {
+        x: any;
+    };
+}>(obj: T): void;
+declare function f81<T extends {
+    a: {
+        x: any;
+    };
+}>(obj: T): T["a"]["x"];
+declare function f82(): void;
+declare function f83<T extends {
+    [x: string]: {
+        x: any;
+    };
+}, K extends keyof T>(obj: T, key: K): T[K]["x"];
+declare function f84(): void;
+declare class C1 {
+    x: number;
+    get<K extends keyof this>(key: K): this[K];
+    set<K extends keyof this>(key: K, value: this[K]): void;
+    foo(): void;
+}
 declare class Base {
     get<K extends keyof this>(prop: K): this[K];
     set<K extends keyof this>(prop: K, value: this[K]): void;
@@ -723,12 +855,12 @@ declare class Base {
 declare class Person extends Base {
     parts: number;
     constructor(parts: number);
-    getParts(): number;
+    getParts(): this["parts"];
 }
 declare class OtherPerson {
     parts: number;
     constructor(parts: number);
-    getParts(): number;
+    getParts(): this["parts"];
 }
 declare function path<T, K1 extends keyof T>(obj: T, key1: K1): T[K1];
 declare function path<T, K1 extends keyof T, K2 extends keyof T[K1]>(obj: T, key1: K1, key2: K2): T[K1][K2];
@@ -776,3 +908,15 @@ interface R {
     p: number;
 }
 declare function f<K extends keyof R>(p: K): void;
+declare type MethodDescriptor = {
+    name: string;
+    args: any[];
+    returnValue: any;
+};
+declare function dispatchMethod<M extends MethodDescriptor>(name: M['name'], args: M['args']): M['returnValue'];
+declare type SomeMethodDescriptor = {
+    name: "someMethod";
+    args: [string, number];
+    returnValue: string[];
+};
+declare let result: string[];
diff --git a/tests/baselines/reference/keyofAndIndexedAccess.symbols b/tests/baselines/reference/keyofAndIndexedAccess.symbols
index 3d90b4ca276..1c69d4959d8 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess.symbols
+++ b/tests/baselines/reference/keyofAndIndexedAccess.symbols
@@ -951,396 +951,628 @@ function f74(func: <T, U, K extends keyof (T | U)>(x: T, y: U, k: K) => (T | U)[
 >b : Symbol(b, Decl(keyofAndIndexedAccess.ts, 247, 46))
 }
 
+function f80<T extends { a: { x: any } }>(obj: T) {
+>f80 : Symbol(f80, Decl(keyofAndIndexedAccess.ts, 248, 1))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 250, 13))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 250, 29))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 250, 13))
+
+    let a1 = obj.a;  // { x: any }
+>a1 : Symbol(a1, Decl(keyofAndIndexedAccess.ts, 251, 7))
+>obj.a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+
+    let a2 = obj['a'];  // { x: any }
+>a2 : Symbol(a2, Decl(keyofAndIndexedAccess.ts, 252, 7))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>'a' : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+
+    let a3 = obj['a'] as T['a'];  // T["a"]
+>a3 : Symbol(a3, Decl(keyofAndIndexedAccess.ts, 253, 7))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>'a' : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 250, 13))
+
+    let x1 = obj.a.x;  // any
+>x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 254, 7))
+>obj.a.x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 250, 29))
+>obj.a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 250, 29))
+
+    let x2 = obj['a']['x'];  // any
+>x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 255, 7))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>'a' : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>'x' : Symbol(x, Decl(keyofAndIndexedAccess.ts, 250, 29))
+
+    let x3 = obj['a']['x'] as T['a']['x'];  // T["a"]["x"]
+>x3 : Symbol(x3, Decl(keyofAndIndexedAccess.ts, 256, 7))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 250, 42))
+>'a' : Symbol(a, Decl(keyofAndIndexedAccess.ts, 250, 24))
+>'x' : Symbol(x, Decl(keyofAndIndexedAccess.ts, 250, 29))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 250, 13))
+}
+
+function f81<T extends { a: { x: any } }>(obj: T) {
+>f81 : Symbol(f81, Decl(keyofAndIndexedAccess.ts, 257, 1))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 259, 13))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 259, 24))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 259, 29))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 259, 42))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 259, 13))
+
+    return obj['a']['x'] as T['a']['x'];
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 259, 42))
+>'a' : Symbol(a, Decl(keyofAndIndexedAccess.ts, 259, 24))
+>'x' : Symbol(x, Decl(keyofAndIndexedAccess.ts, 259, 29))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 259, 13))
+}
+
+function f82() {
+>f82 : Symbol(f82, Decl(keyofAndIndexedAccess.ts, 261, 1))
+
+    let x1 = f81({ a: { x: "hello" } });  // string
+>x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 264, 7))
+>f81 : Symbol(f81, Decl(keyofAndIndexedAccess.ts, 257, 1))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 264, 18))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 264, 23))
+
+    let x2 = f81({ a: { x: 42 } });  // number
+>x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 265, 7))
+>f81 : Symbol(f81, Decl(keyofAndIndexedAccess.ts, 257, 1))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 265, 18))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 265, 23))
+}
+
+function f83<T extends { [x: string]: { x: any } }, K extends keyof T>(obj: T, key: K) {
+>f83 : Symbol(f83, Decl(keyofAndIndexedAccess.ts, 266, 1))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 268, 13))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 268, 26))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 268, 39))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 268, 51))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 268, 13))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 268, 71))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 268, 13))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 268, 78))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 268, 51))
+
+    return obj[key]['x'] as T[K]['x'];
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 268, 71))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 268, 78))
+>'x' : Symbol(x, Decl(keyofAndIndexedAccess.ts, 268, 39))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 268, 13))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 268, 51))
+}
+
+function f84() {
+>f84 : Symbol(f84, Decl(keyofAndIndexedAccess.ts, 270, 1))
+
+    let x1 = f83({ foo: { x: "hello" } }, "foo");  // string
+>x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 273, 7))
+>f83 : Symbol(f83, Decl(keyofAndIndexedAccess.ts, 266, 1))
+>foo : Symbol(foo, Decl(keyofAndIndexedAccess.ts, 273, 18))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 273, 25))
+
+    let x2 = f83({ bar: { x: 42 } }, "bar");  // number
+>x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 274, 7))
+>f83 : Symbol(f83, Decl(keyofAndIndexedAccess.ts, 266, 1))
+>bar : Symbol(bar, Decl(keyofAndIndexedAccess.ts, 274, 18))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 274, 25))
+}
+
+class C1 {
+>C1 : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+
+    x: number;
+>x : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+
+    get<K extends keyof this>(key: K) {
+>get : Symbol(C1.get, Decl(keyofAndIndexedAccess.ts, 278, 14))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 279, 8))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 279, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 279, 8))
+
+        return this[key];
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 279, 30))
+    }
+    set<K extends keyof this>(key: K, value: this[K]) {
+>set : Symbol(C1.set, Decl(keyofAndIndexedAccess.ts, 281, 5))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 282, 8))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 282, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 282, 8))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 282, 37))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 282, 8))
+
+        this[key] = value;
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 282, 30))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 282, 37))
+    }
+    foo() {
+>foo : Symbol(C1.foo, Decl(keyofAndIndexedAccess.ts, 284, 5))
+
+        let x1 = this.x;  // number
+>x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 286, 11))
+>this.x : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>x : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+
+        let x2 = this["x"];  // number
+>x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 287, 11))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>"x" : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+
+        let x3 = this.get("x");  // this["x"]
+>x3 : Symbol(x3, Decl(keyofAndIndexedAccess.ts, 288, 11))
+>this.get : Symbol(C1.get, Decl(keyofAndIndexedAccess.ts, 278, 14))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>get : Symbol(C1.get, Decl(keyofAndIndexedAccess.ts, 278, 14))
+
+        let x4 = getProperty(this, "x"); // this["x"]
+>x4 : Symbol(x4, Decl(keyofAndIndexedAccess.ts, 289, 11))
+>getProperty : Symbol(getProperty, Decl(keyofAndIndexedAccess.ts, 77, 26))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+
+        this.x = 42;
+>this.x : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>x : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+
+        this["x"] = 42;
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>"x" : Symbol(C1.x, Decl(keyofAndIndexedAccess.ts, 277, 10))
+
+        this.set("x", 42);
+>this.set : Symbol(C1.set, Decl(keyofAndIndexedAccess.ts, 281, 5))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+>set : Symbol(C1.set, Decl(keyofAndIndexedAccess.ts, 281, 5))
+
+        setProperty(this, "x", 42);
+>setProperty : Symbol(setProperty, Decl(keyofAndIndexedAccess.ts, 81, 1))
+>this : Symbol(C1, Decl(keyofAndIndexedAccess.ts, 275, 1))
+    }
+}
+
 // Repros from #12011
 
 class Base {
->Base : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 248, 1))
+>Base : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 295, 1))
 
     get<K extends keyof this>(prop: K) {
->get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 252, 12))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 253, 8))
->prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 253, 30))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 253, 8))
+>get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 299, 12))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 300, 8))
+>prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 300, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 300, 8))
 
         return this[prop];
->this : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 248, 1))
->prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 253, 30))
+>this : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 295, 1))
+>prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 300, 30))
     }
     set<K extends keyof this>(prop: K, value: this[K]) {
->set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 255, 5))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 256, 8))
->prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 256, 30))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 256, 8))
->value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 256, 38))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 256, 8))
+>set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 302, 5))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 303, 8))
+>prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 303, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 303, 8))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 303, 38))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 303, 8))
 
         this[prop] = value;
->this : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 248, 1))
->prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 256, 30))
->value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 256, 38))
+>this : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 295, 1))
+>prop : Symbol(prop, Decl(keyofAndIndexedAccess.ts, 303, 30))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 303, 38))
     }
 }
 
 class Person extends Base {
->Person : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 259, 1))
->Base : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 248, 1))
+>Person : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 306, 1))
+>Base : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 295, 1))
 
     parts: number;
->parts : Symbol(Person.parts, Decl(keyofAndIndexedAccess.ts, 261, 27))
+>parts : Symbol(Person.parts, Decl(keyofAndIndexedAccess.ts, 308, 27))
 
     constructor(parts: number) {
->parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 263, 16))
+>parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 310, 16))
 
         super();
->super : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 248, 1))
+>super : Symbol(Base, Decl(keyofAndIndexedAccess.ts, 295, 1))
 
         this.set("parts", parts);
->this.set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 255, 5))
->this : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 259, 1))
->set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 255, 5))
->parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 263, 16))
+>this.set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 302, 5))
+>this : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 306, 1))
+>set : Symbol(Base.set, Decl(keyofAndIndexedAccess.ts, 302, 5))
+>parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 310, 16))
     }
     getParts() {
->getParts : Symbol(Person.getParts, Decl(keyofAndIndexedAccess.ts, 266, 5))
+>getParts : Symbol(Person.getParts, Decl(keyofAndIndexedAccess.ts, 313, 5))
 
         return this.get("parts")
->this.get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 252, 12))
->this : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 259, 1))
->get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 252, 12))
+>this.get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 299, 12))
+>this : Symbol(Person, Decl(keyofAndIndexedAccess.ts, 306, 1))
+>get : Symbol(Base.get, Decl(keyofAndIndexedAccess.ts, 299, 12))
     }
 }
 
 class OtherPerson {
->OtherPerson : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 270, 1))
+>OtherPerson : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 317, 1))
 
     parts: number;
->parts : Symbol(OtherPerson.parts, Decl(keyofAndIndexedAccess.ts, 272, 19))
+>parts : Symbol(OtherPerson.parts, Decl(keyofAndIndexedAccess.ts, 319, 19))
 
     constructor(parts: number) {
->parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 274, 16))
+>parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 321, 16))
 
         setProperty(this, "parts", parts);
 >setProperty : Symbol(setProperty, Decl(keyofAndIndexedAccess.ts, 81, 1))
->this : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 270, 1))
->parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 274, 16))
+>this : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 317, 1))
+>parts : Symbol(parts, Decl(keyofAndIndexedAccess.ts, 321, 16))
     }
     getParts() {
->getParts : Symbol(OtherPerson.getParts, Decl(keyofAndIndexedAccess.ts, 276, 5))
+>getParts : Symbol(OtherPerson.getParts, Decl(keyofAndIndexedAccess.ts, 323, 5))
 
         return getProperty(this, "parts")
 >getProperty : Symbol(getProperty, Decl(keyofAndIndexedAccess.ts, 77, 26))
->this : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 270, 1))
+>this : Symbol(OtherPerson, Decl(keyofAndIndexedAccess.ts, 317, 1))
     }
 }
 
 // Modified repro from #12544
 
 function path<T, K1 extends keyof T>(obj: T, key1: K1): T[K1];
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 284, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 284, 16))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 284, 14))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 284, 37))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 284, 14))
->key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 284, 44))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 284, 16))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 284, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 284, 16))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 331, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 331, 16))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 331, 14))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 331, 37))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 331, 14))
+>key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 331, 44))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 331, 16))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 331, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 331, 16))
 
 function path<T, K1 extends keyof T, K2 extends keyof T[K1]>(obj: T, key1: K1, key2: K2): T[K1][K2];
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 285, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 285, 16))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 285, 14))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 285, 36))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 285, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 285, 16))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 285, 61))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 285, 14))
->key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 285, 68))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 285, 16))
->key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 285, 78))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 285, 36))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 285, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 285, 16))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 285, 36))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 332, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 332, 16))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 332, 14))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 332, 36))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 332, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 332, 16))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 332, 61))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 332, 14))
+>key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 332, 68))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 332, 16))
+>key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 332, 78))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 332, 36))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 332, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 332, 16))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 332, 36))
 
 function path<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2]>(obj: T, key1: K1, key2: K2, key3: K3): T[K1][K2][K3];
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 286, 16))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 286, 36))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 286, 16))
->K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 286, 60))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 286, 16))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 286, 36))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 286, 89))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 286, 96))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 286, 16))
->key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 286, 106))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 286, 36))
->key3 : Symbol(key3, Decl(keyofAndIndexedAccess.ts, 286, 116))
->K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 286, 60))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 286, 14))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 286, 16))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 286, 36))
->K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 286, 60))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 333, 16))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 333, 36))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 333, 16))
+>K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 333, 60))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 333, 16))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 333, 36))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 333, 89))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 333, 96))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 333, 16))
+>key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 333, 106))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 333, 36))
+>key3 : Symbol(key3, Decl(keyofAndIndexedAccess.ts, 333, 116))
+>K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 333, 60))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 333, 14))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 333, 16))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 333, 36))
+>K3 : Symbol(K3, Decl(keyofAndIndexedAccess.ts, 333, 60))
 
 function path(obj: any, ...keys: (string | number)[]): any;
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 287, 14))
->keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 287, 23))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 334, 14))
+>keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 334, 23))
 
 function path(obj: any, ...keys: (string | number)[]): any {
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 288, 14))
->keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 288, 23))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 335, 14))
+>keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 335, 23))
 
     let result = obj;
->result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 289, 7))
->obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 288, 14))
+>result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 336, 7))
+>obj : Symbol(obj, Decl(keyofAndIndexedAccess.ts, 335, 14))
 
     for (let k of keys) {
->k : Symbol(k, Decl(keyofAndIndexedAccess.ts, 290, 12))
->keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 288, 23))
+>k : Symbol(k, Decl(keyofAndIndexedAccess.ts, 337, 12))
+>keys : Symbol(keys, Decl(keyofAndIndexedAccess.ts, 335, 23))
 
         result = result[k];
->result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 289, 7))
->result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 289, 7))
->k : Symbol(k, Decl(keyofAndIndexedAccess.ts, 290, 12))
+>result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 336, 7))
+>result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 336, 7))
+>k : Symbol(k, Decl(keyofAndIndexedAccess.ts, 337, 12))
     }
     return result;
->result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 289, 7))
+>result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 336, 7))
 }
 
 type Thing = {
->Thing : Symbol(Thing, Decl(keyofAndIndexedAccess.ts, 294, 1))
+>Thing : Symbol(Thing, Decl(keyofAndIndexedAccess.ts, 341, 1))
 
     a: { x: number, y: string },
->a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 296, 14))
->x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 297, 8))
->y : Symbol(y, Decl(keyofAndIndexedAccess.ts, 297, 19))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 343, 14))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 344, 8))
+>y : Symbol(y, Decl(keyofAndIndexedAccess.ts, 344, 19))
 
     b: boolean
->b : Symbol(b, Decl(keyofAndIndexedAccess.ts, 297, 32))
+>b : Symbol(b, Decl(keyofAndIndexedAccess.ts, 344, 32))
 
 };
 
 
 function f1(thing: Thing) {
->f1 : Symbol(f1, Decl(keyofAndIndexedAccess.ts, 299, 2))
->thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 302, 12))
->Thing : Symbol(Thing, Decl(keyofAndIndexedAccess.ts, 294, 1))
+>f1 : Symbol(f1, Decl(keyofAndIndexedAccess.ts, 346, 2))
+>thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 349, 12))
+>Thing : Symbol(Thing, Decl(keyofAndIndexedAccess.ts, 341, 1))
 
     let x1 = path(thing, 'a');  // { x: number, y: string }
->x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 303, 7))
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 302, 12))
+>x1 : Symbol(x1, Decl(keyofAndIndexedAccess.ts, 350, 7))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 349, 12))
 
     let x2 = path(thing, 'a', 'y');  // string
->x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 304, 7))
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 302, 12))
+>x2 : Symbol(x2, Decl(keyofAndIndexedAccess.ts, 351, 7))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 349, 12))
 
     let x3 = path(thing, 'b');  // boolean
->x3 : Symbol(x3, Decl(keyofAndIndexedAccess.ts, 305, 7))
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 302, 12))
+>x3 : Symbol(x3, Decl(keyofAndIndexedAccess.ts, 352, 7))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 349, 12))
 
     let x4 = path(thing, ...['a', 'x']);  // any
->x4 : Symbol(x4, Decl(keyofAndIndexedAccess.ts, 306, 7))
->path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 280, 1), Decl(keyofAndIndexedAccess.ts, 284, 62), Decl(keyofAndIndexedAccess.ts, 285, 100), Decl(keyofAndIndexedAccess.ts, 286, 142), Decl(keyofAndIndexedAccess.ts, 287, 59))
->thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 302, 12))
+>x4 : Symbol(x4, Decl(keyofAndIndexedAccess.ts, 353, 7))
+>path : Symbol(path, Decl(keyofAndIndexedAccess.ts, 327, 1), Decl(keyofAndIndexedAccess.ts, 331, 62), Decl(keyofAndIndexedAccess.ts, 332, 100), Decl(keyofAndIndexedAccess.ts, 333, 142), Decl(keyofAndIndexedAccess.ts, 334, 59))
+>thing : Symbol(thing, Decl(keyofAndIndexedAccess.ts, 349, 12))
 }
 
 // Repro from comment in #12114
 
 const assignTo2 = <T, K1 extends keyof T, K2 extends keyof T[K1]>(object: T, key1: K1, key2: K2) =>
->assignTo2 : Symbol(assignTo2, Decl(keyofAndIndexedAccess.ts, 311, 5))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 311, 19))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 311, 21))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 311, 19))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 311, 41))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 311, 19))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 311, 21))
->object : Symbol(object, Decl(keyofAndIndexedAccess.ts, 311, 66))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 311, 19))
->key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 311, 76))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 311, 21))
->key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 311, 86))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 311, 41))
+>assignTo2 : Symbol(assignTo2, Decl(keyofAndIndexedAccess.ts, 358, 5))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 358, 19))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 358, 21))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 358, 19))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 358, 41))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 358, 19))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 358, 21))
+>object : Symbol(object, Decl(keyofAndIndexedAccess.ts, 358, 66))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 358, 19))
+>key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 358, 76))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 358, 21))
+>key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 358, 86))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 358, 41))
 
     (value: T[K1][K2]) => object[key1][key2] = value;
->value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 312, 5))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 311, 19))
->K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 311, 21))
->K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 311, 41))
->object : Symbol(object, Decl(keyofAndIndexedAccess.ts, 311, 66))
->key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 311, 76))
->key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 311, 86))
->value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 312, 5))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 359, 5))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 358, 19))
+>K1 : Symbol(K1, Decl(keyofAndIndexedAccess.ts, 358, 21))
+>K2 : Symbol(K2, Decl(keyofAndIndexedAccess.ts, 358, 41))
+>object : Symbol(object, Decl(keyofAndIndexedAccess.ts, 358, 66))
+>key1 : Symbol(key1, Decl(keyofAndIndexedAccess.ts, 358, 76))
+>key2 : Symbol(key2, Decl(keyofAndIndexedAccess.ts, 358, 86))
+>value : Symbol(value, Decl(keyofAndIndexedAccess.ts, 359, 5))
 
 // Modified repro from #12573
 
 declare function one<T>(handler: (t: T) => void): T
->one : Symbol(one, Decl(keyofAndIndexedAccess.ts, 312, 53))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 316, 21))
->handler : Symbol(handler, Decl(keyofAndIndexedAccess.ts, 316, 24))
->t : Symbol(t, Decl(keyofAndIndexedAccess.ts, 316, 34))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 316, 21))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 316, 21))
+>one : Symbol(one, Decl(keyofAndIndexedAccess.ts, 359, 53))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 363, 21))
+>handler : Symbol(handler, Decl(keyofAndIndexedAccess.ts, 363, 24))
+>t : Symbol(t, Decl(keyofAndIndexedAccess.ts, 363, 34))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 363, 21))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 363, 21))
 
 var empty = one(() => {}) // inferred as {}, expected
->empty : Symbol(empty, Decl(keyofAndIndexedAccess.ts, 317, 3))
->one : Symbol(one, Decl(keyofAndIndexedAccess.ts, 312, 53))
+>empty : Symbol(empty, Decl(keyofAndIndexedAccess.ts, 364, 3))
+>one : Symbol(one, Decl(keyofAndIndexedAccess.ts, 359, 53))
 
 type Handlers<T> = { [K in keyof T]: (t: T[K]) => void }
->Handlers : Symbol(Handlers, Decl(keyofAndIndexedAccess.ts, 317, 25))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 319, 14))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 319, 22))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 319, 14))
->t : Symbol(t, Decl(keyofAndIndexedAccess.ts, 319, 38))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 319, 14))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 319, 22))
+>Handlers : Symbol(Handlers, Decl(keyofAndIndexedAccess.ts, 364, 25))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 366, 14))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 366, 22))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 366, 14))
+>t : Symbol(t, Decl(keyofAndIndexedAccess.ts, 366, 38))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 366, 14))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 366, 22))
 
 declare function on<T>(handlerHash: Handlers<T>): T
->on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 319, 56))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 320, 20))
->handlerHash : Symbol(handlerHash, Decl(keyofAndIndexedAccess.ts, 320, 23))
->Handlers : Symbol(Handlers, Decl(keyofAndIndexedAccess.ts, 317, 25))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 320, 20))
->T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 320, 20))
+>on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 366, 56))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 367, 20))
+>handlerHash : Symbol(handlerHash, Decl(keyofAndIndexedAccess.ts, 367, 23))
+>Handlers : Symbol(Handlers, Decl(keyofAndIndexedAccess.ts, 364, 25))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 367, 20))
+>T : Symbol(T, Decl(keyofAndIndexedAccess.ts, 367, 20))
 
 var hashOfEmpty1 = on({ test: () => {} });  // {}
->hashOfEmpty1 : Symbol(hashOfEmpty1, Decl(keyofAndIndexedAccess.ts, 321, 3))
->on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 319, 56))
->test : Symbol(test, Decl(keyofAndIndexedAccess.ts, 321, 23))
+>hashOfEmpty1 : Symbol(hashOfEmpty1, Decl(keyofAndIndexedAccess.ts, 368, 3))
+>on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 366, 56))
+>test : Symbol(test, Decl(keyofAndIndexedAccess.ts, 368, 23))
 
 var hashOfEmpty2 = on({ test: (x: boolean) => {} });  // { test: boolean }
->hashOfEmpty2 : Symbol(hashOfEmpty2, Decl(keyofAndIndexedAccess.ts, 322, 3))
->on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 319, 56))
->test : Symbol(test, Decl(keyofAndIndexedAccess.ts, 322, 23))
->x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 322, 31))
+>hashOfEmpty2 : Symbol(hashOfEmpty2, Decl(keyofAndIndexedAccess.ts, 369, 3))
+>on : Symbol(on, Decl(keyofAndIndexedAccess.ts, 366, 56))
+>test : Symbol(test, Decl(keyofAndIndexedAccess.ts, 369, 23))
+>x : Symbol(x, Decl(keyofAndIndexedAccess.ts, 369, 31))
 
 // Repro from #12624
 
 interface Options1<Data, Computed> {
->Options1 : Symbol(Options1, Decl(keyofAndIndexedAccess.ts, 322, 52))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 326, 19))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 326, 24))
+>Options1 : Symbol(Options1, Decl(keyofAndIndexedAccess.ts, 369, 52))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 373, 19))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 373, 24))
 
     data?: Data
->data : Symbol(Options1.data, Decl(keyofAndIndexedAccess.ts, 326, 36))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 326, 19))
+>data : Symbol(Options1.data, Decl(keyofAndIndexedAccess.ts, 373, 36))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 373, 19))
 
     computed?: Computed;
->computed : Symbol(Options1.computed, Decl(keyofAndIndexedAccess.ts, 327, 15))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 326, 24))
+>computed : Symbol(Options1.computed, Decl(keyofAndIndexedAccess.ts, 374, 15))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 373, 24))
 }
 
 declare class Component1<Data, Computed> {
->Component1 : Symbol(Component1, Decl(keyofAndIndexedAccess.ts, 329, 1))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 331, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 331, 30))
+>Component1 : Symbol(Component1, Decl(keyofAndIndexedAccess.ts, 376, 1))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 378, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 378, 30))
 
     constructor(options: Options1<Data, Computed>);
->options : Symbol(options, Decl(keyofAndIndexedAccess.ts, 332, 16))
->Options1 : Symbol(Options1, Decl(keyofAndIndexedAccess.ts, 322, 52))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 331, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 331, 30))
+>options : Symbol(options, Decl(keyofAndIndexedAccess.ts, 379, 16))
+>Options1 : Symbol(Options1, Decl(keyofAndIndexedAccess.ts, 369, 52))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 378, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 378, 30))
 
     get<K extends keyof (Data & Computed)>(key: K): (Data & Computed)[K];
->get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 332, 51))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 333, 8))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 331, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 331, 30))
->key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 333, 43))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 333, 8))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 331, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 331, 30))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 333, 8))
+>get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 379, 51))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 380, 8))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 378, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 378, 30))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 380, 43))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 380, 8))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 378, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 378, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 380, 8))
 }
 
 let c1 = new Component1({
->c1 : Symbol(c1, Decl(keyofAndIndexedAccess.ts, 336, 3))
->Component1 : Symbol(Component1, Decl(keyofAndIndexedAccess.ts, 329, 1))
+>c1 : Symbol(c1, Decl(keyofAndIndexedAccess.ts, 383, 3))
+>Component1 : Symbol(Component1, Decl(keyofAndIndexedAccess.ts, 376, 1))
 
     data: {
->data : Symbol(data, Decl(keyofAndIndexedAccess.ts, 336, 25))
+>data : Symbol(data, Decl(keyofAndIndexedAccess.ts, 383, 25))
 
         hello: ""
->hello : Symbol(hello, Decl(keyofAndIndexedAccess.ts, 337, 11))
+>hello : Symbol(hello, Decl(keyofAndIndexedAccess.ts, 384, 11))
     }
 });
 
 c1.get("hello");
->c1.get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 332, 51))
->c1 : Symbol(c1, Decl(keyofAndIndexedAccess.ts, 336, 3))
->get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 332, 51))
+>c1.get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 379, 51))
+>c1 : Symbol(c1, Decl(keyofAndIndexedAccess.ts, 383, 3))
+>get : Symbol(Component1.get, Decl(keyofAndIndexedAccess.ts, 379, 51))
 
 // Repro from #12625
 
 interface Options2<Data, Computed> {
->Options2 : Symbol(Options2, Decl(keyofAndIndexedAccess.ts, 342, 16))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 346, 19))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 346, 24))
+>Options2 : Symbol(Options2, Decl(keyofAndIndexedAccess.ts, 389, 16))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 393, 19))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 393, 24))
 
     data?: Data
->data : Symbol(Options2.data, Decl(keyofAndIndexedAccess.ts, 346, 36))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 346, 19))
+>data : Symbol(Options2.data, Decl(keyofAndIndexedAccess.ts, 393, 36))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 393, 19))
 
     computed?: Computed;
->computed : Symbol(Options2.computed, Decl(keyofAndIndexedAccess.ts, 347, 15))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 346, 24))
+>computed : Symbol(Options2.computed, Decl(keyofAndIndexedAccess.ts, 394, 15))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 393, 24))
 }
 
 declare class Component2<Data, Computed> {
->Component2 : Symbol(Component2, Decl(keyofAndIndexedAccess.ts, 349, 1))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 351, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 351, 30))
+>Component2 : Symbol(Component2, Decl(keyofAndIndexedAccess.ts, 396, 1))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 398, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 398, 30))
 
     constructor(options: Options2<Data, Computed>);
->options : Symbol(options, Decl(keyofAndIndexedAccess.ts, 352, 16))
->Options2 : Symbol(Options2, Decl(keyofAndIndexedAccess.ts, 342, 16))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 351, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 351, 30))
+>options : Symbol(options, Decl(keyofAndIndexedAccess.ts, 399, 16))
+>Options2 : Symbol(Options2, Decl(keyofAndIndexedAccess.ts, 389, 16))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 398, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 398, 30))
 
     get<K extends keyof Data | keyof Computed>(key: K): (Data & Computed)[K];
->get : Symbol(Component2.get, Decl(keyofAndIndexedAccess.ts, 352, 51))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 353, 8))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 351, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 351, 30))
->key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 353, 47))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 353, 8))
->Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 351, 25))
->Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 351, 30))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 353, 8))
+>get : Symbol(Component2.get, Decl(keyofAndIndexedAccess.ts, 399, 51))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 400, 8))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 398, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 398, 30))
+>key : Symbol(key, Decl(keyofAndIndexedAccess.ts, 400, 47))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 400, 8))
+>Data : Symbol(Data, Decl(keyofAndIndexedAccess.ts, 398, 25))
+>Computed : Symbol(Computed, Decl(keyofAndIndexedAccess.ts, 398, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 400, 8))
 }
 
 // Repro from #12641
 
 interface R {
->R : Symbol(R, Decl(keyofAndIndexedAccess.ts, 354, 1))
+>R : Symbol(R, Decl(keyofAndIndexedAccess.ts, 401, 1))
 
     p: number;
->p : Symbol(R.p, Decl(keyofAndIndexedAccess.ts, 358, 13))
+>p : Symbol(R.p, Decl(keyofAndIndexedAccess.ts, 405, 13))
 }
 
 function f<K extends keyof R>(p: K) {
->f : Symbol(f, Decl(keyofAndIndexedAccess.ts, 360, 1))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 362, 11))
->R : Symbol(R, Decl(keyofAndIndexedAccess.ts, 354, 1))
->p : Symbol(p, Decl(keyofAndIndexedAccess.ts, 362, 30))
->K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 362, 11))
+>f : Symbol(f, Decl(keyofAndIndexedAccess.ts, 407, 1))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 409, 11))
+>R : Symbol(R, Decl(keyofAndIndexedAccess.ts, 401, 1))
+>p : Symbol(p, Decl(keyofAndIndexedAccess.ts, 409, 30))
+>K : Symbol(K, Decl(keyofAndIndexedAccess.ts, 409, 11))
 
     let a: any;
->a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 363, 7))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 410, 7))
 
     a[p].add;  // any
->a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 363, 7))
->p : Symbol(p, Decl(keyofAndIndexedAccess.ts, 362, 30))
+>a : Symbol(a, Decl(keyofAndIndexedAccess.ts, 410, 7))
+>p : Symbol(p, Decl(keyofAndIndexedAccess.ts, 409, 30))
 }
+
+// Repro from #12651
+
+type MethodDescriptor = {
+>MethodDescriptor : Symbol(MethodDescriptor, Decl(keyofAndIndexedAccess.ts, 412, 1))
+
+	name: string;
+>name : Symbol(name, Decl(keyofAndIndexedAccess.ts, 416, 25))
+
+	args: any[];
+>args : Symbol(args, Decl(keyofAndIndexedAccess.ts, 417, 14))
+
+	returnValue: any;
+>returnValue : Symbol(returnValue, Decl(keyofAndIndexedAccess.ts, 418, 13))
+}
+
+declare function dispatchMethod<M extends MethodDescriptor>(name: M['name'], args: M['args']): M['returnValue'];
+>dispatchMethod : Symbol(dispatchMethod, Decl(keyofAndIndexedAccess.ts, 420, 1))
+>M : Symbol(M, Decl(keyofAndIndexedAccess.ts, 422, 32))
+>MethodDescriptor : Symbol(MethodDescriptor, Decl(keyofAndIndexedAccess.ts, 412, 1))
+>name : Symbol(name, Decl(keyofAndIndexedAccess.ts, 422, 60))
+>M : Symbol(M, Decl(keyofAndIndexedAccess.ts, 422, 32))
+>args : Symbol(args, Decl(keyofAndIndexedAccess.ts, 422, 76))
+>M : Symbol(M, Decl(keyofAndIndexedAccess.ts, 422, 32))
+>M : Symbol(M, Decl(keyofAndIndexedAccess.ts, 422, 32))
+
+type SomeMethodDescriptor = {
+>SomeMethodDescriptor : Symbol(SomeMethodDescriptor, Decl(keyofAndIndexedAccess.ts, 422, 112))
+
+	name: "someMethod";
+>name : Symbol(name, Decl(keyofAndIndexedAccess.ts, 424, 29))
+
+	args: [string, number];
+>args : Symbol(args, Decl(keyofAndIndexedAccess.ts, 425, 20))
+
+	returnValue: string[];
+>returnValue : Symbol(returnValue, Decl(keyofAndIndexedAccess.ts, 426, 24))
+}
+
+let result = dispatchMethod<SomeMethodDescriptor>("someMethod", ["hello", 35]);
+>result : Symbol(result, Decl(keyofAndIndexedAccess.ts, 430, 3))
+>dispatchMethod : Symbol(dispatchMethod, Decl(keyofAndIndexedAccess.ts, 420, 1))
+>SomeMethodDescriptor : Symbol(SomeMethodDescriptor, Decl(keyofAndIndexedAccess.ts, 422, 112))
+
diff --git a/tests/baselines/reference/keyofAndIndexedAccess.types b/tests/baselines/reference/keyofAndIndexedAccess.types
index 3b984643776..f94249806f6 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess.types
+++ b/tests/baselines/reference/keyofAndIndexedAccess.types
@@ -626,8 +626,8 @@ function f33<S extends Shape, K extends keyof S>(shape: S, key: K) {
 >K : K
 
     let name = getProperty(shape, "name");
->name : string
->getProperty(shape, "name") : string
+>name : S["name"]
+>getProperty(shape, "name") : S["name"]
 >getProperty : <T, K extends keyof T>(obj: T, key: K) => T[K]
 >shape : S
 >"name" : "name"
@@ -1146,6 +1146,245 @@ function f74(func: <T, U, K extends keyof (T | U)>(x: T, y: U, k: K) => (T | U)[
 >'b' : "b"
 }
 
+function f80<T extends { a: { x: any } }>(obj: T) {
+>f80 : <T extends { a: { x: any; }; }>(obj: T) => void
+>T : T
+>a : { x: any; }
+>x : any
+>obj : T
+>T : T
+
+    let a1 = obj.a;  // { x: any }
+>a1 : { x: any; }
+>obj.a : { x: any; }
+>obj : T
+>a : { x: any; }
+
+    let a2 = obj['a'];  // { x: any }
+>a2 : { x: any; }
+>obj['a'] : { x: any; }
+>obj : T
+>'a' : "a"
+
+    let a3 = obj['a'] as T['a'];  // T["a"]
+>a3 : T["a"]
+>obj['a'] as T['a'] : T["a"]
+>obj['a'] : { x: any; }
+>obj : T
+>'a' : "a"
+>T : T
+
+    let x1 = obj.a.x;  // any
+>x1 : any
+>obj.a.x : any
+>obj.a : { x: any; }
+>obj : T
+>a : { x: any; }
+>x : any
+
+    let x2 = obj['a']['x'];  // any
+>x2 : any
+>obj['a']['x'] : any
+>obj['a'] : { x: any; }
+>obj : T
+>'a' : "a"
+>'x' : "x"
+
+    let x3 = obj['a']['x'] as T['a']['x'];  // T["a"]["x"]
+>x3 : T["a"]["x"]
+>obj['a']['x'] as T['a']['x'] : T["a"]["x"]
+>obj['a']['x'] : any
+>obj['a'] : { x: any; }
+>obj : T
+>'a' : "a"
+>'x' : "x"
+>T : T
+}
+
+function f81<T extends { a: { x: any } }>(obj: T) {
+>f81 : <T extends { a: { x: any; }; }>(obj: T) => T["a"]["x"]
+>T : T
+>a : { x: any; }
+>x : any
+>obj : T
+>T : T
+
+    return obj['a']['x'] as T['a']['x'];
+>obj['a']['x'] as T['a']['x'] : T["a"]["x"]
+>obj['a']['x'] : any
+>obj['a'] : { x: any; }
+>obj : T
+>'a' : "a"
+>'x' : "x"
+>T : T
+}
+
+function f82() {
+>f82 : () => void
+
+    let x1 = f81({ a: { x: "hello" } });  // string
+>x1 : string
+>f81({ a: { x: "hello" } }) : string
+>f81 : <T extends { a: { x: any; }; }>(obj: T) => T["a"]["x"]
+>{ a: { x: "hello" } } : { a: { x: string; }; }
+>a : { x: string; }
+>{ x: "hello" } : { x: string; }
+>x : string
+>"hello" : "hello"
+
+    let x2 = f81({ a: { x: 42 } });  // number
+>x2 : number
+>f81({ a: { x: 42 } }) : number
+>f81 : <T extends { a: { x: any; }; }>(obj: T) => T["a"]["x"]
+>{ a: { x: 42 } } : { a: { x: number; }; }
+>a : { x: number; }
+>{ x: 42 } : { x: number; }
+>x : number
+>42 : 42
+}
+
+function f83<T extends { [x: string]: { x: any } }, K extends keyof T>(obj: T, key: K) {
+>f83 : <T extends { [x: string]: { x: any; }; }, K extends keyof T>(obj: T, key: K) => T[K]["x"]
+>T : T
+>x : string
+>x : any
+>K : K
+>T : T
+>obj : T
+>T : T
+>key : K
+>K : K
+
+    return obj[key]['x'] as T[K]['x'];
+>obj[key]['x'] as T[K]['x'] : T[K]["x"]
+>obj[key]['x'] : any
+>obj[key] : T[K]
+>obj : T
+>key : K
+>'x' : "x"
+>T : T
+>K : K
+}
+
+function f84() {
+>f84 : () => void
+
+    let x1 = f83({ foo: { x: "hello" } }, "foo");  // string
+>x1 : string
+>f83({ foo: { x: "hello" } }, "foo") : string
+>f83 : <T extends { [x: string]: { x: any; }; }, K extends keyof T>(obj: T, key: K) => T[K]["x"]
+>{ foo: { x: "hello" } } : { foo: { x: string; }; }
+>foo : { x: string; }
+>{ x: "hello" } : { x: string; }
+>x : string
+>"hello" : "hello"
+>"foo" : "foo"
+
+    let x2 = f83({ bar: { x: 42 } }, "bar");  // number
+>x2 : number
+>f83({ bar: { x: 42 } }, "bar") : number
+>f83 : <T extends { [x: string]: { x: any; }; }, K extends keyof T>(obj: T, key: K) => T[K]["x"]
+>{ bar: { x: 42 } } : { bar: { x: number; }; }
+>bar : { x: number; }
+>{ x: 42 } : { x: number; }
+>x : number
+>42 : 42
+>"bar" : "bar"
+}
+
+class C1 {
+>C1 : C1
+
+    x: number;
+>x : number
+
+    get<K extends keyof this>(key: K) {
+>get : <K extends keyof this>(key: K) => this[K]
+>K : K
+>key : K
+>K : K
+
+        return this[key];
+>this[key] : this[K]
+>this : this
+>key : K
+    }
+    set<K extends keyof this>(key: K, value: this[K]) {
+>set : <K extends keyof this>(key: K, value: this[K]) => void
+>K : K
+>key : K
+>K : K
+>value : this[K]
+>K : K
+
+        this[key] = value;
+>this[key] = value : this[K]
+>this[key] : this[K]
+>this : this
+>key : K
+>value : this[K]
+    }
+    foo() {
+>foo : () => void
+
+        let x1 = this.x;  // number
+>x1 : number
+>this.x : number
+>this : this
+>x : number
+
+        let x2 = this["x"];  // number
+>x2 : number
+>this["x"] : number
+>this : this
+>"x" : "x"
+
+        let x3 = this.get("x");  // this["x"]
+>x3 : this["x"]
+>this.get("x") : this["x"]
+>this.get : <K extends keyof this>(key: K) => this[K]
+>this : this
+>get : <K extends keyof this>(key: K) => this[K]
+>"x" : "x"
+
+        let x4 = getProperty(this, "x"); // this["x"]
+>x4 : this["x"]
+>getProperty(this, "x") : this["x"]
+>getProperty : <T, K extends keyof T>(obj: T, key: K) => T[K]
+>this : this
+>"x" : "x"
+
+        this.x = 42;
+>this.x = 42 : 42
+>this.x : number
+>this : this
+>x : number
+>42 : 42
+
+        this["x"] = 42;
+>this["x"] = 42 : 42
+>this["x"] : number
+>this : this
+>"x" : "x"
+>42 : 42
+
+        this.set("x", 42);
+>this.set("x", 42) : void
+>this.set : <K extends keyof this>(key: K, value: this[K]) => void
+>this : this
+>set : <K extends keyof this>(key: K, value: this[K]) => void
+>"x" : "x"
+>42 : 42
+
+        setProperty(this, "x", 42);
+>setProperty(this, "x", 42) : void
+>setProperty : <T, K extends keyof T>(obj: T, key: K, value: T[K]) => void
+>this : this
+>"x" : "x"
+>42 : 42
+    }
+}
+
 // Repros from #12011
 
 class Base {
@@ -1202,10 +1441,10 @@ class Person extends Base {
 >parts : number
     }
     getParts() {
->getParts : () => number
+>getParts : () => this["parts"]
 
         return this.get("parts")
->this.get("parts") : number
+>this.get("parts") : this["parts"]
 >this.get : <K extends keyof this>(prop: K) => this[K]
 >this : this
 >get : <K extends keyof this>(prop: K) => this[K]
@@ -1230,10 +1469,10 @@ class OtherPerson {
 >parts : number
     }
     getParts() {
->getParts : () => number
+>getParts : () => this["parts"]
 
         return getProperty(this, "parts")
->getProperty(this, "parts") : number
+>getProperty(this, "parts") : this["parts"]
 >getProperty : <T, K extends keyof T>(obj: T, key: K) => T[K]
 >this : this
 >"parts" : "parts"
@@ -1587,3 +1826,52 @@ function f<K extends keyof R>(p: K) {
 >p : K
 >add : any
 }
+
+// Repro from #12651
+
+type MethodDescriptor = {
+>MethodDescriptor : MethodDescriptor
+
+	name: string;
+>name : string
+
+	args: any[];
+>args : any[]
+
+	returnValue: any;
+>returnValue : any
+}
+
+declare function dispatchMethod<M extends MethodDescriptor>(name: M['name'], args: M['args']): M['returnValue'];
+>dispatchMethod : <M extends MethodDescriptor>(name: M["name"], args: M["args"]) => M["returnValue"]
+>M : M
+>MethodDescriptor : MethodDescriptor
+>name : M["name"]
+>M : M
+>args : M["args"]
+>M : M
+>M : M
+
+type SomeMethodDescriptor = {
+>SomeMethodDescriptor : SomeMethodDescriptor
+
+	name: "someMethod";
+>name : "someMethod"
+
+	args: [string, number];
+>args : [string, number]
+
+	returnValue: string[];
+>returnValue : string[]
+}
+
+let result = dispatchMethod<SomeMethodDescriptor>("someMethod", ["hello", 35]);
+>result : string[]
+>dispatchMethod<SomeMethodDescriptor>("someMethod", ["hello", 35]) : string[]
+>dispatchMethod : <M extends MethodDescriptor>(name: M["name"], args: M["args"]) => M["returnValue"]
+>SomeMethodDescriptor : SomeMethodDescriptor
+>"someMethod" : "someMethod"
+>["hello", 35] : [string, number]
+>"hello" : "hello"
+>35 : 35
+
diff --git a/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt b/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt
index 8424d528b22..c19a13209c9 100644
--- a/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt
+++ b/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt
@@ -1,9 +1,7 @@
 tests/cases/compiler/keyofIsLiteralContexualType.ts(5,9): error TS2322: Type '("a" | "b" | "c")[]' is not assignable to type 'keyof T[]'.
   Type '"a" | "b" | "c"' is not assignable to type 'keyof T'.
-    Type '"a" | "b" | "c"' is not assignable to type '"a" | "b"'.
+    Type '"c"' is not assignable to type 'keyof T'.
       Type '"c"' is not assignable to type '"a" | "b"'.
-        Type '"c"' is not assignable to type 'keyof T'.
-          Type '"c"' is not assignable to type '"a" | "b"'.
 tests/cases/compiler/keyofIsLiteralContexualType.ts(13,11): error TS2339: Property 'b' does not exist on type 'Pick<{ a: number; b: number; c: number; }, "a" | "c">'.
 
 
@@ -16,10 +14,8 @@ tests/cases/compiler/keyofIsLiteralContexualType.ts(13,11): error TS2339: Proper
             ~
 !!! error TS2322: Type '("a" | "b" | "c")[]' is not assignable to type 'keyof T[]'.
 !!! error TS2322:   Type '"a" | "b" | "c"' is not assignable to type 'keyof T'.
-!!! error TS2322:     Type '"a" | "b" | "c"' is not assignable to type '"a" | "b"'.
+!!! error TS2322:     Type '"c"' is not assignable to type 'keyof T'.
 !!! error TS2322:       Type '"c"' is not assignable to type '"a" | "b"'.
-!!! error TS2322:         Type '"c"' is not assignable to type 'keyof T'.
-!!! error TS2322:           Type '"c"' is not assignable to type '"a" | "b"'.
     }
     
     // Repro from #12455
diff --git a/tests/baselines/reference/mappedTypeErrors.errors.txt b/tests/baselines/reference/mappedTypeErrors.errors.txt
index 918c6627f66..17c77fa50a0 100644
--- a/tests/baselines/reference/mappedTypeErrors.errors.txt
+++ b/tests/baselines/reference/mappedTypeErrors.errors.txt
@@ -26,9 +26,19 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(78,59): error TS2345: A
   Object literal may only specify known properties, and 'z' does not exist in type 'Readonly<{ x: number; y: number; }>'.
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(84,58): error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Partial<{ x: number; y: number; }>'.
   Object literal may only specify known properties, and 'z' does not exist in type 'Partial<{ x: number; y: number; }>'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(106,15): error TS2345: Argument of type '{ a: undefined; }' is not assignable to parameter of type 'Pick<Foo, "a">'.
+  Types of property 'a' are incompatible.
+    Type 'undefined' is not assignable to type 'string'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(107,17): error TS2345: Argument of type '{ c: boolean; }' is not assignable to parameter of type 'Pick<Foo, "a" | "b">'.
+  Object literal may only specify known properties, and 'c' does not exist in type 'Pick<Foo, "a" | "b">'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(124,12): error TS2345: Argument of type '{ a: undefined; }' is not assignable to parameter of type 'Pick<Foo, "a">'.
+  Types of property 'a' are incompatible.
+    Type 'undefined' is not assignable to type 'string'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(125,14): error TS2345: Argument of type '{ c: boolean; }' is not assignable to parameter of type 'Pick<Foo, "a" | "b">'.
+  Object literal may only specify known properties, and 'c' does not exist in type 'Pick<Foo, "a" | "b">'.
 
 
-==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (17 errors) ====
+==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (21 errors) ====
     
     interface Shape {
         name: string;
@@ -158,4 +168,59 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(84,58): error TS2345: A
                                                              ~~~~
 !!! error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Partial<{ x: number; y: number; }>'.
 !!! error TS2345:   Object literal may only specify known properties, and 'z' does not exist in type 'Partial<{ x: number; y: number; }>'.
-    }
\ No newline at end of file
+    }
+    
+    // Verify use of Pick<T, K> for setState functions (#12793)
+    
+    interface Foo {
+        a: string;
+        b?: number;
+    }
+    
+    function setState<T, K extends keyof T>(obj: T, props: Pick<T, K>) {
+        for (let k in props) {
+            obj[k] = props[k];
+        }
+    }
+    
+    let foo: Foo = { a: "hello", b: 42 };
+    setState(foo, { a: "test", b: 43 })
+    setState(foo, { a: "hi" });
+    setState(foo, { b: undefined });
+    setState(foo, { });
+    setState(foo, foo);
+    setState(foo, { a: undefined });  // Error
+                  ~~~~~~~~~~~~~~~~
+!!! error TS2345: Argument of type '{ a: undefined; }' is not assignable to parameter of type 'Pick<Foo, "a">'.
+!!! error TS2345:   Types of property 'a' are incompatible.
+!!! error TS2345:     Type 'undefined' is not assignable to type 'string'.
+    setState(foo, { c: true });  // Error
+                    ~~~~~~~
+!!! error TS2345: Argument of type '{ c: boolean; }' is not assignable to parameter of type 'Pick<Foo, "a" | "b">'.
+!!! error TS2345:   Object literal may only specify known properties, and 'c' does not exist in type 'Pick<Foo, "a" | "b">'.
+    
+    class C<T> {
+        state: T;
+        setState<K extends keyof T>(props: Pick<T, K>) {
+            for (let k in props) {
+                this.state[k] = props[k];
+            }
+        }
+    }
+    
+    let c = new C<Foo>();
+    c.setState({ a: "test", b: 43 });
+    c.setState({ a: "hi" });
+    c.setState({ b: undefined });
+    c.setState({ });
+    c.setState(foo);
+    c.setState({ a: undefined });  // Error
+               ~~~~~~~~~~~~~~~~
+!!! error TS2345: Argument of type '{ a: undefined; }' is not assignable to parameter of type 'Pick<Foo, "a">'.
+!!! error TS2345:   Types of property 'a' are incompatible.
+!!! error TS2345:     Type 'undefined' is not assignable to type 'string'.
+    c.setState({ c: true });  // Error
+                 ~~~~~~~
+!!! error TS2345: Argument of type '{ c: boolean; }' is not assignable to parameter of type 'Pick<Foo, "a" | "b">'.
+!!! error TS2345:   Object literal may only specify known properties, and 'c' does not exist in type 'Pick<Foo, "a" | "b">'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeErrors.js b/tests/baselines/reference/mappedTypeErrors.js
index 4d2eadbafe9..e1b458565bb 100644
--- a/tests/baselines/reference/mappedTypeErrors.js
+++ b/tests/baselines/reference/mappedTypeErrors.js
@@ -83,7 +83,48 @@ function f21() {
     let x1 = objAndPartial({ x: 0, y: 0 }, { x: 1 });
     let x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
     let x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
-}
+}
+
+// Verify use of Pick<T, K> for setState functions (#12793)
+
+interface Foo {
+    a: string;
+    b?: number;
+}
+
+function setState<T, K extends keyof T>(obj: T, props: Pick<T, K>) {
+    for (let k in props) {
+        obj[k] = props[k];
+    }
+}
+
+let foo: Foo = { a: "hello", b: 42 };
+setState(foo, { a: "test", b: 43 })
+setState(foo, { a: "hi" });
+setState(foo, { b: undefined });
+setState(foo, { });
+setState(foo, foo);
+setState(foo, { a: undefined });  // Error
+setState(foo, { c: true });  // Error
+
+class C<T> {
+    state: T;
+    setState<K extends keyof T>(props: Pick<T, K>) {
+        for (let k in props) {
+            this.state[k] = props[k];
+        }
+    }
+}
+
+let c = new C<Foo>();
+c.setState({ a: "test", b: 43 });
+c.setState({ a: "hi" });
+c.setState({ b: undefined });
+c.setState({ });
+c.setState(foo);
+c.setState({ a: undefined });  // Error
+c.setState({ c: true });  // Error
+
 
 //// [mappedTypeErrors.js]
 function f1(x) {
@@ -124,6 +165,37 @@ function f21() {
     var x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
     var x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 }); // Error
 }
+function setState(obj, props) {
+    for (var k in props) {
+        obj[k] = props[k];
+    }
+}
+var foo = { a: "hello", b: 42 };
+setState(foo, { a: "test", b: 43 });
+setState(foo, { a: "hi" });
+setState(foo, { b: undefined });
+setState(foo, {});
+setState(foo, foo);
+setState(foo, { a: undefined }); // Error
+setState(foo, { c: true }); // Error
+var C = (function () {
+    function C() {
+    }
+    C.prototype.setState = function (props) {
+        for (var k in props) {
+            this.state[k] = props[k];
+        }
+    };
+    return C;
+}());
+var c = new C();
+c.setState({ a: "test", b: 43 });
+c.setState({ a: "hi" });
+c.setState({ b: undefined });
+c.setState({});
+c.setState(foo);
+c.setState({ a: undefined }); // Error
+c.setState({ c: true }); // Error
 
 
 //// [mappedTypeErrors.d.ts]
@@ -168,3 +240,14 @@ declare function objAndReadonly<T>(primary: T, secondary: Readonly<T>): T;
 declare function objAndPartial<T>(primary: T, secondary: Partial<T>): T;
 declare function f20(): void;
 declare function f21(): void;
+interface Foo {
+    a: string;
+    b?: number;
+}
+declare function setState<T, K extends keyof T>(obj: T, props: Pick<T, K>): void;
+declare let foo: Foo;
+declare class C<T> {
+    state: T;
+    setState<K extends keyof T>(props: Pick<T, K>): void;
+}
+declare let c: C<Foo>;
diff --git a/tests/baselines/reference/mappedTypeModifiers.js b/tests/baselines/reference/mappedTypeModifiers.js
index 8489194f95a..9a99441b814 100644
--- a/tests/baselines/reference/mappedTypeModifiers.js
+++ b/tests/baselines/reference/mappedTypeModifiers.js
@@ -1,145 +1,121 @@
 //// [mappedTypeModifiers.ts]
 
 type T = { a: number, b: string };
-type TU = { a: number | undefined, b: string | undefined };
 type TP = { a?: number, b?: string };
 type TR = { readonly a: number, readonly b: string };
 type TPR = { readonly a?: number, readonly b?: string };
 
-// Validate they all have the same keys
 var v00: "a" | "b";
 var v00: keyof T;
-var v00: keyof TU;
 var v00: keyof TP;
 var v00: keyof TR;
 var v00: keyof TPR;
 
-// Validate that non-isomorphic mapped types strip modifiers
 var v01: T;
-var v01: Pick<TR, keyof T>;
-var v01: Pick<Readonly<T>, keyof T>;
+var v01: { [P in keyof T]: T[P] };
+var v01: Pick<T, keyof T>;
+var v01: Pick<Pick<T, keyof T>, keyof T>;
 
-// Validate that non-isomorphic mapped types strip modifiers
-var v02: TU;
+var v02: TP;
+var v02: { [P in keyof T]?: T[P] };
+var v02: Partial<T>;
 var v02: Pick<TP, keyof T>;
-var v02: Pick<TPR, keyof T>;
-var v02: Pick<Partial<T>, keyof T>;
-var v02: Pick<Partial<Readonly<T>>, keyof T>;
 
-// Validate that isomorphic mapped types preserve optional modifier
-var v03: TP;
-var v03: Partial<T>;
+var v03: TR;
+var v03: { readonly [P in keyof T]: T[P] };
+var v03: Readonly<T>;
+var v03: Pick<TR, keyof T>;
 
-// Validate that isomorphic mapped types preserve readonly modifier
-var v04: TR;
-var v04: Readonly<T>;
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var v05: TPR;
-var v05: Partial<TR>;
-var v05: Readonly<TP>;
-var v05: Partial<Readonly<T>>;
-var v05: Readonly<Partial<T>>;
+var v04: TPR;
+var v04: { readonly [P in keyof T]?: T[P] };
+var v04: Partial<TR>;
+var v04: Readonly<TP>;
+var v04: Partial<Readonly<T>>;
+var v04: Readonly<Partial<T>>;
+var v04: Pick<TPR, keyof T>;
 
 type Boxified<T> = { [P in keyof T]: { x: T[P] } };
 
 type B = { a: { x: number }, b: { x: string } };
-type BU = { a: { x: number } | undefined, b: { x: string } | undefined };
 type BP = { a?: { x: number }, b?: { x: string } };
 type BR = { readonly a: { x: number }, readonly b: { x: string } };
 type BPR = { readonly a?: { x: number }, readonly b?: { x: string } };
 
-// Validate they all have the same keys
 var b00: "a" | "b";
 var b00: keyof B;
-var b00: keyof BU;
 var b00: keyof BP;
 var b00: keyof BR;
 var b00: keyof BPR;
 
-// Validate that non-isomorphic mapped types strip modifiers
 var b01: B;
-var b01: Pick<BR, keyof B>;
-var b01: Pick<Readonly<BR>, keyof B>;
+var b01: { [P in keyof B]: B[P] };
+var b01: Pick<B, keyof B>;
+var b01: Pick<Pick<B, keyof B>, keyof B>;
 
-// Validate that non-isomorphic mapped types strip modifiers
-var b02: BU;
+var b02: BP;
+var b02: { [P in keyof B]?: B[P] };
+var b02: Partial<B>;
 var b02: Pick<BP, keyof B>;
-var b02: Pick<BPR, keyof B>;
-var b02: Pick<Partial<B>, keyof B>;
-var b02: Pick<Partial<Readonly<B>>, keyof B>;
 
-// Validate that isomorphic mapped types preserve optional modifier
-var b03: BP;
-var b03: Partial<B>;
+var b03: BR;
+var b03: { readonly [P in keyof B]: B[P] };
+var b03: Readonly<B>;
+var b03: Pick<BR, keyof B>;
 
-// Validate that isomorphic mapped types preserve readonly modifier
-var b04: BR;
-var b04: Readonly<B>;
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var b05: BPR;
-var b05: Partial<BR>;
-var b05: Readonly<BP>;
-var b05: Partial<Readonly<B>>;
-var b05: Readonly<Partial<B>>;
+var b04: BPR;
+var b04: { readonly [P in keyof B]?: B[P] };
+var b04: Partial<BR>;
+var b04: Readonly<BP>;
+var b04: Partial<Readonly<B>>;
+var b04: Readonly<Partial<B>>;
+var b04: Pick<BPR, keyof B>;
 
 //// [mappedTypeModifiers.js]
-// Validate they all have the same keys
 var v00;
 var v00;
 var v00;
 var v00;
 var v00;
-var v00;
-// Validate that non-isomorphic mapped types strip modifiers
 var v01;
 var v01;
 var v01;
-// Validate that non-isomorphic mapped types strip modifiers
+var v01;
 var v02;
 var v02;
 var v02;
 var v02;
-var v02;
-// Validate that isomorphic mapped types preserve optional modifier
 var v03;
 var v03;
-// Validate that isomorphic mapped types preserve readonly modifier
+var v03;
+var v03;
+var v04;
+var v04;
+var v04;
+var v04;
+var v04;
 var v04;
 var v04;
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var v05;
-var v05;
-var v05;
-var v05;
-var v05;
-// Validate they all have the same keys
 var b00;
 var b00;
 var b00;
 var b00;
 var b00;
-var b00;
-// Validate that non-isomorphic mapped types strip modifiers
 var b01;
 var b01;
 var b01;
-// Validate that non-isomorphic mapped types strip modifiers
+var b01;
 var b02;
 var b02;
 var b02;
 var b02;
-var b02;
-// Validate that isomorphic mapped types preserve optional modifier
 var b03;
 var b03;
-// Validate that isomorphic mapped types preserve readonly modifier
+var b03;
+var b03;
+var b04;
+var b04;
+var b04;
+var b04;
+var b04;
 var b04;
 var b04;
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var b05;
-var b05;
-var b05;
-var b05;
-var b05;
diff --git a/tests/baselines/reference/mappedTypeModifiers.symbols b/tests/baselines/reference/mappedTypeModifiers.symbols
index 8be2f53cec1..907f8ba8246 100644
--- a/tests/baselines/reference/mappedTypeModifiers.symbols
+++ b/tests/baselines/reference/mappedTypeModifiers.symbols
@@ -5,309 +5,309 @@ type T = { a: number, b: string };
 >a : Symbol(a, Decl(mappedTypeModifiers.ts, 1, 10))
 >b : Symbol(b, Decl(mappedTypeModifiers.ts, 1, 21))
 
-type TU = { a: number | undefined, b: string | undefined };
->TU : Symbol(TU, Decl(mappedTypeModifiers.ts, 1, 34))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 2, 11))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 2, 34))
-
 type TP = { a?: number, b?: string };
->TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 2, 59))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 3, 11))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 3, 23))
+>TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 1, 34))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 2, 11))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 2, 23))
 
 type TR = { readonly a: number, readonly b: string };
->TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 3, 37))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 4, 11))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 4, 31))
+>TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 2, 37))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 3, 11))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 3, 31))
 
 type TPR = { readonly a?: number, readonly b?: string };
->TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 4, 53))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 5, 12))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 5, 33))
+>TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 3, 53))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 4, 12))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 4, 33))
 
-// Validate they all have the same keys
 var v00: "a" | "b";
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
+>v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 6, 3), Decl(mappedTypeModifiers.ts, 7, 3), Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3))
 
 var v00: keyof T;
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
+>v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 6, 3), Decl(mappedTypeModifiers.ts, 7, 3), Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-var v00: keyof TU;
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
->TU : Symbol(TU, Decl(mappedTypeModifiers.ts, 1, 34))
-
 var v00: keyof TP;
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
->TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 2, 59))
+>v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 6, 3), Decl(mappedTypeModifiers.ts, 7, 3), Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3))
+>TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 1, 34))
 
 var v00: keyof TR;
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
->TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 3, 37))
+>v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 6, 3), Decl(mappedTypeModifiers.ts, 7, 3), Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3))
+>TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 2, 37))
 
 var v00: keyof TPR;
->v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3), Decl(mappedTypeModifiers.ts, 11, 3), Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3))
->TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 4, 53))
+>v00 : Symbol(v00, Decl(mappedTypeModifiers.ts, 6, 3), Decl(mappedTypeModifiers.ts, 7, 3), Decl(mappedTypeModifiers.ts, 8, 3), Decl(mappedTypeModifiers.ts, 9, 3), Decl(mappedTypeModifiers.ts, 10, 3))
+>TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 3, 53))
 
-// Validate that non-isomorphic mapped types strip modifiers
 var v01: T;
->v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 16, 3), Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3))
+>v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3), Decl(mappedTypeModifiers.ts, 14, 3), Decl(mappedTypeModifiers.ts, 15, 3))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-var v01: Pick<TR, keyof T>;
->v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 16, 3), Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3))
+var v01: { [P in keyof T]: T[P] };
+>v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3), Decl(mappedTypeModifiers.ts, 14, 3), Decl(mappedTypeModifiers.ts, 15, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 13, 12))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 13, 12))
+
+var v01: Pick<T, keyof T>;
+>v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3), Decl(mappedTypeModifiers.ts, 14, 3), Decl(mappedTypeModifiers.ts, 15, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 3, 37))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-var v01: Pick<Readonly<T>, keyof T>;
->v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 16, 3), Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3))
+var v01: Pick<Pick<T, keyof T>, keyof T>;
+>v01 : Symbol(v01, Decl(mappedTypeModifiers.ts, 12, 3), Decl(mappedTypeModifiers.ts, 13, 3), Decl(mappedTypeModifiers.ts, 14, 3), Decl(mappedTypeModifiers.ts, 15, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-// Validate that non-isomorphic mapped types strip modifiers
-var v02: TU;
->v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 21, 3), Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
->TU : Symbol(TU, Decl(mappedTypeModifiers.ts, 1, 34))
+var v02: TP;
+>v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3), Decl(mappedTypeModifiers.ts, 19, 3), Decl(mappedTypeModifiers.ts, 20, 3))
+>TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 1, 34))
+
+var v02: { [P in keyof T]?: T[P] };
+>v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3), Decl(mappedTypeModifiers.ts, 19, 3), Decl(mappedTypeModifiers.ts, 20, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 18, 12))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 18, 12))
+
+var v02: Partial<T>;
+>v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3), Decl(mappedTypeModifiers.ts, 19, 3), Decl(mappedTypeModifiers.ts, 20, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
 var v02: Pick<TP, keyof T>;
->v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 21, 3), Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+>v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 17, 3), Decl(mappedTypeModifiers.ts, 18, 3), Decl(mappedTypeModifiers.ts, 19, 3), Decl(mappedTypeModifiers.ts, 20, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 2, 59))
+>TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 1, 34))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-var v02: Pick<TPR, keyof T>;
->v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 21, 3), Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+var v03: TR;
+>v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+>TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 2, 37))
+
+var v03: { readonly [P in keyof T]: T[P] };
+>v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 23, 21))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 23, 21))
+
+var v03: Readonly<T>;
+>v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+
+var v03: Pick<TR, keyof T>;
+>v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 4, 53))
+>TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 2, 37))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
-var v02: Pick<Partial<T>, keyof T>;
->v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 21, 3), Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
+var v04: TPR;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 3, 53))
+
+var v04: { readonly [P in keyof T]?: T[P] };
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 28, 21))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 28, 21))
+
+var v04: Partial<TR>;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 2, 37))
+
+var v04: Readonly<TP>;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 1, 34))
+
+var v04: Partial<Readonly<T>>;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+
+var v04: Readonly<Partial<T>>;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
+
+var v04: Pick<TPR, keyof T>;
+>v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 27, 3), Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3), Decl(mappedTypeModifiers.ts, 30, 3), Decl(mappedTypeModifiers.ts, 31, 3), Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
-
-var v02: Pick<Partial<Readonly<T>>, keyof T>;
->v02 : Symbol(v02, Decl(mappedTypeModifiers.ts, 21, 3), Decl(mappedTypeModifiers.ts, 22, 3), Decl(mappedTypeModifiers.ts, 23, 3), Decl(mappedTypeModifiers.ts, 24, 3), Decl(mappedTypeModifiers.ts, 25, 3))
->Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
-
-// Validate that isomorphic mapped types preserve optional modifier
-var v03: TP;
->v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3))
->TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 2, 59))
-
-var v03: Partial<T>;
->v03 : Symbol(v03, Decl(mappedTypeModifiers.ts, 28, 3), Decl(mappedTypeModifiers.ts, 29, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
-
-// Validate that isomorphic mapped types preserve readonly modifier
-var v04: TR;
->v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
->TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 3, 37))
-
-var v04: Readonly<T>;
->v04 : Symbol(v04, Decl(mappedTypeModifiers.ts, 32, 3), Decl(mappedTypeModifiers.ts, 33, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var v05: TPR;
->v05 : Symbol(v05, Decl(mappedTypeModifiers.ts, 36, 3), Decl(mappedTypeModifiers.ts, 37, 3), Decl(mappedTypeModifiers.ts, 38, 3), Decl(mappedTypeModifiers.ts, 39, 3), Decl(mappedTypeModifiers.ts, 40, 3))
->TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 4, 53))
-
-var v05: Partial<TR>;
->v05 : Symbol(v05, Decl(mappedTypeModifiers.ts, 36, 3), Decl(mappedTypeModifiers.ts, 37, 3), Decl(mappedTypeModifiers.ts, 38, 3), Decl(mappedTypeModifiers.ts, 39, 3), Decl(mappedTypeModifiers.ts, 40, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->TR : Symbol(TR, Decl(mappedTypeModifiers.ts, 3, 37))
-
-var v05: Readonly<TP>;
->v05 : Symbol(v05, Decl(mappedTypeModifiers.ts, 36, 3), Decl(mappedTypeModifiers.ts, 37, 3), Decl(mappedTypeModifiers.ts, 38, 3), Decl(mappedTypeModifiers.ts, 39, 3), Decl(mappedTypeModifiers.ts, 40, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->TP : Symbol(TP, Decl(mappedTypeModifiers.ts, 2, 59))
-
-var v05: Partial<Readonly<T>>;
->v05 : Symbol(v05, Decl(mappedTypeModifiers.ts, 36, 3), Decl(mappedTypeModifiers.ts, 37, 3), Decl(mappedTypeModifiers.ts, 38, 3), Decl(mappedTypeModifiers.ts, 39, 3), Decl(mappedTypeModifiers.ts, 40, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
-
-var v05: Readonly<Partial<T>>;
->v05 : Symbol(v05, Decl(mappedTypeModifiers.ts, 36, 3), Decl(mappedTypeModifiers.ts, 37, 3), Decl(mappedTypeModifiers.ts, 38, 3), Decl(mappedTypeModifiers.ts, 39, 3), Decl(mappedTypeModifiers.ts, 40, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>TPR : Symbol(TPR, Decl(mappedTypeModifiers.ts, 3, 53))
 >T : Symbol(T, Decl(mappedTypeModifiers.ts, 0, 0))
 
 type Boxified<T> = { [P in keyof T]: { x: T[P] } };
->Boxified : Symbol(Boxified, Decl(mappedTypeModifiers.ts, 40, 30))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 42, 14))
->P : Symbol(P, Decl(mappedTypeModifiers.ts, 42, 22))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 42, 14))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 42, 38))
->T : Symbol(T, Decl(mappedTypeModifiers.ts, 42, 14))
->P : Symbol(P, Decl(mappedTypeModifiers.ts, 42, 22))
+>Boxified : Symbol(Boxified, Decl(mappedTypeModifiers.ts, 33, 28))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 35, 14))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 35, 22))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 35, 14))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 35, 38))
+>T : Symbol(T, Decl(mappedTypeModifiers.ts, 35, 14))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 35, 22))
 
 type B = { a: { x: number }, b: { x: string } };
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 44, 10))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 44, 15))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 44, 28))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 44, 33))
-
-type BU = { a: { x: number } | undefined, b: { x: string } | undefined };
->BU : Symbol(BU, Decl(mappedTypeModifiers.ts, 44, 48))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 45, 11))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 45, 16))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 45, 41))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 45, 46))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 37, 10))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 37, 15))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 37, 28))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 37, 33))
 
 type BP = { a?: { x: number }, b?: { x: string } };
->BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 45, 73))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 46, 11))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 46, 17))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 46, 30))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 46, 36))
+>BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 37, 48))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 38, 11))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 38, 17))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 38, 30))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 38, 36))
 
 type BR = { readonly a: { x: number }, readonly b: { x: string } };
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 47, 11))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 47, 25))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 47, 38))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 47, 52))
+>BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 38, 51))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 39, 11))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 39, 25))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 39, 38))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 39, 52))
 
 type BPR = { readonly a?: { x: number }, readonly b?: { x: string } };
->BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 47, 67))
->a : Symbol(a, Decl(mappedTypeModifiers.ts, 48, 12))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 48, 27))
->b : Symbol(b, Decl(mappedTypeModifiers.ts, 48, 40))
->x : Symbol(x, Decl(mappedTypeModifiers.ts, 48, 55))
+>BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 39, 67))
+>a : Symbol(a, Decl(mappedTypeModifiers.ts, 40, 12))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 40, 27))
+>b : Symbol(b, Decl(mappedTypeModifiers.ts, 40, 40))
+>x : Symbol(x, Decl(mappedTypeModifiers.ts, 40, 55))
 
-// Validate they all have the same keys
 var b00: "a" | "b";
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
+>b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 42, 3), Decl(mappedTypeModifiers.ts, 43, 3), Decl(mappedTypeModifiers.ts, 44, 3), Decl(mappedTypeModifiers.ts, 45, 3), Decl(mappedTypeModifiers.ts, 46, 3))
 
 var b00: keyof B;
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-var b00: keyof BU;
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
->BU : Symbol(BU, Decl(mappedTypeModifiers.ts, 44, 48))
+>b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 42, 3), Decl(mappedTypeModifiers.ts, 43, 3), Decl(mappedTypeModifiers.ts, 44, 3), Decl(mappedTypeModifiers.ts, 45, 3), Decl(mappedTypeModifiers.ts, 46, 3))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
 var b00: keyof BP;
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
->BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 45, 73))
+>b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 42, 3), Decl(mappedTypeModifiers.ts, 43, 3), Decl(mappedTypeModifiers.ts, 44, 3), Decl(mappedTypeModifiers.ts, 45, 3), Decl(mappedTypeModifiers.ts, 46, 3))
+>BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 37, 48))
 
 var b00: keyof BR;
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
+>b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 42, 3), Decl(mappedTypeModifiers.ts, 43, 3), Decl(mappedTypeModifiers.ts, 44, 3), Decl(mappedTypeModifiers.ts, 45, 3), Decl(mappedTypeModifiers.ts, 46, 3))
+>BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 38, 51))
 
 var b00: keyof BPR;
->b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 51, 3), Decl(mappedTypeModifiers.ts, 52, 3), Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
->BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 47, 67))
+>b00 : Symbol(b00, Decl(mappedTypeModifiers.ts, 42, 3), Decl(mappedTypeModifiers.ts, 43, 3), Decl(mappedTypeModifiers.ts, 44, 3), Decl(mappedTypeModifiers.ts, 45, 3), Decl(mappedTypeModifiers.ts, 46, 3))
+>BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 39, 67))
 
-// Validate that non-isomorphic mapped types strip modifiers
 var b01: B;
->b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 48, 3), Decl(mappedTypeModifiers.ts, 49, 3), Decl(mappedTypeModifiers.ts, 50, 3), Decl(mappedTypeModifiers.ts, 51, 3))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
-var b01: Pick<BR, keyof B>;
->b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
+var b01: { [P in keyof B]: B[P] };
+>b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 48, 3), Decl(mappedTypeModifiers.ts, 49, 3), Decl(mappedTypeModifiers.ts, 50, 3), Decl(mappedTypeModifiers.ts, 51, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 49, 12))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 49, 12))
+
+var b01: Pick<B, keyof B>;
+>b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 48, 3), Decl(mappedTypeModifiers.ts, 49, 3), Decl(mappedTypeModifiers.ts, 50, 3), Decl(mappedTypeModifiers.ts, 51, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
-var b01: Pick<Readonly<BR>, keyof B>;
->b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
+var b01: Pick<Pick<B, keyof B>, keyof B>;
+>b01 : Symbol(b01, Decl(mappedTypeModifiers.ts, 48, 3), Decl(mappedTypeModifiers.ts, 49, 3), Decl(mappedTypeModifiers.ts, 50, 3), Decl(mappedTypeModifiers.ts, 51, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
-// Validate that non-isomorphic mapped types strip modifiers
-var b02: BU;
->b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3))
->BU : Symbol(BU, Decl(mappedTypeModifiers.ts, 44, 48))
+var b02: BP;
+>b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
+>BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 37, 48))
+
+var b02: { [P in keyof B]?: B[P] };
+>b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 54, 12))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 54, 12))
+
+var b02: Partial<B>;
+>b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
 var b02: Pick<BP, keyof B>;
->b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3))
+>b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 53, 3), Decl(mappedTypeModifiers.ts, 54, 3), Decl(mappedTypeModifiers.ts, 55, 3), Decl(mappedTypeModifiers.ts, 56, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 45, 73))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 37, 48))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
-var b02: Pick<BPR, keyof B>;
->b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3))
+var b03: BR;
+>b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 58, 3), Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
+>BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 38, 51))
+
+var b03: { readonly [P in keyof B]: B[P] };
+>b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 58, 3), Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 59, 21))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 59, 21))
+
+var b03: Readonly<B>;
+>b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 58, 3), Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+
+var b03: Pick<BR, keyof B>;
+>b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 58, 3), Decl(mappedTypeModifiers.ts, 59, 3), Decl(mappedTypeModifiers.ts, 60, 3), Decl(mappedTypeModifiers.ts, 61, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 47, 67))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 38, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
-var b02: Pick<Partial<B>, keyof B>;
->b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3))
+var b04: BPR;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 39, 67))
+
+var b04: { readonly [P in keyof B]?: B[P] };
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 64, 21))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+>P : Symbol(P, Decl(mappedTypeModifiers.ts, 64, 21))
+
+var b04: Partial<BR>;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 38, 51))
+
+var b04: Readonly<BP>;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 37, 48))
+
+var b04: Partial<Readonly<B>>;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+
+var b04: Readonly<Partial<B>>;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
+>Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
+
+var b04: Pick<BPR, keyof B>;
+>b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 63, 3), Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3), Decl(mappedTypeModifiers.ts, 69, 3))
 >Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-var b02: Pick<Partial<Readonly<B>>, keyof B>;
->b02 : Symbol(b02, Decl(mappedTypeModifiers.ts, 64, 3), Decl(mappedTypeModifiers.ts, 65, 3), Decl(mappedTypeModifiers.ts, 66, 3), Decl(mappedTypeModifiers.ts, 67, 3), Decl(mappedTypeModifiers.ts, 68, 3))
->Pick : Symbol(Pick, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-// Validate that isomorphic mapped types preserve optional modifier
-var b03: BP;
->b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 71, 3), Decl(mappedTypeModifiers.ts, 72, 3))
->BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 45, 73))
-
-var b03: Partial<B>;
->b03 : Symbol(b03, Decl(mappedTypeModifiers.ts, 71, 3), Decl(mappedTypeModifiers.ts, 72, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-// Validate that isomorphic mapped types preserve readonly modifier
-var b04: BR;
->b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 75, 3), Decl(mappedTypeModifiers.ts, 76, 3))
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
-
-var b04: Readonly<B>;
->b04 : Symbol(b04, Decl(mappedTypeModifiers.ts, 75, 3), Decl(mappedTypeModifiers.ts, 76, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var b05: BPR;
->b05 : Symbol(b05, Decl(mappedTypeModifiers.ts, 79, 3), Decl(mappedTypeModifiers.ts, 80, 3), Decl(mappedTypeModifiers.ts, 81, 3), Decl(mappedTypeModifiers.ts, 82, 3), Decl(mappedTypeModifiers.ts, 83, 3))
->BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 47, 67))
-
-var b05: Partial<BR>;
->b05 : Symbol(b05, Decl(mappedTypeModifiers.ts, 79, 3), Decl(mappedTypeModifiers.ts, 80, 3), Decl(mappedTypeModifiers.ts, 81, 3), Decl(mappedTypeModifiers.ts, 82, 3), Decl(mappedTypeModifiers.ts, 83, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->BR : Symbol(BR, Decl(mappedTypeModifiers.ts, 46, 51))
-
-var b05: Readonly<BP>;
->b05 : Symbol(b05, Decl(mappedTypeModifiers.ts, 79, 3), Decl(mappedTypeModifiers.ts, 80, 3), Decl(mappedTypeModifiers.ts, 81, 3), Decl(mappedTypeModifiers.ts, 82, 3), Decl(mappedTypeModifiers.ts, 83, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->BP : Symbol(BP, Decl(mappedTypeModifiers.ts, 45, 73))
-
-var b05: Partial<Readonly<B>>;
->b05 : Symbol(b05, Decl(mappedTypeModifiers.ts, 79, 3), Decl(mappedTypeModifiers.ts, 80, 3), Decl(mappedTypeModifiers.ts, 81, 3), Decl(mappedTypeModifiers.ts, 82, 3), Decl(mappedTypeModifiers.ts, 83, 3))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
-
-var b05: Readonly<Partial<B>>;
->b05 : Symbol(b05, Decl(mappedTypeModifiers.ts, 79, 3), Decl(mappedTypeModifiers.ts, 80, 3), Decl(mappedTypeModifiers.ts, 81, 3), Decl(mappedTypeModifiers.ts, 82, 3), Decl(mappedTypeModifiers.ts, 83, 3))
->Readonly : Symbol(Readonly, Decl(lib.d.ts, --, --))
->Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
->B : Symbol(B, Decl(mappedTypeModifiers.ts, 42, 51))
+>BPR : Symbol(BPR, Decl(mappedTypeModifiers.ts, 39, 67))
+>B : Symbol(B, Decl(mappedTypeModifiers.ts, 35, 51))
 
diff --git a/tests/baselines/reference/mappedTypeModifiers.types b/tests/baselines/reference/mappedTypeModifiers.types
index 61b9bfc03b2..befda7e371e 100644
--- a/tests/baselines/reference/mappedTypeModifiers.types
+++ b/tests/baselines/reference/mappedTypeModifiers.types
@@ -5,11 +5,6 @@ type T = { a: number, b: string };
 >a : number
 >b : string
 
-type TU = { a: number | undefined, b: string | undefined };
->TU : TU
->a : number | undefined
->b : string | undefined
-
 type TP = { a?: number, b?: string };
 >TP : TP
 >a : number | undefined
@@ -25,7 +20,6 @@ type TPR = { readonly a?: number, readonly b?: string };
 >a : number | undefined
 >b : string | undefined
 
-// Validate they all have the same keys
 var v00: "a" | "b";
 >v00 : "a" | "b"
 
@@ -33,10 +27,6 @@ var v00: keyof T;
 >v00 : "a" | "b"
 >T : T
 
-var v00: keyof TU;
->v00 : "a" | "b"
->TU : TU
-
 var v00: keyof TP;
 >v00 : "a" | "b"
 >TP : TP
@@ -49,103 +39,114 @@ var v00: keyof TPR;
 >v00 : "a" | "b"
 >TPR : TPR
 
-// Validate that non-isomorphic mapped types strip modifiers
 var v01: T;
 >v01 : T
 >T : T
 
-var v01: Pick<TR, keyof T>;
+var v01: { [P in keyof T]: T[P] };
 >v01 : T
->Pick : Pick<T, K>
->TR : TR
+>P : P
 >T : T
+>T : T
+>P : P
 
-var v01: Pick<Readonly<T>, keyof T>;
+var v01: Pick<T, keyof T>;
 >v01 : T
 >Pick : Pick<T, K>
->Readonly : Readonly<T>
 >T : T
 >T : T
 
-// Validate that non-isomorphic mapped types strip modifiers
-var v02: TU;
->v02 : TU
->TU : TU
+var v01: Pick<Pick<T, keyof T>, keyof T>;
+>v01 : T
+>Pick : Pick<T, K>
+>Pick : Pick<T, K>
+>T : T
+>T : T
+>T : T
+
+var v02: TP;
+>v02 : TP
+>TP : TP
+
+var v02: { [P in keyof T]?: T[P] };
+>v02 : TP
+>P : P
+>T : T
+>T : T
+>P : P
+
+var v02: Partial<T>;
+>v02 : TP
+>Partial : Partial<T>
+>T : T
 
 var v02: Pick<TP, keyof T>;
->v02 : TU
+>v02 : TP
 >Pick : Pick<T, K>
 >TP : TP
 >T : T
 
-var v02: Pick<TPR, keyof T>;
->v02 : TU
->Pick : Pick<T, K>
->TPR : TPR
->T : T
-
-var v02: Pick<Partial<T>, keyof T>;
->v02 : TU
->Pick : Pick<T, K>
->Partial : Partial<T>
->T : T
->T : T
-
-var v02: Pick<Partial<Readonly<T>>, keyof T>;
->v02 : TU
->Pick : Pick<T, K>
->Partial : Partial<T>
->Readonly : Readonly<T>
->T : T
->T : T
-
-// Validate that isomorphic mapped types preserve optional modifier
-var v03: TP;
->v03 : TP
->TP : TP
-
-var v03: Partial<T>;
->v03 : TP
->Partial : Partial<T>
->T : T
-
-// Validate that isomorphic mapped types preserve readonly modifier
-var v04: TR;
->v04 : TR
+var v03: TR;
+>v03 : TR
 >TR : TR
 
-var v04: Readonly<T>;
->v04 : TR
+var v03: { readonly [P in keyof T]: T[P] };
+>v03 : TR
+>P : P
+>T : T
+>T : T
+>P : P
+
+var v03: Readonly<T>;
+>v03 : TR
 >Readonly : Readonly<T>
 >T : T
 
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var v05: TPR;
->v05 : TPR
+var v03: Pick<TR, keyof T>;
+>v03 : TR
+>Pick : Pick<T, K>
+>TR : TR
+>T : T
+
+var v04: TPR;
+>v04 : TPR
 >TPR : TPR
 
-var v05: Partial<TR>;
->v05 : TPR
+var v04: { readonly [P in keyof T]?: T[P] };
+>v04 : TPR
+>P : P
+>T : T
+>T : T
+>P : P
+
+var v04: Partial<TR>;
+>v04 : TPR
 >Partial : Partial<T>
 >TR : TR
 
-var v05: Readonly<TP>;
->v05 : TPR
+var v04: Readonly<TP>;
+>v04 : TPR
 >Readonly : Readonly<T>
 >TP : TP
 
-var v05: Partial<Readonly<T>>;
->v05 : TPR
+var v04: Partial<Readonly<T>>;
+>v04 : TPR
 >Partial : Partial<T>
 >Readonly : Readonly<T>
 >T : T
 
-var v05: Readonly<Partial<T>>;
->v05 : TPR
+var v04: Readonly<Partial<T>>;
+>v04 : TPR
 >Readonly : Readonly<T>
 >Partial : Partial<T>
 >T : T
 
+var v04: Pick<TPR, keyof T>;
+>v04 : TPR
+>Pick : Pick<T, K>
+>TPR : TPR
+>T : T
+
 type Boxified<T> = { [P in keyof T]: { x: T[P] } };
 >Boxified : Boxified<T>
 >T : T
@@ -162,13 +163,6 @@ type B = { a: { x: number }, b: { x: string } };
 >b : { x: string; }
 >x : string
 
-type BU = { a: { x: number } | undefined, b: { x: string } | undefined };
->BU : BU
->a : { x: number; } | undefined
->x : number
->b : { x: string; } | undefined
->x : string
-
 type BP = { a?: { x: number }, b?: { x: string } };
 >BP : BP
 >a : { x: number; } | undefined
@@ -190,7 +184,6 @@ type BPR = { readonly a?: { x: number }, readonly b?: { x: string } };
 >b : { x: string; } | undefined
 >x : string
 
-// Validate they all have the same keys
 var b00: "a" | "b";
 >b00 : "a" | "b"
 
@@ -198,10 +191,6 @@ var b00: keyof B;
 >b00 : "a" | "b"
 >B : B
 
-var b00: keyof BU;
->b00 : "a" | "b"
->BU : BU
-
 var b00: keyof BP;
 >b00 : "a" | "b"
 >BP : BP
@@ -214,100 +203,111 @@ var b00: keyof BPR;
 >b00 : "a" | "b"
 >BPR : BPR
 
-// Validate that non-isomorphic mapped types strip modifiers
 var b01: B;
 >b01 : B
 >B : B
 
-var b01: Pick<BR, keyof B>;
+var b01: { [P in keyof B]: B[P] };
+>b01 : B
+>P : P
+>B : B
+>B : B
+>P : P
+
+var b01: Pick<B, keyof B>;
 >b01 : B
 >Pick : Pick<T, K>
->BR : BR
+>B : B
 >B : B
 
-var b01: Pick<Readonly<BR>, keyof B>;
+var b01: Pick<Pick<B, keyof B>, keyof B>;
 >b01 : B
 >Pick : Pick<T, K>
->Readonly : Readonly<T>
->BR : BR
+>Pick : Pick<T, K>
+>B : B
+>B : B
 >B : B
 
-// Validate that non-isomorphic mapped types strip modifiers
-var b02: BU;
->b02 : BU
->BU : BU
+var b02: BP;
+>b02 : BP
+>BP : BP
+
+var b02: { [P in keyof B]?: B[P] };
+>b02 : BP
+>P : P
+>B : B
+>B : B
+>P : P
+
+var b02: Partial<B>;
+>b02 : BP
+>Partial : Partial<T>
+>B : B
 
 var b02: Pick<BP, keyof B>;
->b02 : BU
+>b02 : BP
 >Pick : Pick<T, K>
 >BP : BP
 >B : B
 
-var b02: Pick<BPR, keyof B>;
->b02 : BU
+var b03: BR;
+>b03 : BR
+>BR : BR
+
+var b03: { readonly [P in keyof B]: B[P] };
+>b03 : BR
+>P : P
+>B : B
+>B : B
+>P : P
+
+var b03: Readonly<B>;
+>b03 : BR
+>Readonly : Readonly<T>
+>B : B
+
+var b03: Pick<BR, keyof B>;
+>b03 : BR
+>Pick : Pick<T, K>
+>BR : BR
+>B : B
+
+var b04: BPR;
+>b04 : BPR
+>BPR : BPR
+
+var b04: { readonly [P in keyof B]?: B[P] };
+>b04 : BPR
+>P : P
+>B : B
+>B : B
+>P : P
+
+var b04: Partial<BR>;
+>b04 : BPR
+>Partial : Partial<T>
+>BR : BR
+
+var b04: Readonly<BP>;
+>b04 : BPR
+>Readonly : Readonly<T>
+>BP : BP
+
+var b04: Partial<Readonly<B>>;
+>b04 : BPR
+>Partial : Partial<T>
+>Readonly : Readonly<T>
+>B : B
+
+var b04: Readonly<Partial<B>>;
+>b04 : BPR
+>Readonly : Readonly<T>
+>Partial : Partial<T>
+>B : B
+
+var b04: Pick<BPR, keyof B>;
+>b04 : BPR
 >Pick : Pick<T, K>
 >BPR : BPR
 >B : B
 
-var b02: Pick<Partial<B>, keyof B>;
->b02 : BU
->Pick : Pick<T, K>
->Partial : Partial<T>
->B : B
->B : B
-
-var b02: Pick<Partial<Readonly<B>>, keyof B>;
->b02 : BU
->Pick : Pick<T, K>
->Partial : Partial<T>
->Readonly : Readonly<T>
->B : B
->B : B
-
-// Validate that isomorphic mapped types preserve optional modifier
-var b03: BP;
->b03 : BP
->BP : BP
-
-var b03: Partial<B>;
->b03 : BP
->Partial : Partial<T>
->B : B
-
-// Validate that isomorphic mapped types preserve readonly modifier
-var b04: BR;
->b04 : BR
->BR : BR
-
-var b04: Readonly<B>;
->b04 : BR
->Readonly : Readonly<T>
->B : B
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var b05: BPR;
->b05 : BPR
->BPR : BPR
-
-var b05: Partial<BR>;
->b05 : BPR
->Partial : Partial<T>
->BR : BR
-
-var b05: Readonly<BP>;
->b05 : BPR
->Readonly : Readonly<T>
->BP : BP
-
-var b05: Partial<Readonly<B>>;
->b05 : BPR
->Partial : Partial<T>
->Readonly : Readonly<T>
->B : B
-
-var b05: Readonly<Partial<B>>;
->b05 : BPR
->Readonly : Readonly<T>
->Partial : Partial<T>
->B : B
-
diff --git a/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.js b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.js
new file mode 100644
index 00000000000..15d6fc70da5
--- /dev/null
+++ b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.js
@@ -0,0 +1,51 @@
+//// [subSubClassCanAccessProtectedConstructor.ts]
+class Base {
+    protected constructor() { }
+    public instance1 = new Base(); // allowed
+}
+
+class Subclass extends Base {
+    public instance1_1 = new Base(); // allowed
+    public instance1_2 = new Subclass(); // allowed
+}
+
+class SubclassOfSubclass extends Subclass {
+    public instance2_1 = new Base(); // allowed
+    public instance2_2 = new Subclass(); // allowed
+    public instance2_3 = new SubclassOfSubclass(); // allowed
+}
+
+
+//// [subSubClassCanAccessProtectedConstructor.js]
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var Base = (function () {
+    function Base() {
+        this.instance1 = new Base(); // allowed
+    }
+    return Base;
+}());
+var Subclass = (function (_super) {
+    __extends(Subclass, _super);
+    function Subclass() {
+        var _this = _super.apply(this, arguments) || this;
+        _this.instance1_1 = new Base(); // allowed
+        _this.instance1_2 = new Subclass(); // allowed
+        return _this;
+    }
+    return Subclass;
+}(Base));
+var SubclassOfSubclass = (function (_super) {
+    __extends(SubclassOfSubclass, _super);
+    function SubclassOfSubclass() {
+        var _this = _super.apply(this, arguments) || this;
+        _this.instance2_1 = new Base(); // allowed
+        _this.instance2_2 = new Subclass(); // allowed
+        _this.instance2_3 = new SubclassOfSubclass(); // allowed
+        return _this;
+    }
+    return SubclassOfSubclass;
+}(Subclass));
diff --git a/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.symbols b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.symbols
new file mode 100644
index 00000000000..8a09e1d742f
--- /dev/null
+++ b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.symbols
@@ -0,0 +1,40 @@
+=== tests/cases/compiler/subSubClassCanAccessProtectedConstructor.ts ===
+class Base {
+>Base : Symbol(Base, Decl(subSubClassCanAccessProtectedConstructor.ts, 0, 0))
+
+    protected constructor() { }
+    public instance1 = new Base(); // allowed
+>instance1 : Symbol(Base.instance1, Decl(subSubClassCanAccessProtectedConstructor.ts, 1, 31))
+>Base : Symbol(Base, Decl(subSubClassCanAccessProtectedConstructor.ts, 0, 0))
+}
+
+class Subclass extends Base {
+>Subclass : Symbol(Subclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 3, 1))
+>Base : Symbol(Base, Decl(subSubClassCanAccessProtectedConstructor.ts, 0, 0))
+
+    public instance1_1 = new Base(); // allowed
+>instance1_1 : Symbol(Subclass.instance1_1, Decl(subSubClassCanAccessProtectedConstructor.ts, 5, 29))
+>Base : Symbol(Base, Decl(subSubClassCanAccessProtectedConstructor.ts, 0, 0))
+
+    public instance1_2 = new Subclass(); // allowed
+>instance1_2 : Symbol(Subclass.instance1_2, Decl(subSubClassCanAccessProtectedConstructor.ts, 6, 36))
+>Subclass : Symbol(Subclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 3, 1))
+}
+
+class SubclassOfSubclass extends Subclass {
+>SubclassOfSubclass : Symbol(SubclassOfSubclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 8, 1))
+>Subclass : Symbol(Subclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 3, 1))
+
+    public instance2_1 = new Base(); // allowed
+>instance2_1 : Symbol(SubclassOfSubclass.instance2_1, Decl(subSubClassCanAccessProtectedConstructor.ts, 10, 43))
+>Base : Symbol(Base, Decl(subSubClassCanAccessProtectedConstructor.ts, 0, 0))
+
+    public instance2_2 = new Subclass(); // allowed
+>instance2_2 : Symbol(SubclassOfSubclass.instance2_2, Decl(subSubClassCanAccessProtectedConstructor.ts, 11, 36))
+>Subclass : Symbol(Subclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 3, 1))
+
+    public instance2_3 = new SubclassOfSubclass(); // allowed
+>instance2_3 : Symbol(SubclassOfSubclass.instance2_3, Decl(subSubClassCanAccessProtectedConstructor.ts, 12, 40))
+>SubclassOfSubclass : Symbol(SubclassOfSubclass, Decl(subSubClassCanAccessProtectedConstructor.ts, 8, 1))
+}
+
diff --git a/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.types b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.types
new file mode 100644
index 00000000000..253f6221c78
--- /dev/null
+++ b/tests/baselines/reference/subSubClassCanAccessProtectedConstructor.types
@@ -0,0 +1,46 @@
+=== tests/cases/compiler/subSubClassCanAccessProtectedConstructor.ts ===
+class Base {
+>Base : Base
+
+    protected constructor() { }
+    public instance1 = new Base(); // allowed
+>instance1 : Base
+>new Base() : Base
+>Base : typeof Base
+}
+
+class Subclass extends Base {
+>Subclass : Subclass
+>Base : Base
+
+    public instance1_1 = new Base(); // allowed
+>instance1_1 : Base
+>new Base() : Base
+>Base : typeof Base
+
+    public instance1_2 = new Subclass(); // allowed
+>instance1_2 : Subclass
+>new Subclass() : Subclass
+>Subclass : typeof Subclass
+}
+
+class SubclassOfSubclass extends Subclass {
+>SubclassOfSubclass : SubclassOfSubclass
+>Subclass : Subclass
+
+    public instance2_1 = new Base(); // allowed
+>instance2_1 : Base
+>new Base() : Base
+>Base : typeof Base
+
+    public instance2_2 = new Subclass(); // allowed
+>instance2_2 : Subclass
+>new Subclass() : Subclass
+>Subclass : typeof Subclass
+
+    public instance2_3 = new SubclassOfSubclass(); // allowed
+>instance2_3 : SubclassOfSubclass
+>new SubclassOfSubclass() : SubclassOfSubclass
+>SubclassOfSubclass : typeof SubclassOfSubclass
+}
+
diff --git a/tests/baselines/reference/unusedLocalsAndObjectSpread.errors.txt b/tests/baselines/reference/unusedLocalsAndObjectSpread.errors.txt
new file mode 100644
index 00000000000..56f2ba927fb
--- /dev/null
+++ b/tests/baselines/reference/unusedLocalsAndObjectSpread.errors.txt
@@ -0,0 +1,40 @@
+tests/cases/compiler/unusedLocalsAndObjectSpread.ts(21,18): error TS6133: 'bar' is declared but never used.
+tests/cases/compiler/unusedLocalsAndObjectSpread.ts(28,21): error TS6133: 'bar' is declared but never used.
+
+
+==== tests/cases/compiler/unusedLocalsAndObjectSpread.ts (2 errors) ====
+    
+    declare var console: { log(a: any): void };
+    
+    function one() {
+        const foo = { a: 1, b: 2 };
+        // 'a' is declared but never used
+        const {a, ...bar} = foo;
+        console.log(bar);
+    }
+    
+    function two() {
+        const foo = { a: 1, b: 2 };
+        // '_' is declared but never used
+        const {a: _, ...bar} = foo;
+        console.log(bar);
+    }
+    
+    function three() {
+        const foo = { a: 1, b: 2 };
+        // 'a' is declared but never used
+        const {a, ...bar} = foo; // bar should be unused
+                     ~~~
+!!! error TS6133: 'bar' is declared but never used.
+        //console.log(bar);
+    }
+    
+    function four() {
+        const foo = { a: 1, b: 2 };
+        // '_' is declared but never used
+        const {a: _, ...bar} = foo; // bar should be unused
+                        ~~~
+!!! error TS6133: 'bar' is declared but never used.
+        //console.log(bar);
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/unusedLocalsAndObjectSpread.js b/tests/baselines/reference/unusedLocalsAndObjectSpread.js
new file mode 100644
index 00000000000..2878f1b5c5c
--- /dev/null
+++ b/tests/baselines/reference/unusedLocalsAndObjectSpread.js
@@ -0,0 +1,67 @@
+//// [unusedLocalsAndObjectSpread.ts]
+
+declare var console: { log(a: any): void };
+
+function one() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo;
+    console.log(bar);
+}
+
+function two() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo;
+    console.log(bar);
+}
+
+function three() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+function four() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+
+//// [unusedLocalsAndObjectSpread.js]
+var __rest = (this && this.__rest) || function (s, e) {
+    var t = {};
+    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
+        t[p] = s[p];
+    if (s != null && typeof Object.getOwnPropertySymbols === "function")
+        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) if (e.indexOf(p[i]) < 0)
+            t[p[i]] = s[p[i]];
+    return t;
+};
+function one() {
+    var foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    var a = foo.a, bar = __rest(foo, ["a"]);
+    console.log(bar);
+}
+function two() {
+    var foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    var _ = foo.a, bar = __rest(foo, ["a"]);
+    console.log(bar);
+}
+function three() {
+    var foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    var a = foo.a, bar = __rest(foo, ["a"]); // bar should be unused
+    //console.log(bar);
+}
+function four() {
+    var foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    var _ = foo.a, bar = __rest(foo, ["a"]); // bar should be unused
+    //console.log(bar);
+}
diff --git a/tests/cases/compiler/subSubClassCanAccessProtectedConstructor.ts b/tests/cases/compiler/subSubClassCanAccessProtectedConstructor.ts
new file mode 100644
index 00000000000..fc5e9794500
--- /dev/null
+++ b/tests/cases/compiler/subSubClassCanAccessProtectedConstructor.ts
@@ -0,0 +1,15 @@
+class Base {
+    protected constructor() { }
+    public instance1 = new Base(); // allowed
+}
+
+class Subclass extends Base {
+    public instance1_1 = new Base(); // allowed
+    public instance1_2 = new Subclass(); // allowed
+}
+
+class SubclassOfSubclass extends Subclass {
+    public instance2_1 = new Base(); // allowed
+    public instance2_2 = new Subclass(); // allowed
+    public instance2_3 = new SubclassOfSubclass(); // allowed
+}
diff --git a/tests/cases/compiler/unusedLocalsAndObjectSpread.ts b/tests/cases/compiler/unusedLocalsAndObjectSpread.ts
new file mode 100644
index 00000000000..b042b412c8e
--- /dev/null
+++ b/tests/cases/compiler/unusedLocalsAndObjectSpread.ts
@@ -0,0 +1,31 @@
+//@noUnusedLocals:true
+
+declare var console: { log(a: any): void };
+
+function one() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo;
+    console.log(bar);
+}
+
+function two() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo;
+    console.log(bar);
+}
+
+function three() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+function four() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
diff --git a/tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts b/tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts
new file mode 100644
index 00000000000..c01030c7fa9
--- /dev/null
+++ b/tests/cases/conformance/types/keyof/circularIndexedAccessErrors.ts
@@ -0,0 +1,31 @@
+// @declaration: true
+
+type T1 = {
+    x: T1["x"];  // Error
+};
+
+type T2<K extends "x" | "y"> = {
+    x: T2<K>[K];  // Error
+    y: number;
+}
+
+declare let x2: T2<"x">;
+let x2x = x2.x;
+
+interface T3<T extends T3<T>> {
+    x: T["x"];  // Error
+}
+
+interface T4<T extends T4<T>> {
+    x: T4<T>["x"];  // Error
+}
+
+class C1 {
+    x: C1["x"];  // Error
+}
+
+class C2 {
+    x: this["y"];  // Error
+    y: this["z"];  // Error
+    z: this["x"];  // Error
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts b/tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts
index 08bcfb28204..ea6a3c2358a 100644
--- a/tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts
+++ b/tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts
@@ -250,6 +250,53 @@ function f74(func: <T, U, K extends keyof (T | U)>(x: T, y: U, k: K) => (T | U)[
     let b = func({ a: 1, b: "hello" }, { a: 2, b: true }, 'b');  // string | boolean
 }
 
+function f80<T extends { a: { x: any } }>(obj: T) {
+    let a1 = obj.a;  // { x: any }
+    let a2 = obj['a'];  // { x: any }
+    let a3 = obj['a'] as T['a'];  // T["a"]
+    let x1 = obj.a.x;  // any
+    let x2 = obj['a']['x'];  // any
+    let x3 = obj['a']['x'] as T['a']['x'];  // T["a"]["x"]
+}
+
+function f81<T extends { a: { x: any } }>(obj: T) {
+    return obj['a']['x'] as T['a']['x'];
+}
+
+function f82() {
+    let x1 = f81({ a: { x: "hello" } });  // string
+    let x2 = f81({ a: { x: 42 } });  // number
+}
+
+function f83<T extends { [x: string]: { x: any } }, K extends keyof T>(obj: T, key: K) {
+    return obj[key]['x'] as T[K]['x'];
+}
+
+function f84() {
+    let x1 = f83({ foo: { x: "hello" } }, "foo");  // string
+    let x2 = f83({ bar: { x: 42 } }, "bar");  // number
+}
+
+class C1 {
+    x: number;
+    get<K extends keyof this>(key: K) {
+        return this[key];
+    }
+    set<K extends keyof this>(key: K, value: this[K]) {
+        this[key] = value;
+    }
+    foo() {
+        let x1 = this.x;  // number
+        let x2 = this["x"];  // number
+        let x3 = this.get("x");  // this["x"]
+        let x4 = getProperty(this, "x"); // this["x"]
+        this.x = 42;
+        this["x"] = 42;
+        this.set("x", 42);
+        setProperty(this, "x", 42);
+    }
+}
+
 // Repros from #12011
 
 class Base {
@@ -365,4 +412,22 @@ interface R {
 function f<K extends keyof R>(p: K) {
     let a: any;
     a[p].add;  // any
-}
\ No newline at end of file
+}
+
+// Repro from #12651
+
+type MethodDescriptor = {
+	name: string;
+	args: any[];
+	returnValue: any;
+}
+
+declare function dispatchMethod<M extends MethodDescriptor>(name: M['name'], args: M['args']): M['returnValue'];
+
+type SomeMethodDescriptor = {
+	name: "someMethod";
+	args: [string, number];
+	returnValue: string[];
+}
+
+let result = dispatchMethod<SomeMethodDescriptor>("someMethod", ["hello", 35]);
\ No newline at end of file
diff --git a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
index a198d4e0dc1..4be6b6b098d 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
@@ -84,4 +84,44 @@ function f21() {
     let x1 = objAndPartial({ x: 0, y: 0 }, { x: 1 });
     let x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
     let x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
-}
\ No newline at end of file
+}
+
+// Verify use of Pick<T, K> for setState functions (#12793)
+
+interface Foo {
+    a: string;
+    b?: number;
+}
+
+function setState<T, K extends keyof T>(obj: T, props: Pick<T, K>) {
+    for (let k in props) {
+        obj[k] = props[k];
+    }
+}
+
+let foo: Foo = { a: "hello", b: 42 };
+setState(foo, { a: "test", b: 43 })
+setState(foo, { a: "hi" });
+setState(foo, { b: undefined });
+setState(foo, { });
+setState(foo, foo);
+setState(foo, { a: undefined });  // Error
+setState(foo, { c: true });  // Error
+
+class C<T> {
+    state: T;
+    setState<K extends keyof T>(props: Pick<T, K>) {
+        for (let k in props) {
+            this.state[k] = props[k];
+        }
+    }
+}
+
+let c = new C<Foo>();
+c.setState({ a: "test", b: 43 });
+c.setState({ a: "hi" });
+c.setState({ b: undefined });
+c.setState({ });
+c.setState(foo);
+c.setState({ a: undefined });  // Error
+c.setState({ c: true });  // Error
diff --git a/tests/cases/conformance/types/mapped/mappedTypeModifiers.ts b/tests/cases/conformance/types/mapped/mappedTypeModifiers.ts
index 1e76c5b7452..30aa652f0d9 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeModifiers.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeModifiers.ts
@@ -1,85 +1,71 @@
 // @strictNullChecks: true
 
 type T = { a: number, b: string };
-type TU = { a: number | undefined, b: string | undefined };
 type TP = { a?: number, b?: string };
 type TR = { readonly a: number, readonly b: string };
 type TPR = { readonly a?: number, readonly b?: string };
 
-// Validate they all have the same keys
 var v00: "a" | "b";
 var v00: keyof T;
-var v00: keyof TU;
 var v00: keyof TP;
 var v00: keyof TR;
 var v00: keyof TPR;
 
-// Validate that non-isomorphic mapped types strip modifiers
 var v01: T;
-var v01: Pick<TR, keyof T>;
-var v01: Pick<Readonly<T>, keyof T>;
+var v01: { [P in keyof T]: T[P] };
+var v01: Pick<T, keyof T>;
+var v01: Pick<Pick<T, keyof T>, keyof T>;
 
-// Validate that non-isomorphic mapped types strip modifiers
-var v02: TU;
+var v02: TP;
+var v02: { [P in keyof T]?: T[P] };
+var v02: Partial<T>;
 var v02: Pick<TP, keyof T>;
-var v02: Pick<TPR, keyof T>;
-var v02: Pick<Partial<T>, keyof T>;
-var v02: Pick<Partial<Readonly<T>>, keyof T>;
 
-// Validate that isomorphic mapped types preserve optional modifier
-var v03: TP;
-var v03: Partial<T>;
+var v03: TR;
+var v03: { readonly [P in keyof T]: T[P] };
+var v03: Readonly<T>;
+var v03: Pick<TR, keyof T>;
 
-// Validate that isomorphic mapped types preserve readonly modifier
-var v04: TR;
-var v04: Readonly<T>;
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var v05: TPR;
-var v05: Partial<TR>;
-var v05: Readonly<TP>;
-var v05: Partial<Readonly<T>>;
-var v05: Readonly<Partial<T>>;
+var v04: TPR;
+var v04: { readonly [P in keyof T]?: T[P] };
+var v04: Partial<TR>;
+var v04: Readonly<TP>;
+var v04: Partial<Readonly<T>>;
+var v04: Readonly<Partial<T>>;
+var v04: Pick<TPR, keyof T>;
 
 type Boxified<T> = { [P in keyof T]: { x: T[P] } };
 
 type B = { a: { x: number }, b: { x: string } };
-type BU = { a: { x: number } | undefined, b: { x: string } | undefined };
 type BP = { a?: { x: number }, b?: { x: string } };
 type BR = { readonly a: { x: number }, readonly b: { x: string } };
 type BPR = { readonly a?: { x: number }, readonly b?: { x: string } };
 
-// Validate they all have the same keys
 var b00: "a" | "b";
 var b00: keyof B;
-var b00: keyof BU;
 var b00: keyof BP;
 var b00: keyof BR;
 var b00: keyof BPR;
 
-// Validate that non-isomorphic mapped types strip modifiers
 var b01: B;
-var b01: Pick<BR, keyof B>;
-var b01: Pick<Readonly<BR>, keyof B>;
+var b01: { [P in keyof B]: B[P] };
+var b01: Pick<B, keyof B>;
+var b01: Pick<Pick<B, keyof B>, keyof B>;
 
-// Validate that non-isomorphic mapped types strip modifiers
-var b02: BU;
+var b02: BP;
+var b02: { [P in keyof B]?: B[P] };
+var b02: Partial<B>;
 var b02: Pick<BP, keyof B>;
-var b02: Pick<BPR, keyof B>;
-var b02: Pick<Partial<B>, keyof B>;
-var b02: Pick<Partial<Readonly<B>>, keyof B>;
 
-// Validate that isomorphic mapped types preserve optional modifier
-var b03: BP;
-var b03: Partial<B>;
+var b03: BR;
+var b03: { readonly [P in keyof B]: B[P] };
+var b03: Readonly<B>;
+var b03: Pick<BR, keyof B>;
 
-// Validate that isomorphic mapped types preserve readonly modifier
-var b04: BR;
-var b04: Readonly<B>;
-
-// Validate that isomorphic mapped types preserve both partial and readonly modifiers
-var b05: BPR;
-var b05: Partial<BR>;
-var b05: Readonly<BP>;
-var b05: Partial<Readonly<B>>;
-var b05: Readonly<Partial<B>>;
\ No newline at end of file
+var b04: BPR;
+var b04: { readonly [P in keyof B]?: B[P] };
+var b04: Partial<BR>;
+var b04: Readonly<BP>;
+var b04: Partial<Readonly<B>>;
+var b04: Readonly<Partial<B>>;
+var b04: Pick<BPR, keyof B>;
\ No newline at end of file
diff --git a/tests/cases/fourslash/commentBraceCompletionPosition.ts b/tests/cases/fourslash/commentBraceCompletionPosition.ts
index b92c49d3a14..3730a3466c9 100644
--- a/tests/cases/fourslash/commentBraceCompletionPosition.ts
+++ b/tests/cases/fourslash/commentBraceCompletionPosition.ts
@@ -14,10 +14,10 @@
 //// }
 
 goTo.marker('1');
-verify.not.isValidBraceCompletionAtPosition('(');
+verify.isValidBraceCompletionAtPosition('(');
 
 goTo.marker('2');
-verify.not.isValidBraceCompletionAtPosition('(');
+verify.isValidBraceCompletionAtPosition('(');
 
 goTo.marker('3');
-verify.not.isValidBraceCompletionAtPosition('(');
\ No newline at end of file
+verify.isValidBraceCompletionAtPosition('(');
\ No newline at end of file