Merge pull request #11430 from Microsoft/fixAsyncReturnTypeCheck

Fix async return type check

src/compiler/checker.ts

@@ -15546,31 +15546,24 @@ namespace ts {
         }
 
         /**
-          * Checks the return type of an async function to ensure it is a compatible
-          * Promise implementation.
-          * @param node The signature to check
-          * @param returnType The return type for the function
-          * @remarks
-          * This checks that an async function has a valid Promise-compatible return type,
-          * and returns the *awaited type* of the promise. An async function has a valid
-          * Promise-compatible return type if the resolved value of the return type has a
-          * construct signature that takes in an `initializer` function that in turn supplies
-          * a `resolve` function as one of its arguments and results in an object with a
-          * callable `then` signature.
-          */
+         * Checks the return type of an async function to ensure it is a compatible
+         * Promise implementation.
+         *
+         * This checks that an async function has a valid Promise-compatible return type,
+         * and returns the *awaited type* of the promise. An async function has a valid
+         * Promise-compatible return type if the resolved value of the return type has a
+         * construct signature that takes in an `initializer` function that in turn supplies
+         * a `resolve` function as one of its arguments and results in an object with a
+         * callable `then` signature.
+         *
+         * @param node The signature to check
+         */
         function checkAsyncFunctionReturnType(node: FunctionLikeDeclaration): Type {
             if (languageVersion >= ScriptTarget.ES2015) {
                 const returnType = getTypeFromTypeNode(node.type);
                 return checkCorrectPromiseType(returnType, node.type, Diagnostics.The_return_type_of_an_async_function_or_method_must_be_the_global_Promise_T_type);
             }
 
-            const globalPromiseConstructorLikeType = getGlobalPromiseConstructorLikeType();
-            if (globalPromiseConstructorLikeType === emptyObjectType) {
-                // If we couldn't resolve the global PromiseConstructorLike type we cannot verify
-                // compatibility with __awaiter.
-                return unknownType;
-            }
-
             // As part of our emit for an async function, we will need to emit the entity name of
             // the return type annotation as an expression. To meet the necessary runtime semantics
             // for __awaiter, we must also check that the type of the declaration (e.g. the static
@@ -15595,42 +15588,56 @@ namespace ts {
             //      then<U>(...): Promise<U>;
             //  }
             //
-            // When we get the type of the `Promise` symbol here, we get the type of the static
-            // side of the `Promise` class, which would be `{ new <T>(...): Promise<T> }`.
 
+            // Always mark the type node as referenced if it points to a value
+            markTypeNodeAsReferenced(node.type);
+
+            const promiseConstructorName = getEntityNameFromTypeNode(node.type);
             const promiseType = getTypeFromTypeNode(node.type);
-            if (promiseType === unknownType && compilerOptions.isolatedModules) {
-                // If we are compiling with isolatedModules, we may not be able to resolve the
-                // type as a value. As such, we will just return unknownType;
+            if (promiseType === unknownType) {
+                if (!compilerOptions.isolatedModules) {
+                    if (promiseConstructorName) {
+                        error(node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, entityNameToString(promiseConstructorName));
+                    }
+                    else {
+                        error(node.type, Diagnostics.An_async_function_or_method_must_have_a_valid_awaitable_return_type);
+                    }
+                }
                 return unknownType;
             }
 
-            const promiseConstructor = getNodeLinks(node.type).resolvedSymbol;
-            if (!promiseConstructor || !symbolIsValue(promiseConstructor)) {
-                // try to fall back to global promise type.
-                const typeName = promiseConstructor
-                    ? symbolToString(promiseConstructor)
-                    : typeToString(promiseType);
-                return checkCorrectPromiseType(promiseType, node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type, typeName);
+            if (promiseConstructorName === undefined) {
+                error(node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, typeToString(promiseType));
+                return unknownType;
             }
 
-            // If the Promise constructor, resolved locally, is an alias symbol we should mark it as referenced.
-            checkReturnTypeAnnotationAsExpression(node);
+            const promiseConstructorSymbol = resolveEntityName(promiseConstructorName, SymbolFlags.Value, /*ignoreErrors*/ true);
+            const promiseConstructorType = promiseConstructorSymbol ? getTypeOfSymbol(promiseConstructorSymbol) : unknownType;
+            if (promiseConstructorType === unknownType) {
+                error(node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, entityNameToString(promiseConstructorName));
+                return unknownType;
+            }
 
-            // Validate the promise constructor type.
-            const promiseConstructorType = getTypeOfSymbol(promiseConstructor);
-            if (!checkTypeAssignableTo(promiseConstructorType, globalPromiseConstructorLikeType, node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type)) {
+            const globalPromiseConstructorLikeType = getGlobalPromiseConstructorLikeType();
+            if (globalPromiseConstructorLikeType === emptyObjectType) {
+                // If we couldn't resolve the global PromiseConstructorLike type we cannot verify
+                // compatibility with __awaiter.
+                error(node.type, Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value, entityNameToString(promiseConstructorName));
+                return unknownType;
+            }
+
+            if (!checkTypeAssignableTo(promiseConstructorType, globalPromiseConstructorLikeType, node.type,
+                Diagnostics.Type_0_is_not_a_valid_async_function_return_type_in_ES5_SlashES3_because_it_does_not_refer_to_a_Promise_compatible_constructor_value)) {
                 return unknownType;
             }
 
             // Verify there is no local declaration that could collide with the promise constructor.
-            const promiseName = getEntityNameFromTypeNode(node.type);
-            const promiseNameOrNamespaceRoot = getFirstIdentifier(promiseName);
-            const rootSymbol = getSymbol(node.locals, promiseNameOrNamespaceRoot.text, SymbolFlags.Value);
-            if (rootSymbol) {
-                error(rootSymbol.valueDeclaration, Diagnostics.Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions,
-                    promiseNameOrNamespaceRoot.text,
-                    getFullyQualifiedName(promiseConstructor));
+            const rootName = promiseConstructorName && getFirstIdentifier(promiseConstructorName);
+            const collidingSymbol = getSymbol(node.locals, rootName.text, SymbolFlags.Value);
+            if (collidingSymbol) {
+                error(collidingSymbol.valueDeclaration, Diagnostics.Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions,
+                    rootName.text,
+                    entityNameToString(promiseConstructorName));
                 return unknownType;
             }
 
@@ -15688,44 +15695,19 @@ namespace ts {
                 errorInfo);
         }
 
-        /** Checks a type reference node as an expression. */
-        function checkTypeNodeAsExpression(node: TypeNode) {
-            // When we are emitting type metadata for decorators, we need to try to check the type
-            // as if it were an expression so that we can emit the type in a value position when we
-            // serialize the type metadata.
-            if (node && node.kind === SyntaxKind.TypeReference) {
-                const root = getFirstIdentifier((<TypeReferenceNode>node).typeName);
-                const meaning = root.parent.kind === SyntaxKind.TypeReference ? SymbolFlags.Type : SymbolFlags.Namespace;
-                // Resolve type so we know which symbol is referenced
-                const rootSymbol = resolveName(root, root.text, meaning | SymbolFlags.Alias, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined);
-                // Resolved symbol is alias
-                if (rootSymbol && rootSymbol.flags & SymbolFlags.Alias) {
-                    const aliasTarget = resolveAlias(rootSymbol);
-                    // If alias has value symbol - mark alias as referenced
-                    if (aliasTarget.flags & SymbolFlags.Value && !isConstEnumOrConstEnumOnlyModule(resolveAlias(rootSymbol))) {
-                        markAliasSymbolAsReferenced(rootSymbol);
-                    }
-                }
-            }
-        }
-
         /**
-          * Checks the type annotation of an accessor declaration or property declaration as
-          * an expression if it is a type reference to a type with a value declaration.
-          */
-        function checkTypeAnnotationAsExpression(node: VariableLikeDeclaration) {
-            checkTypeNodeAsExpression((<PropertyDeclaration>node).type);
-        }
-
-        function checkReturnTypeAnnotationAsExpression(node: FunctionLikeDeclaration) {
-            checkTypeNodeAsExpression(node.type);
-        }
-
-        /** Checks the type annotation of the parameters of a function/method or the constructor of a class as expressions */
-        function checkParameterTypeAnnotationsAsExpressions(node: FunctionLikeDeclaration) {
-            // ensure all type annotations with a value declaration are checked as an expression
-            for (const parameter of node.parameters) {
-                checkTypeAnnotationAsExpression(parameter);
+         * If a TypeNode can be resolved to a value symbol imported from an external module, it is
+         * marked as referenced to prevent import elision.
+         */
+        function markTypeNodeAsReferenced(node: TypeNode) {
+            const typeName = node && getEntityNameFromTypeNode(node);
+            const rootName = typeName && getFirstIdentifier(typeName);
+            const rootSymbol = rootName && resolveName(rootName, rootName.text, (typeName.kind === SyntaxKind.Identifier ? SymbolFlags.Type : SymbolFlags.Namespace) | SymbolFlags.Alias, /*nameNotFoundMessage*/ undefined, /*nameArg*/ undefined);
+            if (rootSymbol
+                && rootSymbol.flags & SymbolFlags.Alias
+                && symbolIsValue(rootSymbol)
+                && !isConstEnumOrConstEnumOnlyModule(resolveAlias(rootSymbol))) {
+                markAliasSymbolAsReferenced(rootSymbol);
             }
         }
 
@@ -15751,20 +15733,25 @@ namespace ts {
                     case SyntaxKind.ClassDeclaration:
                         const constructor = getFirstConstructorWithBody(<ClassDeclaration>node);
                         if (constructor) {
-                            checkParameterTypeAnnotationsAsExpressions(constructor);
+                            for (const parameter of constructor.parameters) {
+                                markTypeNodeAsReferenced(parameter.type);
+                            }
                         }
                         break;
 
                     case SyntaxKind.MethodDeclaration:
                     case SyntaxKind.GetAccessor:
                     case SyntaxKind.SetAccessor:
-                        checkParameterTypeAnnotationsAsExpressions(<FunctionLikeDeclaration>node);
-                        checkReturnTypeAnnotationAsExpression(<FunctionLikeDeclaration>node);
+                        for (const parameter of (<FunctionLikeDeclaration>node).parameters) {
+                            markTypeNodeAsReferenced(parameter.type);
+                        }
+
+                        markTypeNodeAsReferenced((<FunctionLikeDeclaration>node).type);
                         break;
 
                     case SyntaxKind.PropertyDeclaration:
                     case SyntaxKind.Parameter:
-                        checkTypeAnnotationAsExpression(<PropertyDeclaration | ParameterDeclaration>node);
+                        markTypeNodeAsReferenced((<PropertyDeclaration | ParameterDeclaration>node).type);
                         break;
                 }
             }
@@ -18476,9 +18463,33 @@ namespace ts {
         function getDiagnosticsWorker(sourceFile: SourceFile): Diagnostic[] {
             throwIfNonDiagnosticsProducing();
             if (sourceFile) {
+                // Some global diagnostics are deferred until they are needed and
+                // may not be reported in the firt call to getGlobalDiagnostics.
+                // We should catch these changes and report them.
+                const previousGlobalDiagnostics = diagnostics.getGlobalDiagnostics();
+                const previousGlobalDiagnosticsSize = previousGlobalDiagnostics.length;
+
                 checkSourceFile(sourceFile);
-                return diagnostics.getDiagnostics(sourceFile.fileName);
+
+                const semanticDiagnostics = diagnostics.getDiagnostics(sourceFile.fileName);
+                const currentGlobalDiagnostics = diagnostics.getGlobalDiagnostics();
+                if (currentGlobalDiagnostics !== previousGlobalDiagnostics) {
+                    // If the arrays are not the same reference, new diagnostics were added.
+                    const deferredGlobalDiagnostics = relativeComplement(previousGlobalDiagnostics, currentGlobalDiagnostics, compareDiagnostics);
+                    return concatenate(deferredGlobalDiagnostics, semanticDiagnostics);
+                }
+                else if (previousGlobalDiagnosticsSize === 0 && currentGlobalDiagnostics.length > 0) {
+                    // If the arrays are the same reference, but the length has changed, a single
+                    // new diagnostic was added as DiagnosticCollection attempts to reuse the
+                    // same array.
+                    return concatenate(currentGlobalDiagnostics, semanticDiagnostics);
+                }
+
+                return semanticDiagnostics;
             }
+
+            // Global diagnostics are always added when a file is not provided to
+            // getDiagnostics
             forEach(host.getSourceFiles(), checkSourceFile);
             return diagnostics.getDiagnostics();
         }

src/compiler/core.ts

@@ -446,8 +446,8 @@ namespace ts {
     }
 
     export function concatenate<T>(array1: T[], array2: T[]): T[] {
-        if (!array2 || !array2.length) return array1;
-        if (!array1 || !array1.length) return array2;
+        if (!some(array2)) return array1;
+        if (!some(array1)) return array2;
         return [...array1, ...array2];
     }
 
@@ -527,6 +527,27 @@ namespace ts {
         return result || array;
     }
 
+    /**
+     * Gets the relative complement of `arrayA` with respect to `b`, returning the elements that
+     * are not present in `arrayA` but are present in `arrayB`. Assumes both arrays are sorted
+     * based on the provided comparer.
+     */
+    export function relativeComplement<T>(arrayA: T[] | undefined, arrayB: T[] | undefined, comparer: (x: T, y: T) => Comparison = compareValues, offsetA = 0, offsetB = 0): T[] | undefined {
+        if (!arrayB || !arrayA || arrayB.length === 0 || arrayA.length === 0) return arrayB;
+        const result: T[] = [];
+        outer: for (; offsetB < arrayB.length; offsetB++) {
+            inner: for (; offsetA < arrayA.length; offsetA++) {
+                switch (comparer(arrayB[offsetB], arrayA[offsetA])) {
+                    case Comparison.LessThan: break inner;
+                    case Comparison.EqualTo: continue outer;
+                    case Comparison.GreaterThan: continue inner;
+                }
+            }
+            result.push(arrayB[offsetB]);
+        }
+        return result;
+    }
+
     export function sum(array: any[], prop: string): number {
         let result = 0;
         for (const v of array) {
@@ -626,12 +647,12 @@ namespace ts {
      * @param array A sorted array whose first element must be no larger than number
      * @param number The value to be searched for in the array.
      */
-    export function binarySearch<T>(array: T[], value: T, comparer?: (v1: T, v2: T) => number): number {
+    export function binarySearch<T>(array: T[], value: T, comparer?: (v1: T, v2: T) => number, offset?: number): number {
         if (!array || array.length === 0) {
             return -1;
         }
 
-        let low = 0;
+        let low = offset || 0;
         let high = array.length - 1;
         comparer = comparer !== undefined
             ? comparer

src/compiler/diagnosticMessages.json

@@ -163,7 +163,7 @@
         "category": "Error",
         "code": 1054
     },
-    "Type '{0}' is not a valid async function return type.": {
+    "Type '{0}' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.": {
         "category": "Error",
         "code": 1055
     },

src/compiler/transformers/es2017.ts

@@ -262,7 +262,8 @@ namespace ts {
         }
 
         function transformAsyncFunctionBody(node: FunctionLikeDeclaration): ConciseBody | FunctionBody {
-            const nodeType = node.original ? (<FunctionLikeDeclaration>node.original).type : node.type;
+            const original = getOriginalNode(node, isFunctionLike);
+            const nodeType = original.type;
             const promiseConstructor = languageVersion < ScriptTarget.ES2015 ? getPromiseConstructor(nodeType) : undefined;
             const isArrowFunction = node.kind === SyntaxKind.ArrowFunction;
             const hasLexicalArguments = (resolver.getNodeCheckFlags(node) & NodeCheckFlags.CaptureArguments) !== 0;
@@ -336,15 +337,16 @@ namespace ts {
         }
 
         function getPromiseConstructor(type: TypeNode) {
-            const typeName = getEntityNameFromTypeNode(type);
-            if (typeName && isEntityName(typeName)) {
-                const serializationKind = resolver.getTypeReferenceSerializationKind(typeName);
-                if (serializationKind === TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue
-                    || serializationKind === TypeReferenceSerializationKind.Unknown) {
-                    return typeName;
+            if (type) {
+                const typeName = getEntityNameFromTypeNode(type);
+                if (typeName && isEntityName(typeName)) {
+                    const serializationKind = resolver.getTypeReferenceSerializationKind(typeName);
+                    if (serializationKind === TypeReferenceSerializationKind.TypeWithConstructSignatureAndValue
+                        || serializationKind === TypeReferenceSerializationKind.Unknown) {
+                        return typeName;
+                    }
                 }
             }
-
             return undefined;
         }
 

src/compiler/utilities.ts

@@ -485,6 +485,17 @@ namespace ts {
         return getFullWidth(name) === 0 ? "(Missing)" : getTextOfNode(name);
     }
 
+    export function entityNameToString(name: EntityNameOrEntityNameExpression): string {
+        switch (name.kind) {
+            case SyntaxKind.Identifier:
+                return getFullWidth(name) === 0 ? unescapeIdentifier((<Identifier>name).text) : getTextOfNode(name);
+            case SyntaxKind.QualifiedName:
+                return entityNameToString((<QualifiedName>name).left) + "." + entityNameToString((<QualifiedName>name).right);
+            case SyntaxKind.PropertyAccessExpression:
+                return entityNameToString((<PropertyAccessEntityNameExpression>name).expression) + "." + entityNameToString((<PropertyAccessEntityNameExpression>name).name);
+        }
+    }
+
     export function createDiagnosticForNode(node: Node, message: DiagnosticMessage, arg0?: string | number, arg1?: string | number, arg2?: string | number): Diagnostic {
         const sourceFile = getSourceFileOfNode(node);
         const span = getErrorSpanForNode(sourceFile, node);
@@ -1042,17 +1053,19 @@ namespace ts {
     }
 
     export function getEntityNameFromTypeNode(node: TypeNode): EntityNameOrEntityNameExpression {
-        if (node) {
-            switch (node.kind) {
-                case SyntaxKind.TypeReference:
-                    return (<TypeReferenceNode>node).typeName;
-                case SyntaxKind.ExpressionWithTypeArguments:
-                    Debug.assert(isEntityNameExpression((<ExpressionWithTypeArguments>node).expression));
-                    return <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression;
-                case SyntaxKind.Identifier:
-                case SyntaxKind.QualifiedName:
-                    return (<EntityName><Node>node);
-            }
+        switch (node.kind) {
+            case SyntaxKind.TypeReference:
+            case SyntaxKind.JSDocTypeReference:
+                return (<TypeReferenceNode>node).typeName;
+
+            case SyntaxKind.ExpressionWithTypeArguments:
+                return isEntityNameExpression((<ExpressionWithTypeArguments>node).expression)
+                    ? <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression
+                    : undefined;
+
+            case SyntaxKind.Identifier:
+            case SyntaxKind.QualifiedName:
+                return (<EntityName><Node>node);
         }
 
         return undefined;

tests/baselines/reference/asyncAliasReturnType_es5.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/conformance/async/es5/asyncAliasReturnType_es5.ts(3,21): error TS1055: Type 'PromiseAlias' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+
+
+==== tests/cases/conformance/async/es5/asyncAliasReturnType_es5.ts (1 errors) ====
+    type PromiseAlias<T> = Promise<T>;
+    
+    async function f(): PromiseAlias<void> {
+                        ~~~~~~~~~~~~~~~~~~
+!!! error TS1055: Type 'PromiseAlias' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+    }

tests/baselines/reference/asyncAwaitIsolatedModules_es5.js

@@ -77,6 +77,7 @@ var __generator = (this && this.__generator) || function (thisArg, body) {
     }
 };
 var _this = this;
+var missing_1 = require("missing");
 function f0() {
     return __awaiter(this, void 0, void 0, function () { return __generator(this, function (_a) {
         return [2 /*return*/];

tests/baselines/reference/asyncFunctionDeclaration15_es5.errors.txt

@@ -1,9 +1,9 @@
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(6,23): error TS1055: Type '{}' is not a valid async function return type.
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(7,23): error TS1055: Type 'any' is not a valid async function return type.
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(8,23): error TS1055: Type 'number' is not a valid async function return type.
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(9,23): error TS1055: Type 'PromiseLike' is not a valid async function return type.
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type.
-tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(6,23): error TS1055: Type '{}' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(7,23): error TS1055: Type 'any' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(8,23): error TS1055: Type 'number' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(9,23): error TS1055: Type 'PromiseLike' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
+tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
   Type 'Thenable' is not assignable to type 'PromiseLike<any>'.
     Types of property 'then' are incompatible.
       Type '() => void' is not assignable to type '{ (onfulfilled?: (value: any) => any, onrejected?: (reason: any) => any): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => any, onrejected: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<TResult>; <TResult1, TResult2>(onfulfilled: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2>; }'.
@@ -20,21 +20,21 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
     async function fn1() { } // valid: Promise<void>
     async function fn2(): { } { } // error
                           ~~~
-!!! error TS1055: Type '{}' is not a valid async function return type.
+!!! error TS1055: Type '{}' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
     async function fn3(): any { } // error
                           ~~~
-!!! error TS1055: Type 'any' is not a valid async function return type.
+!!! error TS1055: Type 'any' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
     async function fn4(): number { } // error
                           ~~~~~~
-!!! error TS1055: Type 'number' is not a valid async function return type.
+!!! error TS1055: Type 'number' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
     async function fn5(): PromiseLike<void> { } // error
                           ~~~~~~~~~~~~~~~~~
-!!! error TS1055: Type 'PromiseLike' is not a valid async function return type.
+!!! error TS1055: Type 'PromiseLike' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
     async function fn6(): Thenable { } // error
                           ~~~~~~~~
-!!! error TS1055: Type 'typeof Thenable' is not a valid async function return type.
+!!! error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
                           ~~~~~~~~
-!!! error TS1055: Type 'typeof Thenable' is not a valid async function return type.
+!!! error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
 !!! error TS1055:   Type 'Thenable' is not assignable to type 'PromiseLike<any>'.
 !!! error TS1055:     Types of property 'then' are incompatible.
 !!! error TS1055:       Type '() => void' is not assignable to type '{ (onfulfilled?: (value: any) => any, onrejected?: (reason: any) => any): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => any, onrejected: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<any>; <TResult>(onfulfilled: (value: any) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => TResult | PromiseLike<TResult>): PromiseLike<TResult>; <TResult1, TResult2>(onfulfilled: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2>; }'.