Merge pull request #5429 from Microsoft/fixAsyncTypeAlias

Fix type check for async function with alias return type.

Author: rbuckton

src/compiler/checker.ts

@@ -5479,7 +5479,7 @@ namespace ts {
                 let targetType = getIndexTypeOfType(target, IndexKind.String);
                 if (targetType) {
                     if ((targetType.flags & TypeFlags.Any) && !(originalSource.flags & TypeFlags.Primitive)) {
-                        // non-primitive assignment to any is always allowed, eg 
+                        // non-primitive assignment to any is always allowed, eg
                         //   `var x: { [index: string]: any } = { property: 12 };`
                         return Ternary.True;
                     }
@@ -5509,7 +5509,7 @@ namespace ts {
                 let targetType = getIndexTypeOfType(target, IndexKind.Number);
                 if (targetType) {
                     if ((targetType.flags & TypeFlags.Any) && !(originalSource.flags & TypeFlags.Primitive)) {
-                        // non-primitive assignment to any is always allowed, eg 
+                        // non-primitive assignment to any is always allowed, eg
                         //   `var x: { [index: number]: any } = { property: 12 };`
                         return Ternary.True;
                     }
@@ -6586,9 +6586,9 @@ namespace ts {
                 return;
             }
 
-            // 1. walk from the use site up to the declaration and check 
+            // 1. walk from the use site up to the declaration and check
             // if there is anything function like between declaration and use-site (is binding/class is captured in function).
-            // 2. walk from the declaration up to the boundary of lexical environment and check 
+            // 2. walk from the declaration up to the boundary of lexical environment and check
             // if there is an iteration statement in between declaration and boundary (is binding/class declared inside iteration statement)
 
             let container: Node;
@@ -11639,9 +11639,12 @@ namespace ts {
                 return unknownType;
             }
 
-            let promiseConstructor = getMergedSymbol(promiseType.symbol);
+            let promiseConstructor = getNodeLinks(node.type).resolvedSymbol;
             if (!promiseConstructor || !symbolIsValue(promiseConstructor)) {
-                error(node, Diagnostics.Type_0_is_not_a_valid_async_function_return_type, typeToString(promiseType));
+                let typeName = promiseConstructor
+                    ? symbolToString(promiseConstructor)
+                    : typeToString(promiseType);
+                error(node, Diagnostics.Type_0_is_not_a_valid_async_function_return_type, typeName);
                 return unknownType;
             }
 

tests/baselines/reference/asyncFunctionDeclaration15_es6.errors.txt

@@ -1,7 +1,7 @@
 tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(6,16): error TS1055: Type '{}' is not a valid async function return type.
 tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(7,16): error TS1055: Type 'any' is not a valid async function return type.
 tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(8,16): error TS1055: Type 'number' is not a valid async function return type.
-tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(9,16): error TS1055: Type 'PromiseLike<void>' is not a valid async function return type.
+tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(9,16): error TS1055: Type 'PromiseLike' is not a valid async function return type.
 tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration15_es6.ts(10,16): error TS1055: Type 'typeof Thenable' is not a valid async function return type.
   Type 'Thenable' is not assignable to type 'PromiseLike<any>'.
     Types of property 'then' are incompatible.
@@ -28,7 +28,7 @@ tests/cases/conformance/async/es6/functionDeclarations/asyncFunctionDeclaration1
 !!! error TS1055: Type 'number' is not a valid async function return type.
     async function fn5(): PromiseLike<void> { } // error
                    ~~~
-!!! error TS1055: Type 'PromiseLike<void>' is not a valid async function return type.
+!!! error TS1055: Type 'PromiseLike' is not a valid async function return type.
     async function fn6(): Thenable { } // error
                    ~~~
 !!! error TS1055: Type 'typeof Thenable' is not a valid async function return type.