Use resolvedSymbol instead of mergedSymbol.

Fixes #5333.

Author: rbuckton

src/compiler/checker.ts

@@ -11631,17 +11631,20 @@ namespace ts {
             //
             // 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> }`.
-
+            
             let promiseType = getTypeFromTypeNode(node.type);
             if (promiseType === unknownType && compilerOptions.isolatedModules) {
                 // If we are compiling with isolatedModules, we may not be able to resolve the
                 // type as a value. As such, we will just return unknownType;
                 return unknownType;
             }
-
-            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.