Merge pull request #9056 from Microsoft/async_function_return_type

Contextually type return statement in async function

Author: Andy

src/compiler/checker.ts

@@ -8756,6 +8756,16 @@ namespace ts {
 
         function getContextualTypeForReturnExpression(node: Expression): Type {
             const func = getContainingFunction(node);
+
+            if (isAsyncFunctionLike(func)) {
+                const contextualReturnType = getContextualReturnType(func);
+                if (contextualReturnType) {
+                    return getPromisedType(contextualReturnType);
+                }
+
+                return undefined;
+            }
+
             if (func && !func.asteriskToken) {
                 return getContextualReturnType(func);
             }

tests/baselines/reference/asyncFunctionReturnType.js

@@ -0,0 +1,33 @@
+//// [asyncFunctionReturnType.ts]
+async function fAsync() {
+    // Without explicit type annotation, this is just an array.
+    return [1, true];
+}
+
+async function fAsyncExplicit(): Promise<[number, boolean]> {
+    // This is contextually typed as a tuple.
+    return [1, true];
+}
+
+
+//// [asyncFunctionReturnType.js]
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator.throw(value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments)).next());
+    });
+};
+function fAsync() {
+    return __awaiter(this, void 0, void 0, function* () {
+        // Without explicit type annotation, this is just an array.
+        return [1, true];
+    });
+}
+function fAsyncExplicit() {
+    return __awaiter(this, void 0, void 0, function* () {
+        // This is contextually typed as a tuple.
+        return [1, true];
+    });
+}

tests/baselines/reference/asyncFunctionReturnType.symbols

@@ -0,0 +1,16 @@
+=== tests/cases/compiler/asyncFunctionReturnType.ts ===
+async function fAsync() {
+>fAsync : Symbol(fAsync, Decl(asyncFunctionReturnType.ts, 0, 0))
+
+    // Without explicit type annotation, this is just an array.
+    return [1, true];
+}
+
+async function fAsyncExplicit(): Promise<[number, boolean]> {
+>fAsyncExplicit : Symbol(fAsyncExplicit, Decl(asyncFunctionReturnType.ts, 3, 1))
+>Promise : Symbol(Promise, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --))
+
+    // This is contextually typed as a tuple.
+    return [1, true];
+}
+

tests/baselines/reference/asyncFunctionReturnType.types

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/asyncFunctionReturnType.ts ===
+async function fAsync() {
+>fAsync : () => Promise<(number | boolean)[]>
+
+    // Without explicit type annotation, this is just an array.
+    return [1, true];
+>[1, true] : (number | boolean)[]
+>1 : number
+>true : boolean
+}
+
+async function fAsyncExplicit(): Promise<[number, boolean]> {
+>fAsyncExplicit : () => Promise<[number, boolean]>
+>Promise : Promise<T>
+
+    // This is contextually typed as a tuple.
+    return [1, true];
+>[1, true] : [number, boolean]
+>1 : number
+>true : boolean
+}
+

tests/cases/compiler/asyncFunctionReturnType.ts

@@ -0,0 +1,10 @@
+// @target: ES6
+async function fAsync() {
+    // Without explicit type annotation, this is just an array.
+    return [1, true];
+}
+
+async function fAsyncExplicit(): Promise<[number, boolean]> {
+    // This is contextually typed as a tuple.
+    return [1, true];
+}