Fix7334 Disallow async in functionExpression and ArrowFunction (#9062)

* Error when using async modifier in function-expression and arrow-function when target es5

* Add tests and baselines

Author: yuit

src/compiler/checker.ts

@@ -17794,6 +17794,8 @@ namespace ts {
                 case SyntaxKind.ImportEqualsDeclaration:
                 case SyntaxKind.ExportDeclaration:
                 case SyntaxKind.ExportAssignment:
+                case SyntaxKind.FunctionExpression:
+                case SyntaxKind.ArrowFunction:
                 case SyntaxKind.Parameter:
                     break;
                 case SyntaxKind.FunctionDeclaration:

tests/baselines/reference/disallowAsyncModifierInES5.errors.txt

@@ -0,0 +1,27 @@
+error TS2318: Cannot find global type 'Promise'.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(2,1): error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(2,16): error TS1057: An async function or method must have a valid awaitable return type.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(3,11): error TS1057: An async function or method must have a valid awaitable return type.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(3,11): error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(4,11): error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+tests/cases/compiler/disallowAsyncModifierInES5.ts(4,11): error TS1057: An async function or method must have a valid awaitable return type.
+
+
+!!! error TS2318: Cannot find global type 'Promise'.
+==== tests/cases/compiler/disallowAsyncModifierInES5.ts (6 errors) ====
+    
+    async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
+    ~~~~~
+!!! error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+                   ~~~
+!!! error TS1057: An async function or method must have a valid awaitable return type.
+    let bar = async function () { return 42; } // OK, but should be an error
+              ~~~~~
+!!! error TS1057: An async function or method must have a valid awaitable return type.
+              ~~~~~
+!!! error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+    let baz = async () => 42; // OK, but should be an error
+              ~~~~~
+!!! error TS1311: Async functions are only available when targeting ECMAScript 6 and higher.
+              ~~~~~~~~~~~~~~
+!!! error TS1057: An async function or method must have a valid awaitable return type.

tests/baselines/reference/disallowAsyncModifierInES5.js

@@ -0,0 +1,22 @@
+//// [disallowAsyncModifierInES5.ts]
+
+async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
+let bar = async function () { return 42; } // OK, but should be an error
+let baz = async () => 42; // OK, but should be an error
+
+//// [disallowAsyncModifierInES5.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 foo() {
+    return __awaiter(this, void 0, void 0, function* () { return 42; });
+} // ERROR: Async functions are only available in ES6+
+var bar = function () {
+    return __awaiter(this, void 0, void 0, function* () { return 42; });
+}; // OK, but should be an error
+var baz = function () __awaiter(this, void 0, void 0, function* () { return 42; }); // OK, but should be an error

tests/cases/compiler/disallowAsyncModifierInES5.ts

@@ -0,0 +1,5 @@
+// @target: es5
+
+async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
+let bar = async function () { return 42; } // OK, but should be an error
+let baz = async () => 42; // OK, but should be an error