Forbid await await

src/compiler/parser.ts

@@ -3337,7 +3337,10 @@ namespace ts {
         function parseAwaitExpression() {
             const node = <AwaitExpression>createNode(SyntaxKind.AwaitExpression);
             nextToken();
-            node.expression = parseSimpleUnaryExpression();
+            node.expression = token === SyntaxKind.AwaitKeyword
+                // Forbid `await await`
+                ? <UnaryExpression>createMissingNode(SyntaxKind.Identifier, /*reportAtCurrentPosition*/ false, Diagnostics.Expression_expected)
+                : parseSimpleUnaryExpression();
             return finishNode(node);
         }
 

tests/baselines/reference/awaitAwait.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/compiler/awaitAwait.ts(2,11): error TS1109: Expression expected.
+
+
+==== tests/cases/compiler/awaitAwait.ts (1 errors) ====
+    async function f() {
+        await await 0;
+              ~~~~~
+!!! error TS1109: Expression expected.
+    }
+    

tests/baselines/reference/awaitAwait.js

@@ -0,0 +1,21 @@
+//// [awaitAwait.ts]
+async function f() {
+    await await 0;
+}
+
+
+//// [awaitAwait.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 f() {
+    return __awaiter(this, void 0, void 0, function* () {
+        yield ;
+        yield 0;
+    });
+}

tests/cases/compiler/awaitAwait.ts

@@ -0,0 +1,4 @@
+// @target: es6
+async function f() {
+    await await 0;
+}