Allow await await

Author: Andy Hanson

src/compiler/parser.ts

@@ -3337,10 +3337,7 @@ namespace ts {
         function parseAwaitExpression() {
             const node = <AwaitExpression>createNode(SyntaxKind.AwaitExpression);
             nextToken();
-            node.expression = token === SyntaxKind.AwaitKeyword
-                // Forbid `await await`
-                ? <UnaryExpression>createMissingNode(SyntaxKind.Identifier, /*reportAtCurrentPosition*/ false, Diagnostics.Expression_expected)
-                : parseSimpleUnaryExpression();
+            node.expression = parseSimpleUnaryExpression();
             return finishNode(node);
         }
 
@@ -3389,7 +3386,7 @@ namespace ts {
          *      6) - UnaryExpression[?yield]
          *      7) ~ UnaryExpression[?yield]
          *      8) ! UnaryExpression[?yield]
-         *      9) await AwaitExpression[?yield]
+         *      9) [+Await] await AwaitExpression[?yield]
          */
         function parseSimpleUnaryExpression(): UnaryExpression {
             switch (token) {

tests/baselines/reference/awaitAwait.errors.txt

@@ -4,6 +4,7 @@ async function f() {
     typeof await 0;
     void await 0;
     await void <string> typeof <number> void await 0;
+    await await 0;
 }
 
 
@@ -22,5 +23,6 @@ function f() {
         typeof yield 0;
         void yield 0;
         yield void typeof void yield 0;
+        yield yield 0;
     });
 }

tests/baselines/reference/awaitAwait.js

@@ -6,5 +6,6 @@ async function f() {
     typeof await 0;
     void await 0;
     await void <string> typeof <number> void await 0;
+    await await 0;
 }
 

tests/baselines/reference/castOfAwait.js

@@ -25,6 +25,11 @@ async function f() {
 ><number> void await 0 : number
 >void await 0 : undefined
 >await 0 : number
+>0 : number
+
+    await await 0;
+>await await 0 : number
+>await 0 : number
 >0 : number
 }
 

tests/baselines/reference/castOfAwait.symbols

@@ -4,4 +4,5 @@ async function f() {
     typeof await 0;
     void await 0;
     await void <string> typeof <number> void await 0;
+    await await 0;
 }