Improve parsing in await and yield context (#44680)

* Improve parsing in await and yield context

* Avoid yield and await check in identifier

* Revert "Avoid yield and awaitt check in identifier"

This reverts commit 9644859f29.

* Add some comments

src/compiler/parser.ts

@@ -1501,6 +1501,8 @@ namespace ts {
             if (token() === SyntaxKind.Identifier) {
                 return true;
             }
+
+            // `let await`/`let yield` in [Yield] or [Await] are allowed here and disallowed in the binder.
             return token() > SyntaxKind.LastReservedWord;
         }
 
@@ -6122,15 +6124,15 @@ namespace ts {
             }
         }
 
-        function nextTokenIsIdentifierOrStartOfDestructuring() {
+        function nextTokenIsBindingIdentifierOrStartOfDestructuring() {
             nextToken();
-            return isIdentifier() || token() === SyntaxKind.OpenBraceToken || token() === SyntaxKind.OpenBracketToken;
+            return isBindingIdentifier() || token() === SyntaxKind.OpenBraceToken || token() === SyntaxKind.OpenBracketToken;
         }
 
         function isLetDeclaration() {
             // In ES6 'let' always starts a lexical declaration if followed by an identifier or {
             // or [.
-            return lookAhead(nextTokenIsIdentifierOrStartOfDestructuring);
+            return lookAhead(nextTokenIsBindingIdentifierOrStartOfDestructuring);
         }
 
         function parseStatement(): Statement {

tests/baselines/reference/asyncOrYieldAsBindingIdentifier1.errors.txt

@@ -0,0 +1,68 @@
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(14,9): error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(18,9): error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(22,11): error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(38,9): error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(42,9): error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts(46,11): error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+
+
+==== tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts (6 errors) ====
+    function f_let () {
+        let await = 1
+    }
+    
+    function f1_var () {
+        var await = 1
+    }
+    
+    function f1_const () {
+        const await = 1
+    }
+    
+    async function f2_let () {
+        let await = 1
+            ~~~~~
+!!! error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+    }
+    
+    async function f2_var () {
+        var await = 1
+            ~~~~~
+!!! error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+    }
+    
+    async function f2_const () {
+        const await = 1
+              ~~~~~
+!!! error TS1359: Identifier expected. 'await' is a reserved word that cannot be used here.
+    }
+    
+    function f3_let () {
+        let yield = 2
+    }
+    
+    function f3_var () {
+        var yield = 2
+    }
+    
+    function f3_const () {
+        const yield = 2
+    }
+    
+    function * f4_let () {
+        let yield = 2;
+            ~~~~~
+!!! error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+    }
+    
+    function * f4_var () {
+        var yield = 2;
+            ~~~~~
+!!! error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+    }
+    
+    function * f4_const () {
+        const yield = 2;
+              ~~~~~
+!!! error TS1359: Identifier expected. 'yield' is a reserved word that cannot be used here.
+    }

tests/baselines/reference/asyncOrYieldAsBindingIdentifier1.js

@@ -0,0 +1,86 @@
+//// [asyncOrYieldAsBindingIdentifier1.ts]
+function f_let () {
+    let await = 1
+}
+
+function f1_var () {
+    var await = 1
+}
+
+function f1_const () {
+    const await = 1
+}
+
+async function f2_let () {
+    let await = 1
+}
+
+async function f2_var () {
+    var await = 1
+}
+
+async function f2_const () {
+    const await = 1
+}
+
+function f3_let () {
+    let yield = 2
+}
+
+function f3_var () {
+    var yield = 2
+}
+
+function f3_const () {
+    const yield = 2
+}
+
+function * f4_let () {
+    let yield = 2;
+}
+
+function * f4_var () {
+    var yield = 2;
+}
+
+function * f4_const () {
+    const yield = 2;
+}
+
+//// [asyncOrYieldAsBindingIdentifier1.js]
+function f_let() {
+    let await = 1;
+}
+function f1_var() {
+    var await = 1;
+}
+function f1_const() {
+    const await = 1;
+}
+async function f2_let() {
+    let await = 1;
+}
+async function f2_var() {
+    var await = 1;
+}
+async function f2_const() {
+    const await = 1;
+}
+function f3_let() {
+    let yield = 2;
+}
+function f3_var() {
+    var yield = 2;
+}
+function f3_const() {
+    const yield = 2;
+}
+function* f4_let() {
+    let yield = 2;
+}
+function* f4_var() {
+    var yield = 2;
+}
+function* f4_const() {
+    const yield = 2;
+}

tests/baselines/reference/asyncOrYieldAsBindingIdentifier1.symbols

@@ -0,0 +1,84 @@
+=== tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts ===
+function f_let () {
+>f_let : Symbol(f_let, Decl(asyncOrYieldAsBindingIdentifier1.ts, 0, 0))
+
+    let await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 1, 7))
+}
+
+function f1_var () {
+>f1_var : Symbol(f1_var, Decl(asyncOrYieldAsBindingIdentifier1.ts, 2, 1))
+
+    var await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 5, 7))
+}
+
+function f1_const () {
+>f1_const : Symbol(f1_const, Decl(asyncOrYieldAsBindingIdentifier1.ts, 6, 1))
+
+    const await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 9, 9))
+}
+
+async function f2_let () {
+>f2_let : Symbol(f2_let, Decl(asyncOrYieldAsBindingIdentifier1.ts, 10, 1))
+
+    let await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 13, 7))
+}
+
+async function f2_var () {
+>f2_var : Symbol(f2_var, Decl(asyncOrYieldAsBindingIdentifier1.ts, 14, 1))
+
+    var await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 17, 7))
+}
+
+async function f2_const () {
+>f2_const : Symbol(f2_const, Decl(asyncOrYieldAsBindingIdentifier1.ts, 18, 1))
+
+    const await = 1
+>await : Symbol(await, Decl(asyncOrYieldAsBindingIdentifier1.ts, 21, 9))
+}
+
+function f3_let () {
+>f3_let : Symbol(f3_let, Decl(asyncOrYieldAsBindingIdentifier1.ts, 22, 1))
+
+    let yield = 2
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 25, 7))
+}
+
+function f3_var () {
+>f3_var : Symbol(f3_var, Decl(asyncOrYieldAsBindingIdentifier1.ts, 26, 1))
+
+    var yield = 2
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 29, 7))
+}
+
+function f3_const () {
+>f3_const : Symbol(f3_const, Decl(asyncOrYieldAsBindingIdentifier1.ts, 30, 1))
+
+    const yield = 2
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 33, 9))
+}
+
+function * f4_let () {
+>f4_let : Symbol(f4_let, Decl(asyncOrYieldAsBindingIdentifier1.ts, 34, 1))
+
+    let yield = 2;
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 37, 7))
+}
+
+function * f4_var () {
+>f4_var : Symbol(f4_var, Decl(asyncOrYieldAsBindingIdentifier1.ts, 38, 1))
+
+    var yield = 2;
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 41, 7))
+}
+
+function * f4_const () {
+>f4_const : Symbol(f4_const, Decl(asyncOrYieldAsBindingIdentifier1.ts, 42, 1))
+
+    const yield = 2;
+>yield : Symbol(yield, Decl(asyncOrYieldAsBindingIdentifier1.ts, 45, 9))
+}

tests/baselines/reference/asyncOrYieldAsBindingIdentifier1.types

@@ -0,0 +1,96 @@
+=== tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts ===
+function f_let () {
+>f_let : () => void
+
+    let await = 1
+>await : number
+>1 : 1
+}
+
+function f1_var () {
+>f1_var : () => void
+
+    var await = 1
+>await : number
+>1 : 1
+}
+
+function f1_const () {
+>f1_const : () => void
+
+    const await = 1
+>await : 1
+>1 : 1
+}
+
+async function f2_let () {
+>f2_let : () => Promise<void>
+
+    let await = 1
+>await : number
+>1 : 1
+}
+
+async function f2_var () {
+>f2_var : () => Promise<void>
+
+    var await = 1
+>await : number
+>1 : 1
+}
+
+async function f2_const () {
+>f2_const : () => Promise<void>
+
+    const await = 1
+>await : 1
+>1 : 1
+}
+
+function f3_let () {
+>f3_let : () => void
+
+    let yield = 2
+>yield : number
+>2 : 2
+}
+
+function f3_var () {
+>f3_var : () => void
+
+    var yield = 2
+>yield : number
+>2 : 2
+}
+
+function f3_const () {
+>f3_const : () => void
+
+    const yield = 2
+>yield : 2
+>2 : 2
+}
+
+function * f4_let () {
+>f4_let : () => Generator<never, void, unknown>
+
+    let yield = 2;
+>yield : number
+>2 : 2
+}
+
+function * f4_var () {
+>f4_var : () => Generator<never, void, unknown>
+
+    var yield = 2;
+>yield : number
+>2 : 2
+}
+
+function * f4_const () {
+>f4_const : () => Generator<never, void, unknown>
+
+    const yield = 2;
+>yield : 2
+>2 : 2
+}

tests/cases/conformance/async/es6/functionDeclarations/asyncOrYieldAsBindingIdentifier1.ts

@@ -0,0 +1,49 @@
+// @target: esnext
+
+function f_let () {
+    let await = 1
+}
+
+function f1_var () {
+    var await = 1
+}
+
+function f1_const () {
+    const await = 1
+}
+
+async function f2_let () {
+    let await = 1
+}
+
+async function f2_var () {
+    var await = 1
+}
+
+async function f2_const () {
+    const await = 1
+}
+
+function f3_let () {
+    let yield = 2
+}
+
+function f3_var () {
+    var yield = 2
+}
+
+function f3_const () {
+    const yield = 2
+}
+
+function * f4_let () {
+    let yield = 2;
+}
+
+function * f4_var () {
+    var yield = 2;
+}
+
+function * f4_const () {
+    const yield = 2;
+}