[parser] Clear is_parenthesized on ThisExpression when accessing it

Otherwise (this) will leak into a later this=> making it seem like a valid
arrow function head.

Bug: chromium:941703
Change-Id: I5c3ff70f1d525ec0da53b401a0bfec4c1ee7812f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1601260
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61345}

src/ast/ast.h

@@ -2938,6 +2938,13 @@ class AstNodeFactory final {
   }
 
   class ThisExpression* ThisExpression() {
+    // Clear any previously set "parenthesized" flag on this_expression_ so this
+    // particular token does not inherit the it. The flag is used to check
+    // during arrow function head parsing whether we came from parenthesized
+    // exprssion parsing, since additional arrow function verification was done
+    // there. It does not matter whether a flag is unset after arrow head
+    // verification, so clearing at this point is fine.
+    this_expression_->clear_parenthesized();
     return this_expression_;
   }
 

test/mjsunit/regress/regress-crbug-941703.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertThrows("(this) , this =>", SyntaxError);