[parser] Add early return for declaration error in arrow head

Otherwise the expression scope may be in a weird state and DCHECKs for valid
arrow functions in ValidateAndCreateScope willl unnecessarily fire.

Bug: chromium:1018611
Change-Id: I101b8902dce07c29aacba3e7a5e6f86d66505d5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1879906Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64591}

src/parsing/expression-scope.h

@@ -747,13 +747,14 @@ class ArrowHeadParsingScope : public ExpressionParsingScope<Types> {
 
   DeclarationScope* ValidateAndCreateScope() {
     DCHECK(!this->is_verified());
+    DeclarationScope* result = this->parser()->NewFunctionScope(kind());
     if (declaration_error_location.IsValid()) {
       ExpressionScope<Types>::Report(declaration_error_location,
                                      declaration_error_message);
+      return result;
     }
     this->ValidatePattern();
 
-    DeclarationScope* result = this->parser()->NewFunctionScope(kind());
     if (!has_simple_parameter_list_) result->SetHasNonSimpleParameters();
     VariableKind kind = PARAMETER_VARIABLE;
     VariableMode mode =

test/mjsunit/regress/regress-crbug-1018611.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("(l-(c))=>", SyntaxError);