[parser] Don't verify arrow declaration positions if we're going to fail

Bug: chromium:1018611
Change-Id: I2d48673aa5a05e67a6b253415e66af975b693af5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1936470
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65171}

src/parsing/expression-scope.h

@@ -766,8 +766,11 @@ class ArrowHeadParsingScope : public ExpressionParsingScope<Types> {
     }
 
 #ifdef DEBUG
-    for (auto declaration : *result->declarations()) {
-      DCHECK_NE(declaration->var()->initializer_position(), kNoSourcePosition);
+    if (!this->has_error()) {
+      for (auto declaration : *result->declarations()) {
+        DCHECK_NE(declaration->var()->initializer_position(),
+                  kNoSourcePosition);
+      }
     }
 #endif  // DEBUG
 

test/mjsunit/regress/regress-crbug-1018611-2.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("({a, b: ([c])}) => { }", SyntaxError);