[parser] Allow same-named labelled blocks in if/else statements

Bug: chromium:917215
Change-Id: Ie2127953a8ce3d6da4feb3478039ba318774e21d
Reviewed-on: https://chromium-review.googlesource.com/c/1404176Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58755}

src/parsing/parser-base.h

@@ -4777,7 +4777,13 @@ typename ParserBase<Impl>::StatementT ParserBase<Impl>::ParseIfStatement(
   StatementT then_statement = impl()->NullStatement();
   {
     SourceRangeScope range_scope(scanner(), &then_range);
-    then_statement = ParseScopedStatement(labels);
+    // Make a copy of {labels} to avoid conflicts with any
+    // labels that may be applied to the else clause below.
+    auto labels_copy =
+        labels == nullptr
+            ? labels
+            : new (zone()) ZonePtrList<const AstRawString>(*labels, zone());
+    then_statement = ParseScopedStatement(labels_copy);
   }
 
   StatementT else_statement = impl()->NullStatement();

test/mjsunit/regress/regress-917215.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.
+
+a: if (true) b: { break a; break b; }
+else b: { break a; break b; }