[parser] Fix conflict detection loop early exit

During conflict detection, we want to early exit the scope loop when we
find a non-conflict, but continue looking at the other declarations in
the scope.

Bug: chromium:1038588
Change-Id: Ia2a19b02222fbd13cec70d3a60d2f5bae4ce245b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1985991
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65602}

src/ast/scopes.cc

@@ -1187,7 +1187,7 @@ Declaration* DeclarationScope::CheckConflictingVarDeclarations() {
         // anything, so we can't conflict with anything either. The one
         // exception is the binding variable in catch scopes, which is handled
         // by the if above.
-        if (!IsLexicalVariableMode(other_var->mode())) return nullptr;
+        if (!IsLexicalVariableMode(other_var->mode())) break;
         return decl;
       }
       current = current->outer_scope();

test/mjsunit/regress/regress-1038588.js

+// Copyright 2020 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.
+
+function foo(arg){
+  const x = 0;
+  eval("var arg, x;");
+}
+assertThrows(foo, SyntaxError);