[parser] Fix rewinding logic for destructuring in arrow params

Only rewind destructuring assignments if we actually preparsed
the arrow function. For the case of single-expression bodies,
we don't preparse, but we were previously erroneously rewinding.

Bug: v8:6970
Change-Id: I38e15a8a5bdb05abee3bafe7bbd7736b55a6950b
Reviewed-on: https://chromium-review.googlesource.com/733950Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48883}

src/parsing/parser-base.h

@@ -4358,6 +4358,13 @@ ParserBase<Impl>::ParseArrowFunctionLiteral(
         formal_parameters.scope->ResetAfterPreparsing(ast_value_factory_,
                                                       false);
 
+        // Discard any queued destructuring assignments which appeared
+        // in this function's parameter list.
+        FunctionState* parent_state = function_state.outer();
+        DCHECK_NOT_NULL(parent_state);
+        DCHECK_GE(parent_state->destructuring_assignments_to_rewrite().length(),
+                  rewritable_length);
+        parent_state->RewindDestructuringAssignments(rewritable_length);
       } else {
         Consume(Token::LBRACE);
         body = impl()->NewStatementList(8);
@@ -4394,14 +4401,6 @@ ParserBase<Impl>::ParseArrowFunctionLiteral(
     }
     impl()->CheckConflictingVarDeclarations(formal_parameters.scope, CHECK_OK);
 
-    if (is_lazy_top_level_function) {
-      FunctionState* parent_state = function_state.outer();
-      DCHECK_NOT_NULL(parent_state);
-      DCHECK_GE(parent_state->destructuring_assignments_to_rewrite().length(),
-                rewritable_length);
-      parent_state->RewindDestructuringAssignments(rewritable_length);
-    }
-
     impl()->RewriteDestructuringAssignments();
   }
 

test/mjsunit/regress/regress-6970.js

+// Copyright 2017 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.
+
+assertEquals(42, (({a = {b} = {b: 42}}) => a.b)({}));
+assertEquals(42, b);