Fix bug with re-scoping arrow function parameter initializers

src/parsing/parameter-initializer-rewriter.cc

Index: src/parsing/parameter-initializer-rewriter.cc
diff --git a/src/parsing/parameter-initializer-rewriter.cc b/src/parsing/parameter-initializer-rewriter.cc
index 6362c63f8ec65863e97ffac96f362a1e6ced4c9d..bb4e6eeca51b06cdddbd6b24859539d0273d47c7 100644
--- a/src/parsing/parameter-initializer-rewriter.cc
+++ b/src/parsing/parameter-initializer-rewriter.cc
@@ -55,8 +55,9 @@ Rewriter::~Rewriter() {
     // Ensure that we add temporaries in the order they appeared in old_scope_.
     std::sort(temps_.begin(), temps_.end(), LessThanSecond());
     for (auto var_and_index : temps_) {
-      var_and_index.first->set_scope(new_scope_);
-      new_scope_->AddTemporary(var_and_index.first);
+      Scope* scope = new_scope_->ClosureScope();
+      var_and_index.first->set_scope(scope);
+      scope->AddTemporary(var_and_index.first);
     }
   }
 }
@@ -90,11 +91,11 @@ void Rewriter::VisitVariableProxy(VariableProxy* proxy) {
   if (proxy->is_resolved()) {
     Variable* var = proxy->var();
     if (var->mode() != TEMPORARY) return;
-    // For rewriting inside the same ClosureScope (e.g., putting default
-    // parameter values in their own inner scope in certain cases), refrain
-    // from invalidly moving temporaries to a block scope.
-    if (var->scope()->ClosureScope() == new_scope_->ClosureScope()) return;
-    int index = old_scope_->RemoveTemporary(var);
+    // Temporaries are only placed in ClosureScopes.
+    DCHECK_EQ(var->scope(), var->scope()->ClosureScope());
+    // If the temporary is already where it should be, return quickly.
+    if (var->scope() == new_scope_->ClosureScope()) return;
+    int index = old_scope_->ClosureScope()->RemoveTemporary(var);

[adamk, 2016/06/27 18:49:31]

What if the rewriter computed the ClosureScopes of both the old and new scopes
during construction? It's a scope chain walk every time otherwise...

[nickie, 2016/06/28 09:26:37]

Done.

     if (index >= 0) {
       temps_.push_back(std::make_pair(var, index));
     }