Do not ignore an empty context with extension when creating a scope object.

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index d0ee8ec7f21af11758a2cb3c49dcf9f8a2aa6b18..b03ea3d00777a93b3366365d611c1b325abb7c9f 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -137,7 +137,8 @@ Scope::Scope(Scope* outer_scope, ScopeType type)
 
 Scope::Scope(Scope* inner_scope,
              ScopeType type,
-             Handle<ScopeInfo> scope_info)
+             Handle<ScopeInfo> scope_info,
+             bool has_context_with_extension)
     : isolate_(Isolate::Current()),
       inner_scopes_(4),
       variables_(),
@@ -147,12 +148,10 @@ Scope::Scope(Scope* inner_scope,
       decls_(4),
       already_resolved_(true) {
   SetDefaults(type, NULL, scope_info);
-  if (!scope_info.is_null()) {
+  if (!scope_info.is_null() && *scope_info != ScopeInfo::Empty()) {

[Kevin Millikin (Chromium), 2012/02/02 08:36:13]

As far as I can tell, there is no reason *at all* that this function should ever
produce a scope indicating there's no context---because there is definitely a
materialized context.  I think it's much more straightforward to write this
function as:

SetDefaults(type, NULL, scope_info);
if (!scope_info.is_null()) {
  num_heap_slots_ = scope_info->ContextLength();
}
// Ensure at least MIN_CONTEXT_SLOTS to indicate a materialized context.
num_heap_slots_ = Max(num_heap_slots_, Context::MIN_CONTEXT_SLOTS);
AddInnerScope(inner_scope);

And no need for the default-valued parameter.  Does that implementation make
sense?

[ulan, 2012/02/02 09:16:19]

Indeed, this constructor is called only from DeserializeScopeChain.

     num_heap_slots_ = scope_info_->ContextLength();
-    if (*scope_info != ScopeInfo::Empty()) {

[Kevin Millikin (Chromium), 2012/02/02 08:36:13]

As a simple cleanup, you can get rid of this completely (SetDefaults already set
language_mode for all scopes, no need to overwrite it---unless I'm reading the
code wrong).

-      language_mode_ = scope_info->language_mode();
-    }
-  } else if (is_with_scope()) {
+    language_mode_ = scope_info->language_mode();
+  } else if (is_with_scope() || has_context_with_extension) {
     num_heap_slots_ = Context::MIN_CONTEXT_SLOTS;
   }
   AddInnerScope(inner_scope);
@@ -235,7 +234,8 @@ Scope* Scope::DeserializeScopeChain(Context* context, Scope* global_scope) {
       ScopeInfo* scope_info = context->closure()->shared()->scope_info();
       current_scope = new Scope(current_scope,
                                 FUNCTION_SCOPE,
-                                Handle<ScopeInfo>(scope_info));
+                                Handle<ScopeInfo>(scope_info),
+                                context->extension() != NULL);
     } else if (context->IsBlockContext()) {
       ScopeInfo* scope_info = ScopeInfo::cast(context->extension());
       current_scope = new Scope(current_scope,