[es6] Correct length for functions with default parameters

src/scopes.cc

 // Copyright 2012 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.
 
 #include "src/scopes.h"
 
 #include "src/accessors.h"
 #include "src/bootstrapper.h"
 #include "src/messages.h"
 #include "src/parser.h"
@@ skipping 162 matching lines @@
   inner_scope_uses_arguments_ = false;
   force_eager_compilation_ = false;
   force_context_allocation_ = (outer_scope != NULL && !is_function_scope())
       ? outer_scope->has_forced_context_allocation() : false;
   num_var_or_const_ = 0;
   num_stack_slots_ = 0;
   num_heap_slots_ = 0;
   num_global_slots_ = 0;
   num_modules_ = 0;
   module_var_ = NULL;
+  arity_ = 0;
   has_simple_parameters_ = true;
   rest_parameter_ = NULL;
   rest_index_ = -1;
   scope_info_ = scope_info;
   start_position_ = RelocInfo::kNoPosition;
   end_position_ = RelocInfo::kNoPosition;
   if (!scope_info.is_null()) {
     scope_calls_eval_ = scope_info->CallsEval();
     language_mode_ = scope_info->language_mode();
     is_declaration_scope_ = scope_info->is_declaration_scope();
@@ skipping 265 matching lines @@
   for (Scope* scope = this;
        scope != NULL;
        scope = scope->outer_scope()) {
     Variable* var = scope->LookupLocal(name);
     if (var != NULL) return var;
   }
   return NULL;
 }
 
 
-Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
-                                  bool is_rest, bool* is_duplicate) {
+Variable* Scope::DeclareParameter(
+    const AstRawString* name, VariableMode mode,
+    bool is_optional, bool is_rest, bool* is_duplicate) {
   DCHECK(!already_resolved());
   DCHECK(is_function_scope());
+  DCHECK(!is_optional || !is_rest);
   Variable* var;
   if (mode == TEMPORARY) {
     var = NewTemporary(name);
     has_simple_parameters_ = false;
   } else {
     var = variables_.Declare(this, name, mode, Variable::NORMAL,
                              kCreatedInitialized);
     // TODO(wingo): Avoid O(n^2) check.
     *is_duplicate = IsDeclaredParameter(name);
   }
+  if (!is_optional && !is_rest && arity_ == params_.length()) {
+    ++arity_;
+  }
   if (is_rest) {
     DCHECK_NULL(rest_parameter_);
     rest_parameter_ = var;
     rest_index_ = num_parameters();
     has_simple_parameters_ = false;
   }
   params_.Add(var, zone());
   return var;
 }
 
@@ skipping 1124 matching lines @@
       function_ != NULL && function_->proxy()->var()->IsContextSlot();
   return num_heap_slots() - Context::MIN_CONTEXT_SLOTS - num_global_slots() -
          (is_function_var_in_context ? 1 : 0);
 }
 
 
 int Scope::ContextGlobalCount() const { return num_global_slots(); }
 
 }  // namespace internal
 }  // namespace v8