Add Persistent::Upcast.

test/cctest/test-api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2865 matching lines @@
   String* str = global.ClearAndLeak();
   CHECK(global.IsEmpty());
   CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count + 1);
   v8::Persistent<String>* new_global =
       reinterpret_cast<v8::Persistent<String>*>(&str);
   new_global->Dispose();
   CHECK_EQ(global_handles->NumberOfGlobalHandles(), initial_handle_count);
 }
 
 
+THREADED_TEST(GlobalHandleUpcast) {
+  v8::Isolate* isolate = v8::Isolate::GetCurrent();
+  v8::HandleScope scope(isolate);
+  v8::Local<String> local = v8::Local<String>::New(v8_str("str"));
+  v8::Persistent<String> global_string(isolate, local);
+  v8::Persistent<Value>& global_value =
+      v8::Persistent<Value>::Upcast(global_string);
+  CHECK(global_value->IsString());
+  CHECK(global_string == v8::Persistent<String>::Cast(global_value));
+  global_string.Dispose();
+}
+
+
 THREADED_TEST(LocalHandle) {
   v8::HandleScope scope(v8::Isolate::GetCurrent());
   v8::Local<String> local = v8::Local<String>::New(v8_str("str"));
   CHECK_EQ(local->Length(), 3);
 
   local = v8::Local<String>::New(v8::Isolate::GetCurrent(), v8_str("str"));
   CHECK_EQ(local->Length(), 3);
 }
 
 
@@ skipping 16198 matching lines @@
   i::Semaphore* sem_;
   volatile int sem_value_;
 };
 
 
 THREADED_TEST(SemaphoreInterruption) {
   ThreadInterruptTest().RunTest();
 }
 
 #endif  // WIN32