scoped_refptr: support opaque pointed-to classes via extern template

base/test/opaque_ref_counted.h

Index: base/test/opaque_ref_counted.h
diff --git a/base/test/opaque_ref_counted.h b/base/test/opaque_ref_counted.h
new file mode 100644
index 0000000000000000000000000000000000000000..faf6a650fd6069135d77bb4a9f827295bd81afa3
--- /dev/null
+++ b/base/test/opaque_ref_counted.h
@@ -0,0 +1,24 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TEST_OPAQUE_REF_COUNTED_H_
+#define BASE_TEST_OPAQUE_REF_COUNTED_H_
+
+#include "base/memory/ref_counted.h"
+
+namespace base {
+
+// OpaqueRefCounted is a test class for scoped_refptr to ensure it still works
+// when the pointed-to type is opaque (i.e., incomplete).
+class OpaqueRefCounted;
+
+// Test functions that return and accept scoped_refptr<OpaqueRefCounted> values.
+scoped_refptr<OpaqueRefCounted> MakeOpaqueRefCounted();
+void TestOpaqueRefCounted(scoped_refptr<OpaqueRefCounted> p);
+
+}  // namespace base
+
+extern template class scoped_refptr<base::OpaqueRefCounted>;
+
+#endif  // BASE_TEST_OPAQUE_REF_COUNTED_H_