Type check when upcasting PassOwnPtr/PassOwnArrayPtr to avoid possible function call ambiguousness

Source/wtf/PassOwnPtr.h

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 32 matching lines @@
         typedef T ValueType;
         typedef ValueType* PtrType;
 
         PassOwnPtr() : m_ptr(0) { }
         PassOwnPtr(std::nullptr_t) : m_ptr(0) { }
 
         // It somewhat breaks the type system to allow transfer of ownership out of
         // a const PassOwnPtr. However, it makes it much easier to work with PassOwnPtr
         // temporaries, and we don't have a need to use real const PassOwnPtrs anyway.
         PassOwnPtr(const PassOwnPtr& o) : m_ptr(o.leakPtr()) { }
-        template<typename U> PassOwnPtr(const PassOwnPtr<U>& o) : m_ptr(o.leakPtr()) { }
+        template<typename U> PassOwnPtr(const PassOwnPtr<U>& o, EnsurePtrConvertibleArgDecl(U, T)) : m_ptr(o.leakPtr()) { }
 
         ~PassOwnPtr() { deleteOwnedPtr(m_ptr); }
 
         PtrType get() const { return m_ptr; }
 
         PtrType leakPtr() const WARN_UNUSED_RETURN;
 
         ValueType& operator*() const { ASSERT(m_ptr); return *m_ptr; }
         PtrType operator->() const { ASSERT(m_ptr); return m_ptr; }
 
@@ skipping 98 matching lines @@
     }
 
 } // namespace WTF
 
 using WTF::PassOwnPtr;
 using WTF::adoptPtr;
 using WTF::const_pointer_cast;
 using WTF::static_pointer_cast;
 
 #endif // WTF_PassOwnPtr_h