Ill-formed C++0x code or compiler bug (GCC) ?

SG <>
Fri, 28 Jan 2011 02:36:08 -0800 (PST)

In the following C++0x code I tried to clone an object by using a
clone member function (if it exists) and falling back on a copy

   #include <type_traits>

   struct use_copy_ctor {};
   struct prefer_clone_func : use_copy_ctor {};

   template<class T>
   auto clone(T const* ptr, prefer_clone_func)
   -> decltype(ptr->clone())
   { return ptr->clone(); }

   template<class T>
   auto clone(T const* ptr, use_copy_ctor)
   -> decltype(new T(*ptr))
   { return new T(*ptr); }

   struct abc {
     virtual ~abc() {}
     virtual abc* clone() const =0;

   struct derived : abc
     derived* clone() const { return new derived(*this); }

   int main()
     derived d;
     abc* p = &d;
     abc* q = clone(p,prefer_clone_func());
     delete q;

The idea is to use auto...->decltype(expr) to weed out ill-formed
expressions as part of the template argument deduction (SFINAE) and to
resolve a possible ambiguity between both clone function templates via
partial ordering w.r.t. the second function parameter.

Unfortunately, GCC 4.5.1 doesn't accept this program:

test.cpp: In function 'int main()':
test.cpp:30:39: error: cannot allocate an object of abstract type
test.cpp:16:12: note: because the following virtual functions are
pure within 'abc':
test.cpp:18:16: note: virtual abc* abc::clone() const
test.cpp:30:39: error: cannot allocate an object of abstract type
test.cpp:16:12: note: since type 'abc' has pure virtual functions

Now, the question is, is this a compiler bug or was I wrong to assume
that SFINAE applies here?


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