[Bug c++/47513] New: [C++0x] [SFINAE] compiler rejects valid code

[s.gesemann at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47513

           Summary: [C++0x] [SFINAE] compiler rejects valid code
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: s.gesemann@gmail.com


I believe the following code should compile using the option -std=c++0x:

   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;
   }

As far as I can tell SFINAE applies during template argument deduction where
abc is substituted for T in decltype(new T(*ptr)). GCC complains about abc
being abstract -- which it is, of course -- but it should instead simply ignore
the second function template.

Related note: Assuming std::is_constructible is implemented in terms of
decltype, this would explain why std::is_constructible doesn't work either on
abstract class types.