[Bug c++/51336] New: [C++11] is_abstract and sfinae

["marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51336

             Bug #: 51336
           Summary: [C++11] is_abstract and sfinae
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: marc.glisse@normalesup.org


#include <type_traits>
template<class T>
struct A {
        template<class=typename
std::enable_if<std::is_same<T,int>::value>::type>
                A(A const&){}
};
constexpr bool b = std::is_abstract<A<double>>::value;

$ g++ -std=c++0x n.cc -c
n.cc: In instantiation of 'struct A<double>':
/tmp/gcc/inst/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/type_traits:530:12:
  required from 'struct std::is_abstract<A<double> >'
n.cc:7:47:   required from here
n.cc:5:3: error: no type named 'type' in 'struct std::enable_if<false, void>'

I am not sure what is supposed to happen (that's why I tried), but this result
doesn't seem right. Filed under C++ because is_abstract directly forwards to
the __is_abstract builtin, but feel free to reassign to libstdc++ if you think
the problem is there somehow.

I first tried it with is_copy_constructible (which indirectly calls is_abstract
through is_destructible) to check the value it would give (clang+libc++ says
is_copy_constructible is true).