[Bug c++/53039] [4.7/4.8 Regression] including <functional> breaks std::is_convertible with template-pack expansion

["daniel.kruegler at googlemail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53039

--- Comment #7 from Daniel KrÃgler <daniel.kruegler at googlemail dot com> 2012-04-20 06:48:39 UTC ---
I'm pretty sure that this is not related with <functional>, but instead with
some interaction between the <tuple> header and std::is_convertible. The
following variant still demonstrates the problem (I deliberately changed some
of the original names used in the code to be sure that they are not cause of
the problem):

#include <tuple>

template<bool, class T>
struct enable_if
{
  typedef T type;
};

template <bool...>
struct Bs
{
  static const bool value = true;
};

template <typename... Ts>
struct AType {
  template <typename... Us,
            typename enable_if<
               Bs<std::is_convertible<Us, Ts>::value...>::value,
               bool>::type = false
  >
  int foo(Us&&...)
  {
    return sizeof...(Us);
  }
};

int main() {
  AType<int, int> t;
  return t.foo(1, 1);
}

(You can also add <type_traits>, but it is already added via <tuple>). If we
now add upfront

template <class, class>
struct is_convertible
{
  static const bool value = true;
};

and replace the usage of std::is_convertible by is_convertible, the error does
no longer occur.