  template<typename _Res, typename... _Args _GLIBCXX_NOEXCEPT_PARM>
    struct __is_referenceable<_Res(_Args...) _GLIBCXX_NOEXCEPT_QUAL>
    : public true_type
    { };

  template<typename _Res, typename... _Args _GLIBCXX_NOEXCEPT_PARM>
    struct __is_referenceable<_Res(_Args......) _GLIBCXX_NOEXCEPT_QUAL>
    : public true_type
    { };