Help needed debugging std::is_convertible problem (PR 65760)

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65760

I don't understand why commenting out *any one* of the lines marked
(1), (2), (3), (4) causes this to compile:

#include <functional>

struct C {
    C() = default;

    C(std::function<C(int)>);  // (1)
    C(std::function<C(int, int)>); // (2)
    template <class T> C operator () (T&&); // (3)
    template <class T> C operator () (T&&, T&&); // (4)
};

int main() {
    C c = C();
}

My understanding is that the SFINAE constraint on the std::function
constructor needs to check std::is_convertible<C, C> which requires C
to be complete, but why does it only complain that C is incomplete
when all of (1) - (4) are present?

G++, Clang and EDG all agree on this behaviour, so I don't think it's
a compiler bug. The libc++ std::function works fine in this case, I
don't know what it does differently so that it works.

To fix it I am considering short-circuiting the constraint to not use
is_convertible<C, C>, by doing:

__or_<is_same<From, To>, is_convertible<From, To>>

which doesn't require the types to be complete when they're the same.
Before doing that I'd really like to understand the problem properly.

I've attached a reduced version with no header dependencies, based on
the relevant parts of <type_traits> and <functional> from 4.9.2.

namespace std
{
  typedef decltype(sizeof(0)) size_t;

  template<typename _Tp, _Tp __v>
    struct integral_constant
    {
      static constexpr _Tp value = __v;
      typedef integral_constant<_Tp, __v> type;
    };

  typedef integral_constant<bool, true> true_type;
  typedef integral_constant<bool, false> false_type;

  template<bool, typename, typename>
    struct conditional;

  template<typename...>
    struct __or_;

  template<>
    struct __or_<>
    : public false_type
    { };

  template<typename _B1>
    struct __or_<_B1>
    : public _B1
    { };

  template<typename _B1, typename _B2>
    struct __or_<_B1, _B2>
    : public conditional<_B1::value, _B1, _B2>::type
    { };

  template<typename _B1, typename _B2, typename _B3, typename... _Bn>
    struct __or_<_B1, _B2, _B3, _Bn...>
    : public conditional<_B1::value, _B1, __or_<_B2, _B3, _Bn...>>::type
    { };

  template<typename...>
    struct __and_;

  template<>
    struct __and_<>
    : public true_type
    { };

  template<typename _B1>
    struct __and_<_B1>
    : public _B1
    { };

  template<typename _B1, typename _B2>
    struct __and_<_B1, _B2>
    : public conditional<_B1::value, _B2, _B1>::type
    { };

  template<typename _B1, typename _B2, typename _B3, typename... _Bn>
    struct __and_<_B1, _B2, _B3, _Bn...>
    : public conditional<_B1::value, __and_<_B2, _B3, _Bn...>, _B1>::type
    { };

  template<typename _Pp>
    struct __not_
    : public integral_constant<bool, !_Pp::value>
    { };


  template<typename>
    struct remove_cv;

  template<typename>
    struct __is_void_helper
    : public false_type { };

  template<>
    struct __is_void_helper<void>
    : public true_type { };

  template<typename _Tp>
    struct is_void
    : public __is_void_helper<typename remove_cv<_Tp>::type>::type
    { };


  template<typename>
    struct is_array
    : public false_type { };

  template<typename _Tp, std::size_t _Size>
    struct is_array<_Tp[_Size]>
    : public true_type { };

  template<typename _Tp>
    struct is_array<_Tp[]>
    : public true_type { };

  template<typename>
    struct __is_pointer_helper
    : public false_type { };

  template<typename _Tp>
    struct __is_pointer_helper<_Tp*>
    : public true_type { };


  template<typename>
    struct is_lvalue_reference
    : public false_type { };

  template<typename _Tp>
    struct is_lvalue_reference<_Tp&>
    : public true_type { };


  template<typename>
    struct is_rvalue_reference
    : public false_type { };

  template<typename _Tp>
    struct is_rvalue_reference<_Tp&&>
    : public true_type { };

  template<typename>
    struct is_function;

  template<typename>
    struct is_function
    : public false_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...)>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......)>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) volatile>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) volatile &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) volatile &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) volatile>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) volatile &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) volatile &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const volatile>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const volatile &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes...) const volatile &&>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const volatile>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const volatile &>
    : public true_type { };

  template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) const volatile &&>
    : public true_type { };


  template<typename _Tp>
    struct is_reference
    : public __or_<is_lvalue_reference<_Tp>,
                   is_rvalue_reference<_Tp>>::type
    { };


  template<typename _Tp>
    struct is_object
    : public __not_<__or_<is_function<_Tp>, is_reference<_Tp>,
                          is_void<_Tp>>>::type
    { };


  template<typename _Tp>
    struct __is_referenceable
    : public __or_<is_object<_Tp>, is_reference<_Tp>>::type
    { };

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

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


  template<typename>
    struct add_rvalue_reference;

  template<typename _Tp>
    typename add_rvalue_reference<_Tp>::type declval() noexcept;


  template<typename, typename>
    struct is_same;

  template<typename, typename>
    struct is_same
    : public false_type { };

  template<typename _Tp>
    struct is_same<_Tp, _Tp>
    : public true_type { };


  template<typename _From, typename _To,
           bool = __or_<is_void<_From>, is_function<_To>,
                        is_array<_To>>::value>
    struct __is_convertible_helper
    { typedef typename is_void<_To>::type type; };

  template<typename _From, typename _To>
    class __is_convertible_helper<_From, _To, false>
    {
      template<typename _To1>
        static void __test_aux(_To1);

      template<typename _From1, typename _To1,
        typename = decltype(__test_aux<_To1>(std::declval<_From1>()))>
          static true_type
          __test(int);

      template<typename, typename>
        static false_type
        __test(...);

    public:
      typedef decltype(__test<_From, _To>(0)) type;
    };

  template<typename _From, typename _To>
    struct is_convertible
    : public __is_convertible_helper<_From, _To>::type
    { };

  template<typename _Tp>
    struct remove_const
    { typedef _Tp type; };

  template<typename _Tp>
    struct remove_const<_Tp const>
    { typedef _Tp type; };


  template<typename _Tp>
    struct remove_volatile
    { typedef _Tp type; };

  template<typename _Tp>
    struct remove_volatile<_Tp volatile>
    { typedef _Tp type; };


  template<typename _Tp>
    struct remove_cv
    {
      typedef typename
      remove_const<typename remove_volatile<_Tp>::type>::type type;
    };


  template<typename _Tp, bool = __is_referenceable<_Tp>::value>
    struct __add_rvalue_reference_helper
    { typedef _Tp type; };

  template<typename _Tp>
    struct __add_rvalue_reference_helper<_Tp, true>
    { typedef _Tp&& type; };


  template<typename _Tp>
    struct add_rvalue_reference
    : public __add_rvalue_reference_helper<_Tp>
    { };


  template<bool, typename _Tp = void>
    struct enable_if
    { };


  template<typename _Tp>
    struct enable_if<true, _Tp>
    { typedef _Tp type; };


  template<bool _Cond, typename _Iftrue, typename _Iffalse>
    struct conditional
    { typedef _Iftrue type; };


  template<typename _Iftrue, typename _Iffalse>
    struct conditional<false, _Iftrue, _Iffalse>
    { typedef _Iffalse type; };


  template<typename _Tp>
    struct __declval_protector
    {
      static const bool __stop = false;
      static typename add_rvalue_reference<_Tp>::type __delegate();
    };

  template<typename _Tp>
    inline typename add_rvalue_reference<_Tp>::type
    declval() noexcept
    {
      static_assert(__declval_protector<_Tp>::__stop,
      "declval() must not be used!");
      return __declval_protector<_Tp>::__delegate();
    }


  template<typename _Functor>
    inline _Functor&
    __callable_functor(_Functor& __f)
    { return __f; }

  template<typename _Signature>
    class function;

  template<typename _From, typename _To>
    using __check_func_return_type
      = __or_<is_void<_To>, is_convertible<_From, _To>>;


  template<typename _Res, typename... _ArgTypes>
    class function<_Res(_ArgTypes...)>
    {
      typedef _Res _Signature_type(_ArgTypes...);

      template<typename _Functor>
        using _Invoke = decltype(__callable_functor(std::declval<_Functor&>())
            (std::declval<_ArgTypes>()...) );

      template<typename _Tp>
        using _NotSelf = __not_<is_same<_Tp, function>>;

      template<typename _Functor>
        using _Callable
        = __and_<_NotSelf<_Functor>,
        __check_func_return_type<_Invoke<_Functor>, _Res>>;

      template<typename _Cond, typename _Tp>
        using _Requires = typename enable_if<_Cond::value, _Tp>::type;

    public:
      typedef _Res result_type;

      function() noexcept { }

      template<typename _Functor,
               typename = _Requires<_Callable<_Functor>, void>>
        function(_Functor);
  };

}

struct C {
    C() = default;

    C(std::function<C(int)>);
    C(std::function<C(int, int)>);
    template <class T> C operator () (T&&);
    template <class T> C operator () (T&&, T&&);
};

int main() {
    C c = C();
}