[Bug libstdc++/96710] __int128 vs <type_traits>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96710

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> Our definitions of is_scalar depends on is_arithmetic, so
> is_scalar<__int128> is false, and therefore is_object<__int128> is false.
> This is clearly nonsense.

Hmm, what I wrote is nonsense. Our is_object does not depend on is_scalar:

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

So is_object<__int128> is always true.

But is_scalar<__int128> still depends on __STRICT_ANSI__ which seems wrong.
It's not a compound type, so it's scalar.


Currently we define is_scalar as:

  template<typename _Tp>
    struct is_scalar
    : public __or_<is_arithmetic<_Tp>, is_enum<_Tp>, is_pointer<_Tp>,
                   is_member_pointer<_Tp>, is_null_pointer<_Tp>>::type
    { };

I think a better definition would be:

  template<typename _Tp>
    struct is_scalar
    : public __and_<is_object<_Tp>, __not_<is_array<_Tp>>,
                                    __not_<is_class<_Tp>>,
                                    __not_<is_union<_Tp>>>
    { };

Which could be optimized using partial specializations for the array cases:

  template<typename _Tp>
    struct is_scalar
    : public __and_<is_object<_Tp>, __not_<is_class<_Tp>>,
                                    __not_<is_union<_Tp>>>
    { };

  template<typename _Tp>
    struct is_scalar<_Tp[]>
    : public false_type
    { };

  template<typename _Tp, size_t _Num>
    struct is_scalar<_Tp[_Num]>
    : public false_type
    { };