`<atomic>`: `atomic_ref<void*>` and `atomic<void*>` lack `difference_type`

[StephanTLavavej]

While updating our libcxx test suite, I found that https://github.com/llvm/llvm-project/blob/af6812085cc7a7251a3095acbc96343ce660f135/libcxx/test/std/atomics/atomics.ref/member_types.pass.cpp is failing with:

error: no type named 'difference_type' in 'std::atomic_ref<void *>'

AFAICT, this happens because atomic_ref:

STL/stl/inc/atomic

Lines 2323 to 2324 in 63354c3

	_EXPORT_STD template <class _Ty>
	struct atomic_ref : _Choose_atomic_base_t<_Ty, _Ty&> { // atomic reference

and atomic:

STL/stl/inc/atomic

Lines 2143 to 2144 in 63354c3

	_EXPORT_STD template <class _Ty>
	struct atomic : _Choose_atomic_base_t<_Ty> { // atomic value

check for is_pointer_v<_TVal> && is_object_v<remove_pointer_t<_TVal>>:

STL/stl/inc/atomic

Lines 2128 to 2141 in 63354c3

	template <class _TVal, class _Ty = _TVal>
	using _Choose_atomic_base2_t =
	typename _Select<is_integral_v<_TVal> && !is_same_v<bool, _TVal>>::template _Apply<_Atomic_integral_facade<_Ty>,
	typename _Select<is_pointer_v<_TVal> && is_object_v<remove_pointer_t<_TVal>>>::template _Apply<
	_Atomic_pointer<_Ty>, _Atomic_storage<_Ty>>>;
	#if _HAS_CXX20
	template <class _TVal, class _Ty = _TVal>
	using _Choose_atomic_base_t =
	_Select<is_floating_point_v<_TVal>>::template _Apply<_Atomic_floating<_Ty>, _Choose_atomic_base2_t<_TVal, _Ty>>;
	#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
	template <class _TVal, class _Ty = _TVal>
	using _Choose_atomic_base_t = _Choose_atomic_base2_t<_TVal, _Ty>;
	#endif // ^^^ !_HAS_CXX20 ^^^

Yet WG21-N4981 [atomics.ref.pointer] depicts:

template<class T> struct atomic_ref<T*> {
private:
  T** ptr; // exposition only

public:
  using value_type = T*;
  using difference_type = ptrdiff_t;

and [atomics.types.pointer] depicts:

template<class T> struct atomic<T*> {
  using value_type = T*;
  using difference_type = ptrdiff_t;

without making any special cases for void*.