[libc++] Do not define views::all as a CPO since it isn't one

[ldionne]

Summary

Per http://eel.is/c++draft/range.adaptors#range.all.general-2, views::all
is a range adaptor object, but not a CPO. We don't need to define it like
a CPO to avoid name collisions with hidden friend functions of the same
name.

[• Quuxplusone]

https://eel.is/c++draft/range.adaptor.object#2

A range adaptor object is a customization point object...

[ldionne]

In D107169#2916640, @Quuxplusone wrote:

https://eel.is/c++draft/range.adaptor.object#2

A range adaptor object is a customization point object...

Hmm, fair enough, I missed that. However, I still stand by the fact that we don't need to use the inline namespace trick because we're not trying to "overload" a name that may already exist in the same namespace. In other words, views::all and ranges::swap are fundamentally different, because views::all is never expected to be customized by users. By the way, that makes me question why range adaptors are considered CPOs - am I missing something or do they really not need this inline namespace dance?

So I can change the commit message to not lie about views::all not being a CPO, but I'd still like someone to explain to me why a function object like views::all needs to be in an inline namespace. I understand why ranges::swap is, but not why views::all is. Anyone?

[ldionne]

I just talked to Arthur and was convinced that even though we don't *need* views::all to be implemented as a CPO, it's better to do it for consistency. Abandoning.

libcxx/include/__ranges/all.h

 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
 namespace views {
 
-namespace __all {
-  struct __fn {
-    template<class _Tp>
-      requires ranges::view<decay_t<_Tp>>
-    _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Tp&& __t) const
-      noexcept(noexcept(_VSTD::__decay_copy(_VSTD::forward<_Tp>(__t))))
-    {
-      return _VSTD::forward<_Tp>(__t);
-    }
+struct __all_adaptor {
 
-    template<class _Tp>
-      requires (!ranges::view<decay_t<_Tp>>) &&
-               requires (_Tp&& __t) { ranges::ref_view{_VSTD::forward<_Tp>(__t)}; }
-    _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Tp&& __t) const
-      noexcept(noexcept(ranges::ref_view{_VSTD::forward<_Tp>(__t)}))
-    {
-      return ranges::ref_view{_VSTD::forward<_Tp>(__t)};
-    }
 
-    template<class _Tp>
-      requires (!ranges::view<decay_t<_Tp>> &&
-                !requires (_Tp&& __t) { ranges::ref_view{_VSTD::forward<_Tp>(__t)}; } &&
-                 requires (_Tp&& __t) { ranges::subrange{_VSTD::forward<_Tp>(__t)}; })
-    _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Tp&& __t) const
-      noexcept(noexcept(ranges::subrange{_VSTD::forward<_Tp>(__t)}))
-    {
-      return ranges::subrange{_VSTD::forward<_Tp>(__t)};
-    }
-  };
-}
 
-inline namespace __cpo {
-  inline constexpr auto all = __all::__fn{};
-} // namespace __cpo
+inline constexpr auto all = __all_adaptor{};
 
 template<ranges::viewable_range _Range>
 using all_t = decltype(views::all(declval<_Range>()));
 
 } // namespace views
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___RANGES_ALL_H