[libc++] static_cast in `ranges::__crend::__fn` is not required

[Fznamznon]

I discovered this while working on D152003 which is a fix for #62863 . Per C++20 static cast to array of unknown bound U[] should yield the type U[1].
Now D152003 breaks libc++ tests, since the following static assertion fails with it:

#include <ranges>

using RangeCREndT = decltype(std::ranges::crend);
static_assert(!std::is_invocable_v<RangeCREndT, int (&)[]>);

source/llvm-project/build/bin/clang++ -std=c++20 t.cpp -stdlib=libc++ -c
t.cpp:4:15: error: static assertion failed due to requirement '!std::is_invocable_v<const std::ranges::__crend::__fn, int (&)[]>'
    4 | static_assert(!std::is_invocable_v<RangeCREndT, int (&)[]>);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

I noticed that that in libc++ there is a bunch of explicit static casts in ranges::__crend::__fn that endup transformed:

namespace ranges {
namespace __crend {                                                                    
struct __fn {
  template <class _Tp>
    requires is_lvalue_reference_v<_Tp&&>                                              
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Tp&& __t) const
    noexcept(noexcept(ranges::rend(static_cast<const remove_reference_t<_Tp>&>(__t))))
    -> decltype(      ranges::rend(static_cast<const remove_reference_t<_Tp>&>(__t)))  
    { return          ranges::rend(static_cast<const remove_reference_t<_Tp>&>(__t)); }
                                                                                       
  template <class _Tp>                                                                 
    requires is_rvalue_reference_v<_Tp&&>                                              
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Tp&& __t) const
    noexcept(noexcept(ranges::rend(static_cast<const _Tp&&>(__t))))                    
    -> decltype(      ranges::rend(static_cast<const _Tp&&>(__t)))
    { return          ranges::rend(static_cast<const _Tp&&>(__t)); }
};
} // namespace __crend

Per discussion in D152003 we agreed that it is a bug in libc++ so I'm filing this issue so it won't be lost/forgotten.

cc @mordante @philnik777

[AaronBallman]

CC @ldionne -- I believe this is blocking progress on landing the changes in https://reviews.llvm.org/D152003, any chance someone in libc++ can pick this one up (I'm mostly trying to clear the Phab backlog, so if this is a Big Ask, "no" is a perfectly fine answer).

[mordante]

@var-const @philnik777 do you have time to look at @AaronBallman's question?

[ldionne]

Otherwise perhaps @cjdb, who implemented these functions IIRC.

It would be awesome to fix this in order to unblock Clang (by the way, thanks Clang folks for the heads up on this issue and waiting for us to fix our bug to avoid breaking our build, this is appreciated).

[CaseyCarter]

I think this is not a bug. The target type of these static_casts is always a reference type, and therefore never an array type (of known or unknown bound).

[CaseyCarter]

Yeah, I think the bug here is that D152003 looks at the type of the static_cast expression itself (ignoring value category) instead of the target type of the conversion. static_cast<T>(meow) has type remove_reference_t<T>, but decltype(static_cast<T>(meow)) is T. In other words, the fix should apply only when T is an array of unknown bound (static_cast<T>(meow) is a prvalue array of unknown bound) and not when T is a reference to an array of unknown bound (static_cast<T>(meow) is a glvalue array of unknown bound).