`<vector>`: `vector`s with fancy pointers cause `ranges::equal` to fail STL-internal checks

[StephanTLavavej]

Found while updating the libcxx test suite. I'm looking into a fix.

ranges::equal doesn't tolerate vectors with fancy pointers when STL-internal checks are enabled:

C:\Temp>type meow.cpp

#include "min_allocator.h"
#include <algorithm>
#include <cassert>
#include <vector>

int main() {
    const std::vector<int, min_allocator<int>> v{11, 22, 33};
    const int arr[]{11, 22, 33};
    assert(std::ranges::equal(v, arr));
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /D_ENABLE_STL_INTERNAL_CHECK /I D:\GitHub\stl\llvm-project\libcxx\test\support meow.cpp
meow.cpp
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33218\include\xutility(2576): error C2338: static_assert failed: 'same_as<decltype(_STD declval<_Ty>()._Unchecked_begin()), _Unwrapped_iterator_t<_Ty>>'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33218\include\xutility(2576): note: the template instantiation context (the oldest one first) is
meow.cpp(9): note: see reference to function template instantiation 'bool std::ranges::_Equal_fn::operator ()<const std::vector<int,min_allocator<int>>&,const int(&)[3],std::ranges::equal_to,std::identity,std::identity>(_Rng1,_Rng2,_Pr,_Pj1,_Pj2) const' being compiled
        with
        [
            _Rng1=const std::vector<int,min_allocator<int>> &,
            _Rng2=const int (&)[3],
            _Pr=std::ranges::equal_to,
            _Pj1=std::identity,
            _Pj2=std::identity
        ]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33218\include\xutility(2592): note: see reference to variable template 'const std::_Choice_t<std::ranges::_Unchecked_begin::_Cpo::_St> std::ranges::_Unchecked_begin::_Cpo::_Choice<std::vector<int,min_allocator<int> > const &>' being compiled
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33218\include\xutility(2588): note: see reference to function template instantiation 'std::_Choice_t<std::ranges::_Unchecked_begin::_Cpo::_St> std::ranges::_Unchecked_begin::_Cpo::_Choose<const std::vector<int,min_allocator<int>>&>(void) noexcept' being compiled

This is a good check:

STL/stl/inc/xutility

Lines 2571 to 2572 in 0403d19

	_STL_INTERNAL_STATIC_ASSERT(
	same_as<decltype(_STD declval<_Ty>()._Unchecked_begin()), _Unwrapped_iterator_t<_Ty>>);

_Vector_const_iterator unwraps to raw pointers:

STL/stl/inc/vector

Lines 216 to 222 in 0403d19

	_NODISCARD _CONSTEXPR20 const value_type* _Unwrapped() const noexcept {
	return _Unfancy(_Ptr);
	}
	_CONSTEXPR20 void _Seek_to(const value_type* _It) noexcept {
	_Ptr = _Refancy<_Tptr>(const_cast<value_type*>(_It));
	}

But vector::_Unchecked_begin() returns fancy pointers:

STL/stl/inc/vector

Lines 1858 to 1860 in 0403d19

	_NODISCARD _CONSTEXPR20 pointer _Unchecked_begin() noexcept {
	return _Mypair._Myval2._Myfirst;
	}

I think we should resolve this by making vector::_Unchecked_begin() and _Unchecked_end() return raw pointers, but I haven't verified this yet.