`<xmemory>`: `allocate_at_least` optimization broken by `private` inheritance from `std::allocator`

[StephanTLavavej]

After #3864, VSO-1852860 / AB#1852860 "[RWC][prod/fe][std:c++latest][Regression] Catch2 failed with error C2247 and error C2248" was reported, due to a custom allocator privately inheriting from std::allocator (see https://github.com/catchorg/Catch2/blob/4acc51828f7f93f3b2058a63f54d112af4034503/tests/SelfTest/UsageTests/Matchers.tests.cpp#L229-L254 ):

D:\GitHub\STL\out\x64>type meow.cpp

#include <cassert>
#include <memory>
#include <vector>
using namespace std;

template <typename T>
struct CustomAllocator : private allocator<T> {
    using value_type = T;

    CustomAllocator() = default;

    template <typename U>
    CustomAllocator(const CustomAllocator<U>&) {}

    using allocator<T>::allocate;
    using allocator<T>::deallocate;

    template <typename U>
    bool operator==(const CustomAllocator<U>&) const {
        return true;
    }
};

int main() {
    vector<int, CustomAllocator<int>> v;
    v.push_back(1729);
    assert(v.back() == 1729);
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++20 /MTd /Od meow.cpp && meow
meow.cpp

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od meow.cpp && meow
meow.cpp
D:\GitHub\STL\out\x64\out\inc\xmemory(2197): error C2247: 'std::allocator<int>::allocate_at_least' not accessible because 'CustomAllocator<int>' uses 'private' to inherit from 'std::allocator<int>'
D:\GitHub\STL\out\x64\out\inc\xmemory(978): note: see declaration of 'std::allocator<int>::allocate_at_least'
meow.cpp(25): note: see declaration of 'CustomAllocator<int>'
D:\GitHub\STL\out\x64\out\inc\xmemory(1049): note: see declaration of 'std::allocator<int>'
D:\GitHub\STL\out\x64\out\inc\vector(831): note: see reference to function template instantiation 'int *std::_Allocate_at_least_helper<CustomAllocator<int>>(_Alloc &,unsigned __int64 &)' being compiled
        with
        [
            _Alloc=CustomAllocator<int>
        ]
D:\GitHub\STL\out\x64\out\inc\vector(785): note: see reference to function template instantiation 'int *std::vector<int,CustomAllocator<int>>::_Emplace_reallocate<_Ty>(int *const ,_Ty &&)' being compiled
        with
        [
            _Ty=int
        ]
D:\GitHub\STL\out\x64\out\inc\vector(878): note: see reference to function template instantiation '_Ty &std::vector<_Ty,CustomAllocator<int>>::_Emplace_one_at_back<int>(int &&)' being compiled
        with
        [
            _Ty=int
        ]
D:\GitHub\STL\out\x64\out\inc\vector(876): note: while compiling class template member function 'void std::vector<int,CustomAllocator<int>>::push_back(_Ty &&)'
        with
        [
            _Ty=int
        ]
meow.cpp(26): note: see the first reference to 'std::vector<int,CustomAllocator<int>>::push_back' in 'main'
meow.cpp(25): note: see reference to class template instantiation 'std::vector<int,CustomAllocator<int>>' being compiled
D:\GitHub\STL\out\x64\out\inc\xmemory(2197): error C2248: 'std::allocator<int>::allocate_at_least': cannot access private member declared in class 'CustomAllocator<int>'
D:\GitHub\STL\out\x64\out\inc\xmemory(978): note: see declaration of 'std::allocator<int>::allocate_at_least'
meow.cpp(25): note: see declaration of 'CustomAllocator<int>'

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W4 /std:c++20 /MTd /Od meow.cpp && meow

D:\GitHub\STL\out\x64>clang-cl /EHsc /nologo /W4 /std:c++latest /MTd /Od meow.cpp && meow

D:\GitHub\STL\out\x64>

This appears to involve an MSVC compiler bug in SFINAE, because Clang accepts this code. For MSVC, it appears that classic SFINAE is properly detecting that typename _Ty::_From_primary is inaccessible, but then Expression SFINAE is incorrectly detecting that allocate_at_least is accessible:

STL/stl/inc/xmemory

Lines 473 to 480 in f51733c

	#if _HAS_CXX23
	template <class _Alloc, class _SizeTy, class = void>
	inline constexpr bool _Has_member_allocate_at_least = false;
	template <class _Alloc, class _SizeTy>
	inline constexpr bool _Has_member_allocate_at_least<_Alloc, _SizeTy,
	void_t<decltype(_STD declval<_Alloc&>().allocate_at_least(_STD declval<const _SizeTy&>()))>> = true;
	#endif // _HAS_CXX23

STL/stl/inc/xmemory

Lines 2177 to 2190 in f51733c

	#if _HAS_CXX23
	template <class _Ty, class = void>
	inline constexpr bool _Has_member_from_primary = false;
	template <class _Ty>
	inline constexpr bool _Has_member_from_primary<_Ty, void_t<typename _Ty::_From_primary>> = true;
	// Avoid using allocate_at_least when the allocator publicly derives from std::allocator:
	// "old" allocators might hide allocate and deallocate but fail to hide allocate_at_least.
	// Also avoid using allocate_at_least from std::allocator itself because it currently doesn't do anything useful.
	template <class _Alloc>
	inline constexpr bool _Should_allocate_at_least =
	!_Has_member_from_primary<_Alloc>
	&& _Has_member_allocate_at_least<_Alloc, typename allocator_traits<_Alloc>::size_type>;
	#endif // _HAS_CXX23

We should report this compiler bug, and then we may need to work around it with is_base_of to bluntly forbid all std::allocator inheritance (is_base_of sees through private and ambiguous bases).

[CaseyCarter]

I've filed the compiler bug as DevCom-10419218 "Expression SFINAE sometimes ignores access with two-phase lookup".