<xhash>: Some member functions of transparent hash containers fail to work with initializer lists

[frederick-vs-ja]

Describe the bug

From DevCom-10819520.

The following program don't compile with MSVC STL.

#include <unordered_map>
#include <unordered_set>

struct elem { 
    elem(unsigned val) : value(val) {}
    unsigned value = 0;
};
struct hash {
    using is_transparent = void;
    unsigned operator()(elem e) const { return e.value; }
};
struct equal {
    using is_transparent = void;
    unsigned operator()(elem e1, elem e2) const { return e1.value == e2.value; }
};

int main() {
    std::unordered_map<elem, int, hash, equal> map;
    (void)map.find(1u);   // OK
    (void)map.find({1u}); // MSVC: compile error, GCC: OK
    std::unordered_set<elem, hash, equal> set;
    (void)set.find(1u);   // OK
    (void)set.find({1u}); // MSVC: compile error, GCC: OK
}

Command-line test case

Godbolt link

Expected behavior

The program compiles. No "reference to cv void" is attempted to formed.

I think this bug is due to wrong _Deduce_key type.

STL/stl/inc/xhash

Lines 108 to 121 in 9082000

	#if _HAS_CXX20
	template <class _Kty, class _Hasher, class _Keyeq>
	requires _Is_transparent_v<_Hasher> && _Is_transparent_v<_Keyeq>
	struct _Uhash_choose_transparency<_Kty, _Hasher, _Keyeq> {
	// transparency selector for transparent hashed containers
	template <class _Keyty>
	using _Deduce_key = const _Keyty&;
	template <class _Container, class _Kx>
	static constexpr bool _Supports_transparency =
	!disjunction_v<is_convertible<_Kx, typename _Container::const_iterator>,
	is_convertible<_Kx, typename _Container::iterator>>;
	};
	#endif // _HAS_CXX20

When brace initializer list is used as argument, the implementation should (as-if) fallback to the non-transparent overloads. But the wrong const void& type is attempted to be formed now.

STL version

All existing versions since VS 2019 16.5 (which got WG21-P1690R1 implemented).

Additional context

BTW, MSVC STL currently accepts the following bogus program even before C++20 (Godbolt link).

#include <unordered_set>

int main()
{
    std::unordered_set<int> m{1, 2, 3};
    auto it = m.find<void()>(1);
    return it == m.end();
}

Although this may still be conforming due to [member.functions]/2.