<atomic>: atomic_ref<const T> fails to compile

[mocabe]

Description
n4868/[atomic.ref.generic] says template parameter T of atomic_ref<T> should satisfy is_trivially_copyable_v.
Current atomic_ref<T> has static_assert for is_trivially_copyable_v<_Ty> && is_copy_constructible_v<_Ty> && is_move_constructible_v<_Ty> && is_copy_assignable_v<_Ty> && is_move_assignable_v<_Ty>, which is probably copy-pasted from atomic<T>.

This makes atomic_ref<const T> fail to compile since const T is not assignable, but I believe this is valid use case.

Test code

#include <atomic>

auto load_as_atomic(const std::atomic<int>& i) 
{
    return i.load();
}

auto load_as_atomic(const int& i) 
{
    // should compile
    return std::atomic_ref(i).load();
}

int main() {}

This compiles with libstdc++ (Godbolt), but not with STL.

Possible Solution

Change static_assert to match standard wording.

[StephanTLavavej]

Yep, this is a bug, thanks for reporting it. We missed the copy-pasting during code review.

WG21-N4868 31.8.1 [atomics.types.generic.general]/1: "The program is ill-formed if any of is_­trivially_­copyable_­v<T>, is_­copy_­constructible_­v<T>, is_­move_­constructible_­v<T>, is_­copy_­assignable_­v<T>, or is_­move_­assignable_­v<T> is false."

STL/stl/inc/atomic

Lines 2165 to 2168 in 19c683d

	static_assert(is_trivially_copyable_v<_Ty> && is_copy_constructible_v<_Ty> && is_move_constructible_v<_Ty>
	&& is_copy_assignable_v<_Ty> && is_move_assignable_v<_Ty>,
	"atomic<T> requires T to be trivially copyable, copy constructible, move constructible, copy assignable, "
	"and move assignable.");

WG21-N4868 31.7.1 [atomics.ref.generic.general]/2: "The program is ill-formed if is_­trivially_­copyable_­v<T> is false."

STL/stl/inc/atomic

Lines 2350 to 2353 in 19c683d

	static_assert(is_trivially_copyable_v<_Ty> && is_copy_constructible_v<_Ty> && is_move_constructible_v<_Ty>
	&& is_copy_assignable_v<_Ty> && is_move_assignable_v<_Ty>,
	"atomic_ref<T> requires T to be trivially copyable, copy constructible, move constructible, copy assignable, "
	"and move assignable.");

This is trivial to fix in the product code 😹 but we should also add test coverage for a non-(copy/move)-(constructible/assignable) type. (const int is copy constructible, but not copy assignable.)