[libcxx-commits] [libcxx] [libc++] Implement C++20 atomic_ref (PR #76647)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,34 @@
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// static constexpr size_t required_alignment;
+
+#include <atomic>
+
+template <typename T>
+void checkRequiredAlignment() {
+  static_assert(std::atomic_ref<T>::required_alignment >= alignof(T));
+}
----------------
ldionne wrote:

I think I would do this instead:

```c++
template <class T>
constexpr void check_required_alignment() {
  assert(std::atomic_ref<T>::required_alignment >= alignof(T));
}
```

And then do the usual trick where we return a `bool` in a `static_assert`. That way we actually reference `required_alignment` at runtime too.

https://github.com/llvm/llvm-project/pull/76647