[libcxx-commits] [libcxx] [libcxx][P1831R1] Deprecating volatile: library (PR #101439)

[A. Jiang via libcxx-commits]

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@@ -27,6 +27,20 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if _LIBCPP_STD_VER >= 20
+template <class _Tp, bool __lock_free>
+inline constexpr bool __deprecated_if_not_always_lock_free = true;
+
+template <class _Tp>
+[[deprecated("volatile atomic operations are deprecated when std::atomic<T>::is_always_lock_free is false")]]
+inline constexpr bool __deprecated_if_not_always_lock_free<_Tp, false> = true;
+#  define _LIBCPP_DEPRECATED_NOT_ALWAYS_LOCK_FREE(_Tp, __is_always_lock_free)                                          \
+    static_assert(__deprecated_if_not_always_lock_free<_Tp, __is_always_lock_free>)
+#else
+#  define _LIBCPP_DEPRECATED_NOT_ALWAYS_LOCK_FREE(_Tp, __is_always_lock_free)                                          \
+    {}
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frederick-vs-ja wrote:

> I thought it was UB.

It's definitely not now per [P2809R3](https://wg21.link/p2809r3).

However..., shouldn't this be `do { } while(0)`?

https://github.com/llvm/llvm-project/pull/101439