[libcxx-commits] [libcxx] [libc++] Add `__exchange` as a C++11 utility (PR #187953)

[A. Jiang via libcxx-commits]

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@@ -24,14 +27,21 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 14
 template <class _T1, class _T2 = _T1>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _T1 exchange(_T1& __obj, _T2&& __new_value) noexcept(
-    is_nothrow_move_constructible<_T1>::value && is_nothrow_assignable<_T1&, _T2>::value) {
+[[nodiscard]] _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _T1
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frederick-vs-ja wrote:

A user should use plain assignment if the return value of `exchange` is not desired. In any case, I believe we should enhance the documentation and keep using `[[nodiscard]]`.

BTW, it seems that we should use __uglified `[[__nodiscard__]]` for pre-C++17 code.

https://github.com/llvm/llvm-project/pull/187953