[libcxx-commits] [libcxx] [libc++] Add the __is_replaceable type trait (PR #132408)

[Mark de Wever via libcxx-commits]

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@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_trivially_copyable.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is replaceable if `x = std::move(y)` is equivalent to:
+//
+//  std::destroy_at(&x)
+//  std::construct_at(&x, std::move(y))
+//
+// This allows turning a move-assignment into a sequence of destroy + move-construct, which
+// is often more efficient. This is especially relevant when the move-construct is in fact
+// part of a trivial relocation from somewhere else, in which case there is a huge win.
+//
+// Note that this requires language support in order to be really effective, but we
+// currently emulate the base template with something very conservative.
+template <class _Tp, class = void>
+struct __is_replaceable : is_trivially_copyable<_Tp> {};
+
+template <class _Tp>
+struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {};
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mordante wrote:

I'd like to see an `__is_replacable_v` so we can directly use that.

https://github.com/llvm/llvm-project/pull/132408