[libcxx-commits] [libcxx] [libc++][RFC] Rewrite CPOs with resolver functions (PR #209104)

[via libcxx-commits]

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@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___UTILITY_CPO_H
+#define _LIBCPP___UTILITY_CPO_H
+
+#include <__config>
+#include <__utility/forward.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 20
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __string {
+  consteval __string(const char*) {}
+};
+
+template <__string>
+struct __diagnostic {};
+
+// This is a utility to do overload resolution via a resolver function. That resolver function returns either a
+// `__diagnostic` if overload resolution fails or a callable object (usually a stateless lambda).
+// For expression-equivalence the callable object has to be marked `noexcept` if the expression inside is `noexcept`.
+// The resolver function should be marked `noexcept` unconditionally and be `consteval`.
+template <auto __resolver>
+struct _CPO {
+  template <class... _Args>
+  [[nodiscard]] static constexpr auto operator()(_Args&&... __args) noexcept(
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huixie90 wrote:

Does it mean all CPOs must be `[[nodiscard]]` ?

https://github.com/llvm/llvm-project/pull/209104