[libcxx-commits] [libcxx] [libc++] Implement `std::function_ref` (PR #186692)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,126 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___FUNCTIONAL_FUNCTION_REF_COMMON_H
+#define _LIBCPP___FUNCTIONAL_FUNCTION_REF_COMMON_H
+
+#include <__config>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_trivially_copyable.h>
+#include <__type_traits/remove_pointer.h>
+#include <__utility/constant_wrapper.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <class...>
+class function_ref;
+
+template <class _Fn, bool _NoExcept1, class _Rp, class... _ArgTypes>
+struct __is_convertible_from_specialization : false_type {};
+
+// use a union instead of a plain `void*` to avoid dropping const qualifiers and casting function pointers to data
+// pointers
+// todo: libstdc++ does not support volatile objects. shall we support it? the standard does not say it should not be
+// supported
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ldionne wrote:

I think I'd be okay with not supporting them. It adds essentially no value and the spec isn't clearly saying we should.

https://github.com/llvm/llvm-project/pull/186692