[libcxx-commits] [libcxx] [libc++] Partially implement P2846R6: `reserve_hint` (PR #206385)

[A. Jiang via libcxx-commits]

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@@ -0,0 +1,80 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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___RANGES_RESERVE_HINT_H
+#define _LIBCPP___RANGES_RESERVE_HINT_H
+
+#include <__concepts/class_or_enum.h>
+#include <__config>
+#include <__iterator/concepts.h>
+#include <__ranges/size.h>
+#include <__type_traits/remove_cvref.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 26
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// [range.prim.size.hint]
+
+namespace ranges {
+namespace __reserve_hint {
+void reserve_hint() = delete;
+
+template <typename _Tp>
+concept __sized = requires(_Tp&& __t) { ranges::size(__t); };
+
+template <typename _Tp>
+concept __member_reserve_hint = !__sized<_Tp> && requires(_Tp&& __t) {
+  { auto(__t.reserve_hint()) } -> __integer_like;
+};
+
+template <typename _Tp>
+concept __unqualified_reserve_hint =
+    !__sized<_Tp> && !__member_reserve_hint<_Tp> && __class_or_enum<remove_cvref_t<_Tp>> && requires(_Tp&& __t) {
+      { auto(reserve_hint(__t)) } -> __integer_like;
+    };
+
+struct __fn {
+  // `[range.prim.size.hint]`: `ranges::size(t)` is a valid expression
+  template <__sized _Tp>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __integer_like auto operator()(_Tp&& __t) const
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frederick-vs-ja wrote:

I believe these `operator()`'s should be static member functions. Was there any reason making them non-static?

https://github.com/llvm/llvm-project/pull/206385