[llvm] [libc++] Implement ranges::contains (PR #65148)

[Konstantin Varlamov via llvm-commits]

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@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___ALGORITHM_RANGES_CONTAINS_H
+#define _LIBCPP___ALGORITHM_RANGES_CONTAINS_H
+
+#include <__algorithm/in_in_result.h>
+#include <__algorithm/ranges_starts_with.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/indirectly_comparable.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+namespace __contains {
+struct __fn {
+  template <class _Iter, class _Sent, class _Type, class _Proj>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool
+  __compare_single_element(_Iter __first, _Sent __last, const _Type& __value, _Proj& __proj = {}) const {
+    auto __pred = [&](auto&& __e) { return std::forward<decltype(__e)>(__e) == __value; };
+    for (; __first != __last; ++__first) {
+      if (std::invoke(std::move(__pred), std::invoke(std::move(__proj), *__first)))
+        return true;
+    }
+    return false;
+  }
+
+  template <input_iterator _Iter, sentinel_for<_Iter> _Sent, class _Type, class _Proj = identity>
+    requires indirect_binary_predicate<ranges::equal_to, projected<_Iter, _Proj>, const _Type*>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool
+  operator()(_Iter __first, _Sent __last, const _Type& __value, _Proj __proj = {}) const {
+    return __compare_single_element(std::move(__first), std::move(__last), __value, __proj);
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var-const wrote:

+1. You might need to wrap the projection in `std::ref` and perhaps move the iterators.

In general, I would suggest delegating to an existing algorithm when doing so is straightforward. It usually makes the code easier to read and lets us keep optimizations in one place.

https://github.com/llvm/llvm-project/pull/65148