1 files changed, 99 insertions, 0 deletions
diff --git a/libcxx/include/__ranges/non_propagating_cache.h b/libcxx/include/__ranges/non_propagating_cache.h
new file mode 100644
index 000000000000..878f7070a07f
--- /dev/null
+++ b/libcxx/include/__ranges/non_propagating_cache.h
@@ -0,0 +1,99 @@
+// -*- 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_NON_PROPAGATING_CACHE_H
+#define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
+
+#include <__config>
+#include <__iterator/concepts.h>        // indirectly_readable
+#include <__iterator/iterator_traits.h> // iter_reference_t
+#include <__memory/addressof.h>
+#include <concepts>                     // constructible_from
+#include <optional>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// clang-format off
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+namespace ranges {
+  // __non_propagating_cache is a helper type that allows storing an optional value in it,
+  // but which does not copy the source's value when it is copy constructed/assigned to,
+  // and which resets the source's value when it is moved-from.
+  //
+  // This type is used as an implementation detail of some views that need to cache the
+  // result of `begin()` in order to provide an amortized O(1) begin() method. Typically,
+  // we don't want to propagate the value of the cache upon copy because the cached iterator
+  // may refer to internal details of the source view.
+  template<class _Tp>
+    requires is_object_v<_Tp>
+  class _LIBCPP_TEMPLATE_VIS __non_propagating_cache {
+    optional<_Tp> __value_ = nullopt;
+
+  public:
+    _LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
+      : __value_(nullopt)
+    { }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
+      : __value_(nullopt)
+    {
+      __other.__value_.reset();
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
+      if (this != _VSTD::addressof(__other)) {
+        __value_.reset();
+      }
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
+      __value_.reset();
+      __other.__value_.reset();
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _Tp& operator*() { return *__value_; }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _Tp const& operator*() const { return *__value_; }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr bool __has_value() const { return __value_.has_value(); }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr void __set(_Tp const& __value) { __value_.emplace(__value); }
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr void __set(_Tp&& __value) { __value_.emplace(_VSTD::move(__value)); }
+  };
+
+  struct __empty_cache { };
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H