[libcxx][ranges] Add `unreachable_sentinel`.

Differential Revision: https://reviews.llvm.org/D107920

1 files changed, 38 insertions, 0 deletions

diff --git a/libcxx/include/__iterator/unreachable_sentinel.h b/libcxx/include/__iterator/unreachable_sentinel.h
new file mode 100644
index 000000000000..cbbccd7bb288
--- /dev/null
+++ b/libcxx/include/__iterator/unreachable_sentinel.h
@@ -0,0 +1,38 @@
+// -*- 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___ITERATOR_UNREACHABLE_SENTINEL_H
+#define _LIBCPP___ITERATOR_UNREACHABLE_SENTINEL_H
+
+#include <__config>
+#include <__iterator/concepts.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+struct unreachable_sentinel_t {
+  template<weakly_incrementable _Iter>
+  _LIBCPP_HIDE_FROM_ABI
+  friend constexpr bool operator==(unreachable_sentinel_t, const _Iter&) noexcept {
+    return false;
+  }
+};
+
+inline constexpr unreachable_sentinel_t unreachable_sentinel{};
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ITERATOR_UNREACHABLE_SENTINEL_H