[libcxx][ranges] Add `default_sentinel` and `default_sentinel_t`.

Refs https://eel.is/c++draft/default.sentinel and https://eel.is/c++draft/iterator.synopsis

Differential Revision: https://reviews.llvm.org/D103487

libcxx/docs/OneRangesProposalStatus.csv

@@ -31,7 +31,7 @@ bidirectional_iterator: `D100278 <https://llvm.org/D100278>`_",
 [predef.iterators],Updates to predefined iterators.,"[iterator.concepts], [iterator.cust.swap], [iterator.cust.move]",,,
 [move.sentinel],,[concepts] … Note: for testing it may be beneficial to have completed [predef.iterators]. ,,,
 [common.iterator],,"[iterator.concepts], [iterator.cust.swap], [iterator.cust.move]",Zoe Carver,,
-[default.sentinels],The empty std::default_sentinel_t.,,,,
+[default.sentinels],The empty std::default_sentinel_t.,,Zoe Carver,,✅
 [counted.iterator],,"[iterator.concepts], [iterator.cust.swap], [iterator.cust.move], [default.sentinels]",,,
 [stream.iterators],,[default.sentinels],,,
 [ranges.syn]: pt. 1,All the stuff not specified elsewhere. ,"[range.access], [iterator.concepts], [range.all], [range.subrange], unreachable, [range.empty]",,,

libcxx/include/CMakeLists.txt

@@ -16,6 +16,7 @@ set(files
   __hash_table
   __iterator/advance.h
   __iterator/concepts.h
+  __iterator/default_sentinel.h
   __iterator/incrementable_traits.h
   __iterator/indirect_concepts.h
   __iterator/iter_move.h

libcxx/include/__iterator/default_sentinel.h

@@ -0,0 +1,35 @@
+// -*- 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_DEFAULT_SENTINEL_H
+#define _LIBCPP___ITERATOR_DEFAULT_SENTINEL_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+struct default_sentinel_t { };
+inline constexpr default_sentinel_t default_sentinel{};
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___ITERATOR_DEFAULT_SENTINEL_H

libcxx/include/iterator

@@ -383,6 +383,9 @@ constexpr move_iterator<Iterator> operator+(   // constexpr in C++17
 template <class Iterator>   // constexpr in C++17
 constexpr  move_iterator<Iterator> make_move_iterator(const Iterator& i);
 
+// [default.sentinel], default sentinel
+struct default_sentinel_t;
+inline constexpr default_sentinel_t default_sentinel{};
 
 template <class T, class charT = char, class traits = char_traits<charT>, class Distance = ptrdiff_t>
 class istream_iterator
@@ -554,6 +557,7 @@ template <class E> constexpr const E* data(initializer_list<E> il) noexcept;
 #include <__functional_base>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
+#include <__iterator/default_sentinel.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/indirect_concepts.h>
 #include <__iterator/iter_move.h>

libcxx/test/std/iterators/predef.iterators/default.sentinel/default.sentinel.pass.cpp

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// struct default_sentinel_t;
+// inline constexpr default_sentinel_t default_sentinel;
+
+#include <iterator>
+
+#include <concepts>
+#include <type_traits>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  static_assert(std::is_empty_v<std::default_sentinel_t>);
+  static_assert(std::semiregular<std::default_sentinel_t>);
+
+  static_assert(std::same_as<decltype(std::default_sentinel), const std::default_sentinel_t>);
+
+  std::default_sentinel_t s1;
+  auto s2 = std::default_sentinel_t{};
+  s2 = s1;
+
+  return 0;
+}