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

[zoecarver]

Summary

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

[• Quuxplusone]

LGTM % that one additional test.

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

16

#include <concepts>
#include <type_traits>

Actually, if you replaced same_as with is_same_v and semiregular with is_copy_constructible_v, could you remove the UNSUPPORTED: libcpp-no-concepts line so that this would be testable in more situations?
If de-concept-ifying this test turns into a rabbit hole, then never mind; but if it's trivial to simplify, then it'd be nice.

24

I'd also like to see here that

std::default_sentinel_t s1;
auto s2 = std::default_sentinel_t{};
(void)s1;
(void)s2;

both compile. (C++20 seems to require that these compile. Unlike some types, e.g. std::nullopt_t, which are not supposed to default-construct.)

[zoecarver]

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

24

Doesn't semiregular check that? Or is this a default constructible vs default initializable thing?

[• Quuxplusone]

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

24

If we don't know, that's a great reason to add the test. :)
The snippet I suggested will test both "default initialization" (s1) and "construction from nothing" (s2).

[cjdb]

LGTM

[ldionne]

LGTM post commit.

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

16

FWIW, libcpp-no-concepts is something that will die eventually as we move to newer compilers. IMO there's little benefit in doing any work to improve the situation on compilers that do not support concepts.

libcxx/docs/OneRangesProposalStatus.csv

 [std.iterator.tags],,[iterator.traits],,,
 [range.iter.op.advance],ranges::advance,[iterator.concepts],Christopher Di Bella,`D101922 <https://llvm.org/D101922>`_,
 [range.iter.op.distance],ranges::distance,"[iterator.concepts], [range.range], [range.sized]",Christopher Di Bella,`D102789 <https://llvm.org/D102789>`_,
 [range.iter.op.next],ranges::next,[iterator.concepts],Christopher Di Bella,`D102563 <https://llvm.org/D102563>`_,
 [range.iter.op.prev],ranges::prev,[iterator.concepts],Christopher Di Bella,`D102564 <https://llvm.org/D102564>`_,
 [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]",,,
 [ranges.syn]: pt. 2,"iterator_t, sentinel_t, and safe_iterator_t",[range.access],Christopher Di Bella,"`D100255 <https://llvm.org/D100255>`_, `D100269 <https://llvm.org/D100269>`_",
 [range.access],"ranges::begin, end, cbegin, cend, rbegin, rend, crbegin, and crend",[iterator.concepts],Christopher Di Bella,`D100255 <https://llvm.org/D100255>`_,
 [ranges.primitives],"size, empty, data, and cdata",[iterator.concepts],Zoe Carver,,
 [range.range],,[range.access],,,
 [range.sized],ranges::sized_range,"[range.primitives], [range.range]",Christopher Di Bella,`D102434 <https://llvm.org/D102434>`_,✅

libcxx/include/CMakeLists.txt

set(files
   __format/format_parse_context.h
   __function_like.h
   __functional_03
   __functional_base
   __functional_base_03
   __hash_table
   __iterator/advance.h
   __iterator/concepts.h
+  __iterator/default_sentinel.h
   __iterator/incrementable_traits.h
   __iterator/indirect_concepts.h
   __iterator/iter_move.h
   __iterator/iterator_traits.h
   __iterator/next.h
   __iterator/prev.h
   __iterator/projected.h
   __iterator/readable_traits.h

libcxx/include/__iterator/default_sentinel.h

+// -*- 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

 template <class Iterator>
 constexpr move_iterator<Iterator> operator+(   // constexpr in C++17
             typename move_iterator<Iterator>::difference_type n,
             const move_iterator<Iterator>& x);
 
 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
     : public iterator<input_iterator_tag, T, Distance, const T*, const T&> // until C++17
 {
 public:
     typedef input_iterator_tag           iterator_category;
     typedef T                            value_type;

 
 */
 
 #include <__config>
 #include <__debug>
 #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>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/next.h>
 #include <__iterator/prev.h>
 #include <__iterator/projected.h>
 #include <__iterator/readable_traits.h>

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

+//===----------------------------------------------------------------------===//
+//
+// 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 "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>);
+
+    return 0;
+}