[libcxx] const_iterator and c{r,}{begin,end}

Papers:
  - [P2278R4][]: `cbegin` should always return a constant iterator
  - [P2836R1][]: `std::basic_const_iterator` should follow its
    underlying type's convertibility

Issues:
  - [LWG3765][]: `const_sentinel` should be constrained
  - [LWG3766][]: `view_interface::cbegin` is underconstrained
  - [LWG3770][]: `const_sentinel_t` is missing
  - [LWG3769][]: `basic_const_iterator::operator==` causes infinite
    constraint recursion
  - [LWG3853][]: `basic_const_iterator<volatile int*>::operator->` is
    ill-formed
  - [LWG3862][]: `basic_const_iterator`'s `common_type` specialization
    is underconstrained
  - [LWG3872][]: `basic_const_iterator` should have custom `iter_move`
  - [LWG3946][]: The definition of `const_iterator_t` should be reworked
  - [LWG3948][]: _`possibly-const-range`_ and _`as-const-pointer`_
    should be `noexcept`

Implements:

- `basic_const_iterator`, `const_iterator`, and `const_sentinel`
- The const accessors - `ranges::{cbegin,cend,crbegin,crend}`
- Add `cbegin` and `cend` member functions to `view_interface`
- The changes from [P2836R1][].

Still to be done:
- `ranges::as_const_view`, `ranges::views::as_const`
- `cdata`
- The changes to `span`

Drive-by:
- Fix some includes that would have been circular.

[P2278R4]: https://wg21.link/p2278r4
[P2836R1]: https://wg21.link/p2836r1
[LWG3765]: https://wg21.link/lwg3765
[LWG3766]: https://wg21.link/lwg3766
[LWG3770]: https://wg21.link/lwg3770
[LWG3769]: https://wg21.link/lwg3769
[LWG3853]: https://wg21.link/lwg3853
[LWG3862]: https://wg21.link/lwg3862
[LWG3872]: https://wg21.link/lwg3872
[LWG3946]: https://wg21.link/lwg3946
[LWG3948]: https://wg21.link/lwg3948

Author: strega-nil

libcxx/docs/Status/Cxx23Issues.csv

@@ -219,9 +219,9 @@
 "`LWG3761 <https://wg21.link/LWG3761>`__","``cartesian_product_view::iterator::operator-`` should pass by reference","2022-11 (Kona)","","",""
 "`LWG3762 <https://wg21.link/LWG3762>`__","``generator::iterator::operator==`` should pass by reference","2022-11 (Kona)","","",""
 "`LWG3764 <https://wg21.link/LWG3764>`__","``reference_wrapper::operator()`` should propagate noexcept","2022-11 (Kona)","|Complete|","17.0",""
-"`LWG3765 <https://wg21.link/LWG3765>`__","``const_sentinel`` should be constrained","2022-11 (Kona)","","",""
-"`LWG3766 <https://wg21.link/LWG3766>`__","``view_interface::cbegin`` is underconstrained","2022-11 (Kona)","","",""
-"`LWG3770 <https://wg21.link/LWG3770>`__","``const_sentinel_t`` is missing","2022-11 (Kona)","","",""
+"`LWG3765 <https://wg21.link/LWG3765>`__","``const_sentinel`` should be constrained","2022-11 (Kona)","|Complete|","20.0",""
+"`LWG3766 <https://wg21.link/LWG3766>`__","``view_interface::cbegin`` is underconstrained","2022-11 (Kona)","|Complete|","20.0",""
+"`LWG3770 <https://wg21.link/LWG3770>`__","``const_sentinel_t`` is missing","2022-11 (Kona)","|Complete|","20.0",""
 "`LWG3773 <https://wg21.link/LWG3773>`__","``views::zip_transform`` still requires ``F`` to be ``copy_constructible`` when empty pack","2022-11 (Kona)","","",""
 "`LWG3774 <https://wg21.link/LWG3774>`__","``<flat_set>`` should include ``<compare>``","2022-11 (Kona)","","",""
 "`LWG3775 <https://wg21.link/LWG3775>`__","Broken dependencies in the ``Cpp17Allocator`` requirements","2022-11 (Kona)","","",""
@@ -252,7 +252,7 @@
 "`LWG3664 <https://wg21.link/LWG3664>`__","`LWG 3392 <https://wg21.link/LWG3392>`__ ``broke std::ranges::distance(a, a+3)``","2023-02 (Issaquah)","","",""
 "`LWG3720 <https://wg21.link/LWG3720>`__","Restrict the valid types of ``arg-id`` for width and precision in ``std-format-spec``","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3756 <https://wg21.link/LWG3756>`__","Is the ``std::atomic_flag`` class signal-safe?","2023-02 (Issaquah)","","",""
-"`LWG3769 <https://wg21.link/LWG3769>`__","``basic_const_iterator::operator==`` causes infinite constraint recursion","2023-02 (Issaquah)","","",""
+"`LWG3769 <https://wg21.link/LWG3769>`__","``basic_const_iterator::operator==`` causes infinite constraint recursion","2023-02 (Issaquah)","|Complete|","20.0",""
 "`LWG3807 <https://wg21.link/LWG3807>`__","The feature test macro for ``ranges::find_last`` should be renamed","2023-02 (Issaquah)","","",""
 "`LWG3811 <https://wg21.link/LWG3811>`__","``views::as_const`` on ``ref_view<T>`` should return ``ref_view<const T>``","2023-02 (Issaquah)","","",""
 "`LWG3820 <https://wg21.link/LWG3820>`__","``cartesian_product_view::iterator::prev`` is not quite right","2023-02 (Issaquah)","","",""
@@ -271,7 +271,7 @@
 "`LWG3849 <https://wg21.link/LWG3849>`__","``cartesian_product_view::iterator``'s default constructor is overconstrained","2023-02 (Issaquah)","","",""
 "`LWG3850 <https://wg21.link/LWG3850>`__","``views::as_const`` on ``empty_view<T>`` should return ``empty_view<const T>``","2023-02 (Issaquah)","","",""
 "`LWG3851 <https://wg21.link/LWG3851>`__","``chunk_view::inner-iterator`` missing custom ``iter_move`` and ``iter_swap``","2023-02 (Issaquah)","","",""
-"`LWG3853 <https://wg21.link/LWG3853>`__","``basic_const_iterator<volatile int*>::operator->`` is ill-formed","2023-02 (Issaquah)","","",""
+"`LWG3853 <https://wg21.link/LWG3853>`__","``basic_const_iterator<volatile int*>::operator->`` is ill-formed","2023-02 (Issaquah)","|Complete|","20.0",""
 "`LWG3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3866 <https://wg21.link/LWG3866>`__","Bad Mandates for ``expected::transform_error`` overloads","2023-02 (Issaquah)","|Complete|","17.0",""
@@ -293,12 +293,12 @@
 "`LWG3836 <https://wg21.link/LWG3836>`__","``std::expected<bool, E1>`` conversion constructor ``expected(const expected<U, G>&)`` should take precedence over ``expected(U&&)`` with operator ``bool``","2023-02 (Issaquah)","|Complete|","18.0",""
 "`LWG3843 <https://wg21.link/LWG3843>`__","``std::expected<T,E>::value() &`` assumes ``E`` is copy constructible","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3847 <https://wg21.link/LWG3847>`__","``ranges::to`` can still return views","2023-02 (Issaquah)","|Complete|","17.0",""
-"`LWG3862 <https://wg21.link/LWG3862>`__","``basic_const_iterator``'s ``common_type`` specialization is underconstrained","2023-02 (Issaquah)","","",""
+"`LWG3862 <https://wg21.link/LWG3862>`__","``basic_const_iterator``'s ``common_type`` specialization is underconstrained","2023-02 (Issaquah)","|Complete|","20.0",""
 "`LWG3865 <https://wg21.link/LWG3865>`__","Sorting a range of ``pairs``","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3869 <https://wg21.link/LWG3869>`__","Deprecate ``std::errc`` constants related to UNIX STREAMS","2023-02 (Issaquah)","|Complete|","19.0",""
 "`LWG3870 <https://wg21.link/LWG3870>`__","Remove ``voidify``","2023-02 (Issaquah)","","",""
 "`LWG3871 <https://wg21.link/LWG3871>`__","Adjust note about ``terminate``","2023-02 (Issaquah)","","",""
-"`LWG3872 <https://wg21.link/LWG3872>`__","``basic_const_iterator`` should have custom ``iter_move``","2023-02 (Issaquah)","","",""
+"`LWG3872 <https://wg21.link/LWG3872>`__","``basic_const_iterator`` should have custom ``iter_move``","2023-02 (Issaquah)","|Complete|","20.0",""
 "`LWG3875 <https://wg21.link/LWG3875>`__","``std::ranges::repeat_view<T, IntegerClass>::iterator`` may be ill-formed","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3876 <https://wg21.link/LWG3876>`__","Default constructor of ``std::layout_XX::mapping`` misses precondition","2023-02 (Issaquah)","","",""
 "`LWG3877 <https://wg21.link/LWG3877>`__","Incorrect constraints on ``const``-qualified monadic overloads for ``std::expected``","2023-02 (Issaquah)","|Complete|","17.0",""

libcxx/docs/Status/Cxx23Papers.csv

@@ -61,7 +61,7 @@
 "`P1899R3 <https://wg21.link/P1899R3>`__","``stride_view``","2022-07 (Virtual)","","",""
 "`P2093R14 <https://wg21.link/P2093R14>`__","Formatted output","2022-07 (Virtual)","|Complete|","18.0",""
 "`P2165R4 <https://wg21.link/P2165R4>`__","Compatibility between ``tuple``, ``pair`` and ``tuple-like`` objects","2022-07 (Virtual)","","",""
-"`P2278R4 <https://wg21.link/P2278R4>`__","``cbegin`` should always return a constant iterator","2022-07 (Virtual)","","",""
+"`P2278R4 <https://wg21.link/P2278R4>`__","``cbegin`` should always return a constant iterator","2022-07 (Virtual)","|In Progress|","20.0","``basic_const_iterator`` and the changes to the const range access functions (less ``cdata``) are implemented."
 "`P2286R8 <https://wg21.link/P2286R8>`__","Formatting Ranges","2022-07 (Virtual)","|Complete|","16.0",""
 "`P2291R3 <https://wg21.link/P2291R3>`__","Add Constexpr Modifiers to Functions ``to_chars`` and ``from_chars`` for Integral Types in ``<charconv>`` Header","2022-07 (Virtual)","|Complete|","16.0",""
 "`P2302R4 <https://wg21.link/P2302R4>`__","``std::ranges::contains``","2022-07 (Virtual)","|Complete|","19.0",""

libcxx/docs/Status/Cxx2cIssues.csv

@@ -25,9 +25,9 @@
 "`LWG3809 <https://wg21.link/LWG3809>`__","Is ``std::subtract_with_carry_engine<uint16_t>`` supposed to work","2023-11 (Kona)","","",""
 "`LWG3892 <https://wg21.link/LWG3892>`__","Incorrect formatting of nested ranges and tuples","2023-11 (Kona)","|Complete|","17.0",""
 "`LWG3897 <https://wg21.link/LWG3897>`__","``inout_ptr`` will not update raw pointer to 0","2023-11 (Kona)","|Complete|","19.0",""
-"`LWG3946 <https://wg21.link/LWG3946>`__","The definition of ``const_iterator_t`` should be reworked","2023-11 (Kona)","","",""
+"`LWG3946 <https://wg21.link/LWG3946>`__","The definition of ``const_iterator_t`` should be reworked","2023-11 (Kona)","|Complete|","20.0",""
 "`LWG3947 <https://wg21.link/LWG3947>`__","Unexpected constraints on ``adjacent_transform_view::base()``","2023-11 (Kona)","","",""
-"`LWG3948 <https://wg21.link/LWG3948>`__","``possibly-const-range and as-const-pointer`` should be ``noexcept``","2023-11 (Kona)","","",""
+"`LWG3948 <https://wg21.link/LWG3948>`__","``possibly-const-range and as-const-pointer`` should be ``noexcept``","2023-11 (Kona)","|In Progress|","20.0","Implemented for _``possibly-const-range``_, since _``as-const-pointer``_ is not yet implemented."
 "`LWG3949 <https://wg21.link/LWG3949>`__","``std::atomic<bool>``'s trivial destructor dropped in C++17 spec wording","2023-11 (Kona)","","",""
 "`LWG3951 <https://wg21.link/LWG3951>`__","[expected.object.swap]: Using ``value()`` instead of ``has_value()``","2023-11 (Kona)","|Complete|","16.0",""
 "`LWG3953 <https://wg21.link/LWG3953>`__","``iter_move`` for ``common_iterator`` and ``counted_iterator`` should return ``decltype(auto)``","2023-11 (Kona)","","",""

libcxx/docs/Status/Cxx2cPapers.csv

@@ -42,7 +42,7 @@
 "`P2819R2 <https://wg21.link/P2819R2>`__","Add tuple protocol to complex","2023-11 (Kona)","|Complete|","19.0",""
 "`P2937R0 <https://wg21.link/P2937R0>`__","Freestanding: Remove ``strtok``","2023-11 (Kona)","","",""
 "`P2833R2 <https://wg21.link/P2833R2>`__","Freestanding Library: inout expected span","2023-11 (Kona)","","",""
-"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","2023-11 (Kona)","","",""
+"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","2023-11 (Kona)","|Complete|","20.0","Implemented as a DR against C++23. (MSVC STL and libstdc++ will do the same.)"
 "`P2264R7 <https://wg21.link/P2264R7>`__","Make ``assert()`` macro user friendly for C and C++","2023-11 (Kona)","","",""
 "`P1673R13 <https://wg21.link/P1673R13>`__","A free function linear algebra interface based on the BLAS","2023-11 (Kona)","","",""
 "","","","","",""

libcxx/include/CMakeLists.txt

@@ -444,6 +444,7 @@ set(files
   __iterator/bounded_iter.h
   __iterator/common_iterator.h
   __iterator/concepts.h
+  __iterator/const_iterator.h
   __iterator/counted_iterator.h
   __iterator/cpp17_iterator_concepts.h
   __iterator/data.h
@@ -639,6 +640,7 @@ set(files
   __ranges/chunk_by_view.h
   __ranges/common_view.h
   __ranges/concepts.h
+  __ranges/const_access.h
   __ranges/container_compatible_range.h
   __ranges/counted.h
   __ranges/dangling.h

libcxx/include/__algorithm/ranges_reverse_copy.h

@@ -34,6 +34,14 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace ranges {
 
+template <ranges::bidirectional_range _Range>
+_LIBCPP_HIDE_FROM_ABI constexpr ranges::subrange<reverse_iterator<ranges::iterator_t<_Range>>,
+                                                 reverse_iterator<ranges::iterator_t<_Range>>>
+__reverse_range(_Range&& __range) {
+  auto __first = ranges::begin(__range);
+  return {std::make_reverse_iterator(ranges::next(__first, ranges::end(__range))), std::make_reverse_iterator(__first)};
+}
+
 template <class _InIter, class _OutIter>
 using reverse_copy_result = in_out_result<_InIter, _OutIter>;
 
@@ -49,7 +57,7 @@ struct __reverse_copy {
     requires indirectly_copyable<iterator_t<_Range>, _OutIter>
   _LIBCPP_HIDE_FROM_ABI constexpr reverse_copy_result<borrowed_iterator_t<_Range>, _OutIter>
   operator()(_Range&& __range, _OutIter __result) const {
-    auto __ret = ranges::copy(std::__reverse_range(__range), std::move(__result));
+    auto __ret = ranges::copy(std::ranges::__reverse_range(__range), std::move(__result));
     return {ranges::next(ranges::begin(__range), ranges::end(__range)), std::move(__ret.out)};
   }
 };