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

Papers: [P2278R4][], [P2836R1][]
PR: [LLVM-PR-99915][]

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
[LLVM-PR-99915]: llvm#99915

Author: strega-nil

libcxx/docs/Status/Cxx23.rst

@@ -43,6 +43,9 @@ Paper Status
    .. [#note-P0533R9] P0533R9: ``isfinite``, ``isinf``, ``isnan`` and ``isnormal`` are implemented.
    .. [#note-P1413R3] P1413R3: ``std::aligned_storage_t`` and ``std::aligned_union_t`` are marked deprecated, but
       clang doesn't issue a diagnostic for deprecated using template declarations.
+   .. [#note-P2278R4] P2278R4: ``basic_const_iterator`` has been implemented, and the const range accessors
+      (``cbegin``, ``cend``, ``crbegin``, ``crend``) have been updated to return this new type.
+      However, the changes to ``cdata`` and ``span``, and ``const_view``, have not yet been implemented.
    .. [#note-P2520R0] P2520R0: Libc++ implemented this paper as a DR in C++20 as well.
    .. [#note-P2711R1] P2711R1: ``join_with_view`` hasn't been done yet since this type isn't implemented yet.
    .. [#note-P2770R0] P2770R0: ``join_with_view`` hasn't been done yet since this type isn't implemented yet.

libcxx/docs/Status/Cxx23Papers.csv

@@ -61,7 +61,7 @@
 "`P1899R3 <https://wg21.link/P1899R3>`__","``stride_view``","July 2022","","","|ranges|"
 "`P2093R14 <https://wg21.link/P2093R14>`__","Formatted output","July 2022","|Complete|","18.0","|format|"
 "`P2165R4 <https://wg21.link/P2165R4>`__","Compatibility between ``tuple``, ``pair`` and ``tuple-like`` objects","July 2022","","",""
-"`P2278R4 <https://wg21.link/P2278R4>`__","``cbegin`` should always return a constant iterator","July 2022","","","|ranges|"
+"`P2278R4 <https://wg21.link/P2278R4>`__","``cbegin`` should always return a constant iterator","July 2022","|In progress| [#note-P2278R4]","","|ranges|"
 "`P2286R8 <https://wg21.link/P2286R8>`__","Formatting Ranges","July 2022","|Complete|","16.0","|format| |ranges|"
 "`P2291R3 <https://wg21.link/P2291R3>`__","Add Constexpr Modifiers to Functions ``to_chars`` and ``from_chars`` for Integral Types in ``<charconv>`` Header","July 2022","|Complete|","16.0",""
 "`P2302R4 <https://wg21.link/P2302R4>`__","``std::ranges::contains``","July 2022","|Complete|","19.0","|ranges|"

libcxx/docs/Status/Cxx2c.rst

@@ -44,6 +44,8 @@ Paper Status
    .. [#note-P2944R3] Implemented comparisons for ``reference_wrapper`` only.
    .. [#note-P2422R1] Libc++ keeps the ``nodiscard`` attributes as a conforming extension.
    .. [#note-P2997R1] This paper is applied as DR against C++20. (MSVC STL and libstdc++ will do the same.)
+   .. [#note-P2836R1] This paper is applies as DR against C++23 (MSVC STL and libstdc++ have done the same);
+      The feature macro is not yet defined, since other features from the original paper have not been completed.
 
 .. _issues-status-cxx2c:
 

libcxx/docs/Status/Cxx2cPapers.csv

@@ -42,7 +42,7 @@
 "`P2819R2 <https://wg21.link/P2819R2>`__","Add tuple protocol to complex","Kona November 2023","|Complete|","19.0",""
 "`P2937R0 <https://wg21.link/P2937R0>`__","Freestanding: Remove ``strtok``","Kona November 2023","","",""
 "`P2833R2 <https://wg21.link/P2833R2>`__","Freestanding Library: inout expected span","Kona November 2023","","",""
-"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","Kona November 2023","","","|DR|"
+"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","Kona November 2023","|Complete| [#note-P2836R1]","","|DR|"
 "`P2264R7 <https://wg21.link/P2264R7>`__","Make ``assert()`` macro user friendly for C and C++","Kona November 2023","","",""
 "`P1673R13 <https://wg21.link/P1673R13>`__","A free function linear algebra interface based on the BLAS","Kona November 2023","","",""
 "","","","","",""

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)};
   }
 };