[libcxx-commits] [libcxx] [libcxx] P2278R4: implement `{basic_, }const_iterator`, and have `cbegin` et. al. return it (PR #99915)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,340 @@
+// -*- 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_CONST_ITERATOR_H
+#define _LIBCPP___ITERATOR_CONST_ITERATOR_H
+
+#include <__compare/three_way_comparable.h>
+#include <__concepts/common_with.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/different_from.h>
+#include <__concepts/same_as.h>
+#include <__concepts/semiregular.h>
+#include <__concepts/totally_ordered.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iter_move.h>
+#include <__iterator/iterator_traits.h>
+#include <__memory/addressof.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/common_reference.h>
+#include <__type_traits/common_type.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_specialization.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+template <indirectly_readable _Iter>
+using iter_const_reference_t = common_reference_t<const iter_value_t<_Iter>&&, iter_reference_t<_Iter>>;
+
+template <class _Iter>
+concept __constant_iterator = input_iterator<_Iter> && same_as<iter_const_reference_t<_Iter>, iter_reference_t<_Iter>>;
+
+template <input_iterator _Iter>
+class basic_const_iterator;
+
+template <input_iterator _Iter>
+using const_iterator = conditional_t<__constant_iterator<_Iter>, _Iter, basic_const_iterator<_Iter>>;
+template <semiregular _Sent>
+using const_sentinel = conditional_t<input_iterator<_Sent>, const_iterator<_Sent>, _Sent>;
+
+template <class _Iter>
+concept __not_a_const_iterator = !__is_specialization_v<_Iter, basic_const_iterator>;
+
+template <indirectly_readable _Iter>
+using __iter_const_rvalue_reference_t = common_reference_t<const iter_value_t<_Iter>&&, iter_rvalue_reference_t<_Iter>>;
+
+template <class _Iter>
+struct __basic_const_iterator_concept {
+  // clang-format off
+  using iterator_concept =
+    conditional_t<contiguous_iterator<_Iter>,
+      contiguous_iterator_tag,
+    conditional_t<random_access_iterator<_Iter>,
+      random_access_iterator_tag,
+    conditional_t<bidirectional_iterator<_Iter>,
+      bidirectional_iterator_tag,
+    conditional_t<forward_iterator<_Iter>,
+      forward_iterator_tag,
+    // else
+      input_iterator_tag>>>>;
+  // clang-format on
+};
+
+template <class _Iter>
+struct __basic_const_iterator_category : __basic_const_iterator_concept<_Iter> {};
+template <forward_iterator _Iter>
+struct __basic_const_iterator_category<_Iter> : __basic_const_iterator_concept<_Iter> {
+  using iterator_category = __basic_const_iterator_concept<_Iter>::iterator_concept;
+};
+
+template <input_iterator _Iter>
+class _LIBCPP_TEMPLATE_VIS basic_const_iterator : __basic_const_iterator_category<_Iter> {
+  _Iter __current = _Iter();
+
+  using __reference        = iter_const_reference_t<_Iter>;
+  using __rvalue_reference = __iter_const_rvalue_reference_t<_Iter>;
+
+public:
+  using value_type      = iter_value_t<_Iter>;
+  using difference_type = iter_difference_t<_Iter>;
+
+  _LIBCPP_HIDE_FROM_ABI basic_const_iterator()
+    requires default_initializable<_Iter>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(_Iter __cur) : __current(std::move(__cur)) {}
+  template <convertible_to<_Iter> _Type>
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(basic_const_iterator<_Type> __cur)
+      : __current(std::move(__cur.__current)) {}
+  template <__different_from<basic_const_iterator> _Type>
+    requires convertible_to<_Type, _Iter>
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(_Type&& __cur) : __current(std::forward<_Type>(__cur)) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Iter& base() const& noexcept { return __current; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iter base() && { return std::move(__current); }
+
+  constexpr __reference operator*() const { return static_cast<__reference>(*__current); }
+  _LIBCPP_HIDE_FROM_ABI constexpr const auto* operator->() const
+    requires is_lvalue_reference_v<iter_reference_t<_Iter>> &&
+             same_as<remove_cvref_t<iter_reference_t<_Iter>>, value_type>
+  {
+    if constexpr (contiguous_iterator<_Iter>) {
+      return std::to_address(__current);
+    } else {
+      return std::addressof(*__current);
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator++() {
+    ++__current;
+    return *this;
+  }
+  constexpr void operator++(int) { ++__current; }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator operator++(int)
+    requires forward_iterator<_Iter>
+  {
+    auto __tmp = *this;
+    ++__current;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator--()
+    requires bidirectional_iterator<_Iter>
+  {
+    --__current;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator operator--(int)
+    requires bidirectional_iterator<_Iter>
+  {
+    auto __tmp = *this;
+    --__current;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator+=(difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    __current += __n;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator-=(difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    __current -= __n;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __reference operator[](difference_type __n) const
+    requires random_access_iterator<_Iter>
+  {
+    return static_cast<__reference>(__current[__n]);
+  }
+
+  template <sentinel_for<_Iter> _Sent>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator==(const _Sent& __sent) const {
+    return __current == __sent;
+  }
+
+  template <__not_a_const_iterator _ConstIt>
+    requires __constant_iterator<_ConstIt> && convertible_to<_Iter const&, _ConstIt>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator _ConstIt() const& {
+    return __current;
+  }
+  template <__not_a_const_iterator _ConstIt>
+    requires __constant_iterator<_ConstIt> && convertible_to<_Iter, _ConstIt>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator _ConstIt() && {
+    return std::move(__current);
+  }
----------------
frederick-vs-ja wrote:

No change requested for these conversion functions. But the entry for [P2836R1](https://wg21.link/p2836r1) in Cxx2cPapers.csv needs to be updated (to Complete, 20.0?), and the description of this PR should also mention that paper.
https://github.com/llvm/llvm-project/blob/ac1a1e5797388598201511d17f05aa088ef4a2e2/libcxx/docs/Status/Cxx2cPapers.csv#L45

https://github.com/llvm/llvm-project/pull/99915