libstdc++: Implement P2278R4 "cbegin should always return a constant iterator"

This also implements the approved follow-up LWG issues 3765, 3766, 3769,
3770, 3811, 3850, 3853, 3862 and 3872.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (const_iterator_t): Define for C++23.
	(const_sentinel_t): Likewise.
	(range_const_reference_t): Likewise.
	(constant_range): Likewise.
	(__cust_access::__possibly_const_range): Likewise, replacing ...
	(__cust_access::__as_const): ... this.
	(__cust_access::_CBegin::operator()): Redefine for C++23 as per P2278R4.
	(__cust_access::_CEnd::operator()): Likewise.
	(__cust_access::_CRBegin::operator()): Likewise.
	(__cust_access::_CREnd::operator()): Likewise.
	(__cust_access::_CData::operator()): Likewise.
	* include/bits/ranges_util.h (ranges::__detail::__different_from):
	Make it an alias of std::__detail::__different_from.
	(view_interface::cbegin): Define for C++23.
	(view_interface::cend): Likewise.
	* include/bits/stl_iterator.h (__detail::__different_from): Define.
	(iter_const_reference_t): Define for C++23.
	(__detail::__constant_iterator): Likewise.
	(__detail::__is_const_iterator): Likewise.
	(__detail::__not_a_const_iterator): Likewise.
	(__detail::__iter_const_rvalue_reference_t): Likewise.
	(__detail::__basic_const_iter_cat):: Likewise.
	(const_iterator): Likewise.
	(__detail::__const_sentinel): Likewise.
	(const_sentinel): Likewise.
	(basic_const_iterator): Likewise.
	(common_type<basic_const_iterator<_Tp>, _Up>): Likewise.
	(common_type<_Up, basic_const_iterator<_Tp>>): Likewise.
	(common_type<basic_const_iterator<_Tp>, basic_const_iterator<Up>>):
	Likewise.
	(make_const_iterator): Define for C++23.
	(make_const_sentinel): Likewise.
	* include/std/ranges (__cpp_lib_ranges_as_const): Likewise.
	(as_const_view): Likewise.
	(enable_borrowed_range<as_const_view>): Likewise.
	(views::__detail::__is_ref_view): Likewise.
	(views::__detail::__can_is_const_view): Likewise.
	(views::_AsConst, views::as_const): Likewise.
	* include/std/span (span::const_iterator): Likewise.
	(span::const_reverse_iterator): Likewise.
	(span::cbegin): Likewise.
	(span::cend): Likewise.
	(span::crbegin): Likewise.
	(span::crend): Likewise.
	* include/std/version (__cpp_lib_ranges_as_const): Likewise.
	* testsuite/std/ranges/adaptors/join.cc (test06): Adjust to
	behave independently of C++20 vs C++23.
	* testsuite/std/ranges/version_c++23.cc: Verify value of
	__cpp_lib_ranges_as_const macro.
	* testsuite/24_iterators/const_iterator/1.cc: New test.
	* testsuite/std/ranges/adaptors/as_const/1.cc: New test.

3 files changed, 129 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 3f6ff50..283d757 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -8929,6 +8929,112 @@ namespace views::__adaptor
 
     inline constexpr _Enumerate enumerate;
   }
+
+#define __cpp_lib_ranges_as_const 202207L
+
+  template<view _Vp>
+    requires input_range<_Vp>
+  class as_const_view : public view_interface<as_const_view<_Vp>>
+  {
+    _Vp _M_base = _Vp();
+
+  public:
+    as_const_view() requires default_initializable<_Vp> = default;
+
+    constexpr explicit
+    as_const_view(_Vp __base)
+    noexcept(is_nothrow_move_constructible_v<_Vp>)
+    : _M_base(std::move(__base))
+    { }
+
+    constexpr _Vp
+    base() const &
+    noexcept(is_nothrow_copy_constructible_v<_Vp>)
+    requires copy_constructible<_Vp>
+    { return _M_base; }
+
+    constexpr _Vp
+    base() &&
+    noexcept(is_nothrow_move_constructible_v<_Vp>)
+    { return std::move(_M_base); }
+
+    constexpr auto
+    begin() requires (!__detail::__simple_view<_Vp>)
+    { return ranges::cbegin(_M_base); }
+
+    constexpr auto
+    begin() const requires range<const _Vp>
+    { return ranges::cbegin(_M_base); }
+
+    constexpr auto
+    end() requires (!__detail::__simple_view<_Vp>)
+    { return ranges::cend(_M_base); }
+
+    constexpr auto
+    end() const requires range<const _Vp>
+    { return ranges::cend(_M_base); }
+
+    constexpr auto
+    size() requires sized_range<_Vp>
+    { return ranges::size(_M_base); }
+
+    constexpr auto
+    size() const requires sized_range<const _Vp>
+    { return ranges::size(_M_base); }
+  };
+
+  template<typename _Range>
+    as_const_view(_Range&&) -> as_const_view<views::all_t<_Range>>;
+
+  template<typename _Tp>
+    inline constexpr bool enable_borrowed_range<as_const_view<_Tp>>
+      = enable_borrowed_range<_Tp>;
+
+  namespace views
+  {
+    namespace __detail
+    {
+      template<typename _Tp>
+	inline constexpr bool __is_ref_view = false;
+
+      template<typename _Range>
+	inline constexpr bool __is_ref_view<ref_view<_Range>> = true;
+
+      template<typename _Range>
+	concept __can_as_const_view = requires { as_const_view(std::declval<_Range>()); };
+    }
+
+    struct _AsConst : __adaptor::_RangeAdaptorClosure
+    {
+      template<viewable_range _Range>
+      constexpr auto
+      operator()(_Range&& __r) const
+      noexcept(noexcept(as_const_view(std::declval<_Range>())))
+      requires __detail::__can_as_const_view<_Range>
+      {
+	using _Tp = remove_cvref_t<_Range>;
+	using element_type = remove_reference_t<range_reference_t<_Range>>;
+	if constexpr (constant_range<views::all_t<_Range>>)
+	  return views::all(std::forward<_Range>(__r));
+	else if constexpr (__detail::__is_empty_view<_Tp>)
+	  return views::empty<const element_type>;
+	else if constexpr (std::__detail::__is_span<_Tp>)
+	  return span<const element_type, _Tp::extent>(std::forward<_Range>(__r));
+	else if constexpr (__detail::__is_ref_view<_Tp>
+			   && constant_range<const element_type>)
+	  return ref_view(static_cast<const element_type&>
+			  (std::forward<_Range>(__r).base()));
+	else if constexpr (is_lvalue_reference_v<_Range>
+			   && constant_range<_Tp>
+			   && !view<_Tp>)
+	  return ref_view(static_cast<const _Tp&>(__r));
+	else
+	  return as_const_view(std::forward<_Range>(__r));
+      }
+    };
+
+    inline constexpr _AsConst as_const;
+  }
 #endif // C++23
 } // namespace ranges
 
diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 06d5c18..6763389 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -137,6 +137,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using const_reference        = const element_type&;
       using iterator = __gnu_cxx::__normal_iterator<pointer, span>;
       using reverse_iterator       = std::reverse_iterator<iterator>;
+#if __cplusplus > 202002L
+      using const_iterator         = std::const_iterator<iterator>;
+      using const_reverse_iterator = std::const_iterator<reverse_iterator>;
+#endif
 
       // member constants
       static constexpr size_t extent = _Extent;
@@ -301,6 +305,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       rend() const noexcept
       { return reverse_iterator(this->begin()); }
 
+#if __cplusplus > 202002L
+      constexpr const_iterator
+      cbegin() const noexcept
+      { return begin(); }
+
+      constexpr const_iterator
+      cend() const noexcept
+      { return end(); }
+
+      constexpr const_reverse_iterator
+      crbegin() const noexcept
+      { return rbegin(); }
+
+      constexpr const_reverse_iterator
+      crend() const noexcept
+      { return rend(); }
+#endif
+
       // subviews
 
       template<size_t _Count>
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index d233b03..9f31f25 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -339,6 +339,7 @@
 #define __cpp_lib_ranges_stride 202207L
 #define __cpp_lib_ranges_cartesian_product 202207L
 #define __cpp_lib_ranges_as_rvalue 202207L
+#define __cpp_lib_ranges_as_const 202207L
 #define __cpp_lib_ranges_enumerate 202302L
 #define __cpp_lib_fold 202207L
 #if __cpp_constexpr_dynamic_alloc