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.

libstdc++-v3/include/bits/ranges_base.h

@@ -515,6 +515,17 @@ namespace ranges
   template<range _Range>
     using sentinel_t = decltype(ranges::end(std::declval<_Range&>()));
 
+#if __cplusplus > 202002L
+  template<range _Range>
+    using const_iterator_t = const_iterator<iterator_t<_Range>>;
+
+  template<range _Range>
+    using const_sentinel_t = const_sentinel<sentinel_t<_Range>>;
+
+  template<range _Range>
+    using range_const_reference_t = iter_const_reference_t<iterator_t<_Range>>;
+#endif
+
   template<range _Range>
     using range_difference_t = iter_difference_t<iterator_t<_Range>>;
 
@@ -607,8 +618,25 @@ namespace ranges
     concept common_range
       = range<_Tp> && same_as<iterator_t<_Tp>, sentinel_t<_Tp>>;
 
+#if __cplusplus > 202002L
+  template<typename _Tp>
+    concept constant_range
+      = input_range<_Tp> && std::__detail::__constant_iterator<iterator_t<_Tp>>;
+#endif
+
   namespace __cust_access
   {
+#if __cplusplus > 202020L
+    template<typename _Range>
+      constexpr auto&
+      __possibly_const_range(_Range& __r) noexcept
+      {
+	if constexpr (constant_range<const _Range> && !constant_range<_Range>)
+	  return const_cast<const _Range&>(__r);
+	else
+	  return __r;
+      }
+#else
     // If _To is an lvalue-reference, return const _Tp&, otherwise const _Tp&&.
     template<typename _To, typename _Tp>
       constexpr decltype(auto)
@@ -621,9 +649,24 @@ namespace ranges
 	else
 	  return static_cast<const _Tp&&>(__t);
       }
+#endif
 
     struct _CBegin
     {
+#if __cplusplus > 202002L
+      template<__maybe_borrowed_range _Tp>
+	[[nodiscard]]
+	constexpr auto
+	operator()(_Tp&& __t) const
+	noexcept(noexcept(std::make_const_iterator
+			  (ranges::begin(__cust_access::__possibly_const_range(__t)))))
+	requires requires { std::make_const_iterator
+			    (ranges::begin(__cust_access::__possibly_const_range(__t))); }
+	{
+	  auto& __r = __cust_access::__possibly_const_range(__t);
+	  return const_iterator_t<decltype(__r)>(ranges::begin(__r));
+	}
+#else
       template<typename _Tp>
 	[[nodiscard]]
 	constexpr auto
@@ -633,10 +676,25 @@ namespace ranges
 	{
 	  return _Begin{}(__cust_access::__as_const<_Tp>(__e));
 	}
+#endif
     };
 
     struct _CEnd final
     {
+#if __cplusplus > 202002L
+      template<__maybe_borrowed_range _Tp>
+	[[nodiscard]]
+	constexpr auto
+	operator()(_Tp&& __t) const
+	noexcept(noexcept(std::make_const_sentinel
+			  (ranges::end(__cust_access::__possibly_const_range(__t)))))
+	requires requires { std::make_const_sentinel
+			    (ranges::end(__cust_access::__possibly_const_range(__t))); }
+	{
+	  auto& __r = __cust_access::__possibly_const_range(__t);
+	  return const_sentinel_t<decltype(__r)>(ranges::end(__r));
+	}
+#else
       template<typename _Tp>
 	[[nodiscard]]
 	constexpr auto
@@ -646,10 +704,25 @@ namespace ranges
 	{
 	  return _End{}(__cust_access::__as_const<_Tp>(__e));
 	}
+#endif
     };
 
     struct _CRBegin
     {
+#if __cplusplus > 202002L
+      template<__maybe_borrowed_range _Tp>
+	[[nodiscard]]
+	constexpr auto
+	operator()(_Tp&& __t) const
+	noexcept(noexcept(std::make_const_iterator
+			  (ranges::rbegin(__cust_access::__possibly_const_range(__t)))))
+	requires requires { std::make_const_iterator
+			    (ranges::rbegin(__cust_access::__possibly_const_range(__t))); }
+	{
+	  auto& __r = __cust_access::__possibly_const_range(__t);
+	  return const_iterator<decltype(ranges::rbegin(__r))>(ranges::rbegin(__r));
+	}
+#else
       template<typename _Tp>
 	[[nodiscard]]
 	constexpr auto
@@ -659,10 +732,25 @@ namespace ranges
 	{
 	  return _RBegin{}(__cust_access::__as_const<_Tp>(__e));
 	}
+#endif
     };
 
     struct _CREnd
     {
+#if __cplusplus > 202002L
+      template<__maybe_borrowed_range _Tp>
+	[[nodiscard]]
+	constexpr auto
+	operator()(_Tp&& __t) const
+	noexcept(noexcept(std::make_const_sentinel
+			  (ranges::rend(__cust_access::__possibly_const_range(__t)))))
+	requires requires { std::make_const_sentinel
+			    (ranges::rend(__cust_access::__possibly_const_range(__t))); }
+	{
+	  auto& __r = __cust_access::__possibly_const_range(__t);
+	  return const_sentinel<decltype(ranges::rend(__r))>(ranges::rend(__r));
+	}
+#else
       template<typename _Tp>
 	[[nodiscard]]
 	constexpr auto
@@ -672,10 +760,20 @@ namespace ranges
 	{
 	  return _REnd{}(__cust_access::__as_const<_Tp>(__e));
 	}
+#endif
     };
 
     struct _CData
     {
+#if __cplusplus > 202002L
+      template<__maybe_borrowed_range _Tp>
+	[[nodiscard]]
+	constexpr const auto*
+	operator()(_Tp&& __t) const
+	noexcept(noexcept(ranges::data(__cust_access::__possibly_const_range(__t))))
+	requires requires { ranges::data(__cust_access::__possibly_const_range(__t)); }
+	{ return ranges::data(__cust_access::__possibly_const_range(__t)); }
+#else
       template<typename _Tp>
 	[[nodiscard]]
 	constexpr auto
@@ -685,6 +783,7 @@ namespace ranges
 	{
 	  return _Data{}(__cust_access::__as_const<_Tp>(__e));
 	}
+#endif
     };
 
   } // namespace __cust_access

libstdc++-v3/include/bits/ranges_util.h

@@ -53,9 +53,7 @@ namespace ranges
       concept __has_arrow = input_iterator<_It>
 	&& (is_pointer_v<_It> || requires(_It __it) { __it.operator->(); });
 
-    template<typename _Tp, typename _Up>
-      concept __different_from
-	= !same_as<remove_cvref_t<_Tp>, remove_cvref_t<_Up>>;
+    using std::__detail::__different_from;
   } // namespace __detail
 
   /// The ranges::view_interface class template
@@ -192,6 +190,24 @@ namespace ranges
 	constexpr decltype(auto)
 	operator[](range_difference_t<_Range> __n) const
 	{ return ranges::begin(_M_derived())[__n]; }
+
+#if __cplusplus > 202002L
+      constexpr auto
+      cbegin() requires input_range<_Derived>
+      { return ranges::cbegin(_M_derived()); }
+
+      constexpr auto
+      cbegin() const requires input_range<const _Derived>
+      { return ranges::cbegin(_M_derived()); }
+
+      constexpr auto
+      cend() requires input_range<_Derived>
+      { return ranges::cend(_M_derived()); }
+
+      constexpr auto
+      cend() const requires input_range<const _Derived>
+      { return ranges::cend(_M_derived()); }
+#endif
     };
 
   namespace __detail