[libc++][ranges] Implement LWG-3860 range_common_reference_t is missing

[fsb4000]

Summary

https://cplusplus.github.io/LWG/issue3860

[philnik]

libcxx/include/__ranges/concepts.h

76–78

Do we want to back-port this to C++20?

[fsb4000]

libcxx/include/__ranges/concepts.h

76–78

Yes, I we want.

[Mordante]

Thanks, LGTM!

[fsb4000]

I landed https://github.com/llvm/llvm-project/commit/a8ead9194cfb18922408404c8cabfa83c276944f but I forgot to add

Reviewed By: Mordante, philnik, #libc

Differential Revision: https://reviews.llvm.org/D144338

🤦

libcxx/docs/Status/Cxx2bIssues.csv

 "`3841 <https://wg21.link/LWG3841>`__","``<version>`` should not be ""all freestanding""","February 2023","","",""
 "`3842 <https://wg21.link/LWG3842>`__","Unclear wording for ``precision`` in ``chrono-format-spec``","February 2023","","","|format|"
 "`3848 <https://wg21.link/LWG3848>`__","``adjacent_view``, ``adjacent_transform_view`` and ``slide_view`` missing ``base`` accessor","February 2023","","","|ranges|"
 "`3849 <https://wg21.link/LWG3849>`__","``cartesian_product_view::iterator``'s default constructor is overconstrained","February 2023","","","|ranges|"
 "`3850 <https://wg21.link/LWG3850>`__","``views::as_const`` on ``empty_view<T>`` should return ``empty_view<const T>``","February 2023","","","|ranges|"
 "`3851 <https://wg21.link/LWG3851>`__","``chunk_view::inner-iterator`` missing custom ``iter_move`` and ``iter_swap``","February 2023","","","|ranges|"
 "`3853 <https://wg21.link/LWG3853>`__","``basic_const_iterator<volatile int*>::operator->`` is ill-formed","February 2023","","",""
 "`3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","February 2023","|Complete|","17.0",""
-"`3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","February 2023","","","|ranges|"
+"`3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","February 2023","|Complete|","17.0","|ranges|"
 "`3866 <https://wg21.link/LWG3866>`__","Bad Mandates for ``expected::transform_error`` overloads","February 2023","","",""
 "`3867 <https://wg21.link/LWG3867>`__","Should ``std::basic_osyncstream``'s move assignment operator be ``noexcept``?","February 2023","","",""
 "`3441 <https://wg21.link/LWG3441>`__","Misleading note about calls to customization points","February 2023","","",""
 "`3622 <https://wg21.link/LWG3622>`__","Misspecified transitivity of equivalence in §[unord.req.general]","February 2023","","",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","February 2023","|Complete|","15.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","February 2023","|Complete|","14.0",""
 "`3655 <https://wg21.link/LWG3655>`__","The ``INVOKE`` operation and union types","February 2023","","",""
 "`3723 <https://wg21.link/LWG3723>`__","``priority_queue::push_range`` needs to ``append_range``","February 2023","","","|ranges|"

libcxx/include/__ranges/concepts.h

namespace ranges {
   using range_value_t = iter_value_t<iterator_t<_Rp>>;
 
   template <range _Rp>
   using range_reference_t = iter_reference_t<iterator_t<_Rp>>;
 
   template <range _Rp>
   using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<_Rp>>;
 
+  template <range _Rp>
+  using range_common_reference_t = iter_common_reference_t<iterator_t<_Rp>>;
+
   // [range.sized]
   template <class _Tp>
   concept sized_range = range<_Tp> && requires(_Tp& __t) { ranges::size(__t); };
 
   template<sized_range _Rp>
   using range_size_t = decltype(ranges::size(std::declval<_Rp&>()));
 
   // `disable_sized_range` defined in `<__ranges/size.h>`

libcxx/include/ranges

namespace std::ranges {
   template<sized_range R>
     using range_size_t = decltype(ranges::size(declval<R&>()));
   template<range R>
     using range_value_t = iter_value_t<iterator_t<R>>;
   template<range R>
     using range_reference_t = iter_reference_t<iterator_t<R>>;
   template<range R>
     using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>;
+  template <range R>
+    using range_common_reference_t = iter_common_reference_t<iterator_t<R>>;
 
   // [range.sized], sized ranges
   template<class>
     inline constexpr bool disable_sized_range = false;
 
   template<class T>
     concept sized_range = ...;
 

libcxx/test/std/ranges/range.req/range.range/helper_aliases.compile.pass.cpp

 // using range_value_t = iter_value_t<iterator_t<R>>;
 
 // template<range R>
 // using range_reference_t = iter_reference_t<iterator_t<R>>;
 
 // template<range R>
 // using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>;
 
+// template <range R>
+// using range_common_reference_t = iter_common_reference_t<iterator_t<R>>;
+
 #include <ranges>
 
 #include <concepts>
 
 #include "test_iterators.h"
 #include "test_range.h"
 
-
-
 static_assert(std::same_as<std::ranges::range_difference_t<test_range<cpp20_input_iterator> >, std::iter_difference_t<int*> >);
 static_assert(std::same_as<std::ranges::range_value_t<test_range<cpp20_input_iterator> >, std::iter_value_t<int*> >);
 static_assert(std::same_as<std::ranges::range_reference_t<test_range<cpp20_input_iterator> >, std::iter_reference_t<int*> >);
 static_assert(std::same_as<std::ranges::range_rvalue_reference_t<test_range<cpp20_input_iterator> >, std::iter_rvalue_reference_t<int*> >);
+static_assert(std::same_as<std::ranges::range_common_reference_t<test_range<cpp20_input_iterator> >, std::iter_common_reference_t<int*> >);