[libc++] Implement views::join_with (#65536)

* Implement "P2441R2 `views::join_with`" (https://wg21.link/P2441R2),
closes #105185
* Implement LWG4074 (https://wg21.link/LWG4074), closes #105346
* Complete implementation of "P2711R1 Making multi-param constructors of
views explicit" (https://wg21.link/P2711R1), closes #105252
* Complete implementation of "P2770R0 Stashing stashing iterators for
proper flattening" (https://wg21.link/P2770R0), closes #105250

Author: JMazurkiewicz

libcxx/docs/FeatureTestMacroTable.rst

@@ -376,7 +376,7 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_ranges_iota``                                  ``202202L``
     ---------------------------------------------------------- -----------------
-    ``__cpp_lib_ranges_join_with``                             *unimplemented*
+    ``__cpp_lib_ranges_join_with``                             ``202202L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_ranges_repeat``                                ``202207L``
     ---------------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes/21.rst

@@ -47,6 +47,9 @@ Implemented Papers
 - P1222R4: A Standard ``flat_set`` (`Github <https://github.com/llvm/llvm-project/issues/105193>`__)
 - P2897R7: ``aligned_accessor``: An mdspan accessor expressing pointer over-alignment (`Github <https://github.com/llvm/llvm-project/issues/118372>`__)
 - P3247R2: Deprecate the notion of trivial types (`Github <https://github.com/llvm/llvm-project/issues/118387>`__)
+- P2441R2: ``views::join_with`` (`Github <https://github.com/llvm/llvm-project/issues/105185>`__)
+- P2711R1: Making multi-param constructors of ``views`` ``explicit`` (`Github <https://github.com/llvm/llvm-project/issues/105252>`__)
+- P2770R0: Stashing stashing ``iterators`` for proper flattening (`Github <https://github.com/llvm/llvm-project/issues/105250>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx23Papers.csv

@@ -47,7 +47,7 @@
 "`P2273R3 <https://wg21.link/P2273R3>`__","Making ``std::unique_ptr`` constexpr","2022-02 (Virtual)","|Complete|","16",""
 "`P2387R3 <https://wg21.link/P2387R3>`__","Pipe support for user-defined range adaptors","2022-02 (Virtual)","|Complete|","19",""
 "`P2440R1 <https://wg21.link/P2440R1>`__","``ranges::iota``, ``ranges::shift_left`` and ``ranges::shift_right``","2022-02 (Virtual)","|Partial|","","Only ``ranges::iota`` is implemented."
-"`P2441R2 <https://wg21.link/P2441R2>`__","``views::join_with``","2022-02 (Virtual)","|In Progress|","",""
+"`P2441R2 <https://wg21.link/P2441R2>`__","``views::join_with``","2022-02 (Virtual)","|Complete|","21",""
 "`P2442R1 <https://wg21.link/P2442R1>`__","Windowing range adaptors: ``views::chunk`` and ``views::slide``","2022-02 (Virtual)","","",""
 "`P2443R1 <https://wg21.link/P2443R1>`__","``views::chunk_by``","2022-02 (Virtual)","|Complete|","18",""
 "","","","","",""
@@ -103,9 +103,9 @@
 "`P2708R1 <https://wg21.link/P2708R1>`__","No Further Fundamentals TSes","2022-11 (Kona)","|Nothing To Do|","",""
 "","","","","",""
 "`P0290R4 <https://wg21.link/P0290R4>`__","``apply()`` for ``synchronized_value<T>``","2023-02 (Issaquah)","","",""
-"`P2770R0 <https://wg21.link/P2770R0>`__","Stashing stashing ``iterators`` for proper flattening","2023-02 (Issaquah)","|Partial|","","``join_with_view`` hasn't been done yet since this type isn't implemented yet"
+"`P2770R0 <https://wg21.link/P2770R0>`__","Stashing stashing ``iterators`` for proper flattening","2023-02 (Issaquah)","|Complete|","21",""
 "`P2164R9 <https://wg21.link/P2164R9>`__","``views::enumerate``","2023-02 (Issaquah)","","",""
-"`P2711R1 <https://wg21.link/P2711R1>`__","Making multi-param constructors of ``views`` ``explicit``","2023-02 (Issaquah)","|In Progress|","","``join_with_view`` hasn't been done yet since this type isn't implemented yet"
+"`P2711R1 <https://wg21.link/P2711R1>`__","Making multi-param constructors of ``views`` ``explicit``","2023-02 (Issaquah)","|Complete|","21",""
 "`P2609R3 <https://wg21.link/P2609R3>`__","Relaxing Ranges Just A Smidge","2023-02 (Issaquah)","|Complete|","20","Implemented as a DR in C++20. Other implementations will do the same."
 "`P2713R1 <https://wg21.link/P2713R1>`__","Escaping improvements in ``std::format``","2023-02 (Issaquah)","|Complete|","19",""
 "`P2675R1 <https://wg21.link/P2675R1>`__","``format``'s width estimation is too approximate and not forward compatible","2023-02 (Issaquah)","|Complete|","17",""

libcxx/docs/Status/Cxx2cIssues.csv

@@ -66,7 +66,7 @@
 "`LWG4060 <https://wg21.link/LWG4060>`__","``submdspan`` preconditions do not forbid creating invalid pointer","2024-06 (St. Louis)","","",""
 "`LWG4061 <https://wg21.link/LWG4061>`__","Should ``std::basic_format_context`` be default-constructible/copyable/movable?","2024-06 (St. Louis)","|Complete|","19",""
 "`LWG4071 <https://wg21.link/LWG4071>`__","``reference_wrapper`` comparisons are not SFINAE-friendly","2024-06 (St. Louis)","|Complete|","19",""
-"`LWG4074 <https://wg21.link/LWG4074>`__","``compatible-joinable-ranges`` is underconstrained","2024-06 (St. Louis)","","",""
+"`LWG4074 <https://wg21.link/LWG4074>`__","``compatible-joinable-ranges`` is underconstrained","2024-06 (St. Louis)","|Complete|","21",""
 "`LWG4076 <https://wg21.link/LWG4076>`__","``concat_view`` should be freestanding","2024-06 (St. Louis)","","",""
 "`LWG4079 <https://wg21.link/LWG4079>`__","Missing Preconditions in ``concat_view::iterator``\`s conversion constructor","2024-06 (St. Louis)","","",""
 "`LWG4082 <https://wg21.link/LWG4082>`__","``views::concat(r)`` is well-formed when ``r`` is an ``output_range``","2024-06 (St. Louis)","","",""

libcxx/include/CMakeLists.txt

@@ -706,6 +706,7 @@ set(files
   __ranges/iota_view.h
   __ranges/istream_view.h
   __ranges/join_view.h
+  __ranges/join_with_view.h
   __ranges/lazy_split_view.h
   __ranges/movable_box.h
   __ranges/non_propagating_cache.h

libcxx/include/__ranges/concepts.h

@@ -10,7 +10,9 @@
 #ifndef _LIBCPP___RANGES_CONCEPTS_H
 #define _LIBCPP___RANGES_CONCEPTS_H
 
+#include <__concepts/common_reference_with.h>
 #include <__concepts/constructible.h>
+#include <__concepts/convertible_to.h>
 #include <__concepts/movable.h>
 #include <__concepts/same_as.h>
 #include <__config>
@@ -25,6 +27,8 @@
 #include <__ranges/enable_view.h>
 #include <__ranges/size.h>
 #include <__type_traits/add_pointer.h>
+#include <__type_traits/common_reference.h>
+#include <__type_traits/common_type.h>
 #include <__type_traits/is_reference.h>
 #include <__type_traits/remove_cvref.h>
 #include <__type_traits/remove_reference.h>
@@ -133,6 +137,42 @@ concept viewable_range =
       (is_lvalue_reference_v<_Tp> ||
        (movable<remove_reference_t<_Tp>> && !__is_std_initializer_list<remove_cvref_t<_Tp>>))));
 
+#  if _LIBCPP_STD_VER >= 23
+
+template <class... _Rs>
+using __concat_reference_t _LIBCPP_NODEBUG = common_reference_t<range_reference_t<_Rs>...>;
+
+template <class... _Rs>
+using __concat_value_t _LIBCPP_NODEBUG = common_type_t<range_value_t<_Rs>...>;
+
+template <class... _Rs>
+using __concat_rvalue_reference_t _LIBCPP_NODEBUG = common_reference_t<range_rvalue_reference_t<_Rs>...>;
+
+template <class _Ref, class _RRef, class _It>
+concept __concat_indirectly_readable_impl = requires(const _It __it) {
+  { *__it } -> convertible_to<_Ref>;
+  { ranges::iter_move(__it) } -> convertible_to<_RRef>;
+};
+
+template <class... _Rs>
+concept __concat_indirectly_readable =
+    common_reference_with<__concat_reference_t<_Rs...>&&, __concat_value_t<_Rs...>&> &&
+    common_reference_with<__concat_reference_t<_Rs...>&&, __concat_rvalue_reference_t<_Rs...>&&> &&
+    common_reference_with<__concat_rvalue_reference_t<_Rs...>&&, const __concat_value_t<_Rs...>&> &&
+    (__concat_indirectly_readable_impl<__concat_reference_t<_Rs...>,
+                                       __concat_rvalue_reference_t<_Rs...>,
+                                       iterator_t<_Rs>> &&
+     ...);
+
+template <class... _Rs>
+concept __concatable = requires {
+  typename __concat_reference_t<_Rs...>;
+  typename __concat_value_t<_Rs...>;
+  typename __concat_rvalue_reference_t<_Rs...>;
+} && __concat_indirectly_readable<_Rs...>;
+
+#  endif // _LIBCPP_STD_VER >= 23
+
 } // namespace ranges
 
 #endif // _LIBCPP_STD_VER >= 20