[libc++] P2655R3 common_reference_t of reference_wrapper Should Be a Reference Type

libcxx/docs/FeatureTestMacroTable.rst

@@ -198,6 +198,10 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_char8_t``                                      ``201907L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_common_reference``                             ``202302L``
+    ---------------------------------------------------------- -----------------
+    ``__cpp_lib_common_reference_wrapper``                     ``202302L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                                     ``202002L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_algorithms``                         ``201806L``

libcxx/docs/ReleaseNotes/21.rst

@@ -51,6 +51,7 @@ Implemented Papers
 - 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>`__)
+- P2655R3: ``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type (`Github <https://github.com/llvm/llvm-project/issues/105260>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx23Papers.csv

@@ -113,7 +113,7 @@
 "`P2693R1 <https://wg21.link/P2693R1>`__","Formatting ``thread::id`` and ``stacktrace``","2023-02 (Issaquah)","|Partial|","","The formatter for ``stacktrace`` is not implemented, since ``stacktrace`` is not implemented yet"
 "`P2679R2 <https://wg21.link/P2679R2>`__","Fixing ``std::start_lifetime_as`` for arrays","2023-02 (Issaquah)","","",""
 "`P2674R1 <https://wg21.link/P2674R1>`__","A trait for implicit lifetime types","2023-02 (Issaquah)","|Complete|","20",""
-"`P2655R3 <https://wg21.link/P2655R3>`__","``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type","2023-02 (Issaquah)","","",""
+"`P2655R3 <https://wg21.link/P2655R3>`__","``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type","2023-02 (Issaquah)","|Complete|","21","The paper is implemented as a DR to C++20"
 "`P2652R2 <https://wg21.link/P2652R2>`__","Disallow User Specialization of ``allocator_traits``","2023-02 (Issaquah)","|Complete|","19",""
 "`P2787R1 <https://wg21.link/P2787R1>`__","``pmr::generator`` - Promise Types are not Values","2023-02 (Issaquah)","","",""
 "`P2614R2 <https://wg21.link/P2614R2>`__","Deprecate ``numeric_limits::has_denorm``","2023-02 (Issaquah)","|Complete|","18",""

libcxx/include/__functional/reference_wrapper.h

@@ -11,15 +11,18 @@
 #define _LIBCPP___FUNCTIONAL_REFERENCE_WRAPPER_H
 
 #include <__compare/synth_three_way.h>
+#include <__concepts/convertible_to.h>
 #include <__config>
 #include <__functional/weak_result_type.h>
 #include <__memory/addressof.h>
+#include <__type_traits/common_reference.h>
 #include <__type_traits/desugars_to.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_const.h>
 #include <__type_traits/is_core_convertible.h>
 #include <__type_traits/is_same.h>
+#include <__type_traits/is_specialization.h>
 #include <__type_traits/remove_cvref.h>
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>
@@ -156,6 +159,32 @@ template <class _CanonicalTag, class _Operation, class... _Args>
 inline const bool __desugars_to_v<_CanonicalTag, reference_wrapper<_Operation>, _Args...> =
     __desugars_to_v<_CanonicalTag, _Operation, _Args...>;
 
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Tp>
+inline constexpr bool __is_ref_wrapper = __is_specialization_v<_Tp, reference_wrapper>;
+
+template <class _Rp, class _Tp, class _RpQual, class _TpQual>
+concept __ref_wrap_common_reference_exists_with = __is_ref_wrapper<_Rp> && requires {
+  typename common_reference_t<typename _Rp::type&, _TpQual>;
+} && convertible_to<_RpQual, common_reference_t<typename _Rp::type&, _TpQual>>;
+
+template <class _Rp, class _Tp, template <class> class _RpQual, template <class> class _TpQual>
+  requires(__ref_wrap_common_reference_exists_with<_Rp, _Tp, _RpQual<_Rp>, _TpQual<_Tp>> &&
+           !__ref_wrap_common_reference_exists_with<_Tp, _Rp, _TpQual<_Tp>, _RpQual<_Rp>>)
+struct basic_common_reference<_Rp, _Tp, _RpQual, _TpQual> {
+  using type _LIBCPP_NODEBUG = common_reference_t<typename _Rp::type&, _TpQual<_Tp>>;
+};
+
+template <class _Tp, class _Rp, template <class> class _TpQual, template <class> class _RpQual>
+  requires(__ref_wrap_common_reference_exists_with<_Rp, _Tp, _RpQual<_Rp>, _TpQual<_Tp>> &&
+           !__ref_wrap_common_reference_exists_with<_Tp, _Rp, _TpQual<_Tp>, _RpQual<_Rp>>)
+struct basic_common_reference<_Tp, _Rp, _TpQual, _RpQual> {
+  using type _LIBCPP_NODEBUG = common_reference_t<typename _Rp::type&, _TpQual<_Tp>>;
+};
+
+#endif // _LIBCPP_STD_VER >= 20
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FUNCTIONAL_REFERENCE_WRAPPER_H

libcxx/include/__type_traits/common_reference.h

@@ -10,6 +10,7 @@
 #define _LIBCPP___TYPE_TRAITS_COMMON_REFERENCE_H
 
 #include <__config>
+#include <__type_traits/add_pointer.h>
 #include <__type_traits/common_type.h>
 #include <__type_traits/copy_cv.h>
 #include <__type_traits/copy_cvref.h>
@@ -139,13 +140,17 @@ struct __common_reference_sub_bullet2 : __common_reference_sub_bullet3<_Tp, _Up>
 template <class _Tp, class _Up>
 struct __common_reference_sub_bullet1 : __common_reference_sub_bullet2<_Tp, _Up> {};
 
-// sub-bullet 1 - If T1 and T2 are reference types and COMMON-REF(T1, T2) is well-formed, then
-// the member typedef `type` denotes that type.
+// sub-bullet 1 - Let R be COMMON-REF(T1, T2). If T1 and T2 are reference types, R is well-formed, and
+// is_convertible_v<add_pointer_t<T1>, add_pointer_t<R>> && is_convertible_v<add_pointer_t<T2>, add_pointer_t<R>> is
+// true, then the member typedef type denotes R.
+
 template <class _Tp, class _Up>
 struct common_reference<_Tp, _Up> : __common_reference_sub_bullet1<_Tp, _Up> {};
 
 template <class _Tp, class _Up>
-  requires is_reference_v<_Tp> && is_reference_v<_Up> && requires { typename __common_ref_t<_Tp, _Up>; }
+  requires is_reference_v<_Tp> && is_reference_v<_Up> && requires { typename __common_ref_t<_Tp, _Up>; } &&
+           is_convertible_v<add_pointer_t<_Tp>, add_pointer_t<__common_ref_t<_Tp, _Up>>> &&
+           is_convertible_v<add_pointer_t<_Up>, add_pointer_t<__common_ref_t<_Tp, _Up>>>
 struct __common_reference_sub_bullet1<_Tp, _Up> {
   using type _LIBCPP_NODEBUG = __common_ref_t<_Tp, _Up>;
 };

libcxx/include/functional

@@ -565,6 +565,7 @@ POLICY:  For non-variadic implementations, the number of arguments is limited
 #    include <__functional/bind_front.h>
 #    include <__functional/identity.h>
 #    include <__functional/ranges_operations.h>
+#    include <__type_traits/common_reference.h>
 #    include <__type_traits/unwrap_ref.h>
 #  endif
 

libcxx/include/version

@@ -59,6 +59,8 @@ __cpp_lib_char8_t                                       201907L <atomic> <filesy
 __cpp_lib_chrono                                        201611L <chrono>
 __cpp_lib_chrono_udls                                   201304L <chrono>
 __cpp_lib_clamp                                         201603L <algorithm>
+__cpp_lib_common_reference                              202302L <type_traits>
+__cpp_lib_common_reference_wrapper                      202302L <functional>
 __cpp_lib_complex_udls                                  201309L <complex>
 __cpp_lib_concepts                                      202002L <concepts>
 __cpp_lib_constexpr_algorithms                          202306L <algorithm> <utility>
@@ -402,6 +404,8 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # if _LIBCPP_HAS_CHAR8_T
 #   define __cpp_lib_char8_t                            201907L
 # endif
+# define __cpp_lib_common_reference                     202302L
+# define __cpp_lib_common_reference_wrapper             202302L
 # define __cpp_lib_concepts                             202002L
 # define __cpp_lib_constexpr_algorithms                 201806L
 # define __cpp_lib_constexpr_complex                    201711L

libcxx/modules/std/functional.inc

@@ -14,6 +14,10 @@ export namespace std {
   using std::invoke_r;
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+  using std::basic_common_reference;
+#endif
+
   // [refwrap], reference_wrapper
   using std::reference_wrapper;
 

libcxx/test/std/language.support/support.limits/support.limits.general/functional.version.compile.pass.cpp

@@ -32,6 +32,10 @@
 #    error "__cpp_lib_boyer_moore_searcher should not be defined before c++17"
 #  endif
 
+#  ifdef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should not be defined before c++20"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should not be defined before c++20"
 #  endif
@@ -94,6 +98,10 @@
 #    error "__cpp_lib_boyer_moore_searcher should not be defined before c++17"
 #  endif
 
+#  ifdef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should not be defined before c++20"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should not be defined before c++20"
 #  endif
@@ -165,6 +173,10 @@
 #    error "__cpp_lib_boyer_moore_searcher should have the value 201603L in c++17"
 #  endif
 
+#  ifdef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should not be defined before c++20"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should not be defined before c++20"
 #  endif
@@ -245,6 +257,13 @@
 #    error "__cpp_lib_boyer_moore_searcher should have the value 201603L in c++20"
 #  endif
 
+#  ifndef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should be defined in c++20"
+#  endif
+#  if __cpp_lib_common_reference_wrapper != 202302L
+#    error "__cpp_lib_common_reference_wrapper should have the value 202302L in c++20"
+#  endif
+
 #  ifndef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should be defined in c++20"
 #  endif
@@ -337,6 +356,13 @@
 #    error "__cpp_lib_boyer_moore_searcher should have the value 201603L in c++23"
 #  endif
 
+#  ifndef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should be defined in c++23"
+#  endif
+#  if __cpp_lib_common_reference_wrapper != 202302L
+#    error "__cpp_lib_common_reference_wrapper should have the value 202302L in c++23"
+#  endif
+
 #  ifndef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should be defined in c++23"
 #  endif
@@ -441,6 +467,13 @@
 #    error "__cpp_lib_boyer_moore_searcher should have the value 201603L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_common_reference_wrapper
+#    error "__cpp_lib_common_reference_wrapper should be defined in c++26"
+#  endif
+#  if __cpp_lib_common_reference_wrapper != 202302L
+#    error "__cpp_lib_common_reference_wrapper should have the value 202302L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_constexpr_functional
 #    error "__cpp_lib_constexpr_functional should be defined in c++26"
 #  endif

libcxx/test/std/language.support/support.limits/support.limits.general/type_traits.version.compile.pass.cpp

@@ -28,6 +28,10 @@
 #    error "__cpp_lib_bounded_array_traits should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should not be defined before c++20"
+#  endif
+
 #  ifdef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should not be defined before c++17"
 #  endif
@@ -130,6 +134,10 @@
 #    error "__cpp_lib_bounded_array_traits should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should not be defined before c++20"
+#  endif
+
 #  ifdef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should not be defined before c++17"
 #  endif
@@ -250,6 +258,10 @@
 #    error "__cpp_lib_bounded_array_traits should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should not be defined before c++20"
+#  endif
+
 #  ifndef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should be defined in c++17"
 #  endif
@@ -394,6 +406,13 @@
 #    error "__cpp_lib_bounded_array_traits should have the value 201902L in c++20"
 #  endif
 
+#  ifndef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should be defined in c++20"
+#  endif
+#  if __cpp_lib_common_reference != 202302L
+#    error "__cpp_lib_common_reference should have the value 202302L in c++20"
+#  endif
+
 #  ifndef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should be defined in c++20"
 #  endif
@@ -568,6 +587,13 @@
 #    error "__cpp_lib_bounded_array_traits should have the value 201902L in c++23"
 #  endif
 
+#  ifndef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should be defined in c++23"
+#  endif
+#  if __cpp_lib_common_reference != 202302L
+#    error "__cpp_lib_common_reference should have the value 202302L in c++23"
+#  endif
+
 #  ifndef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should be defined in c++23"
 #  endif
@@ -763,6 +789,13 @@
 #    error "__cpp_lib_bounded_array_traits should have the value 201902L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_common_reference
+#    error "__cpp_lib_common_reference should be defined in c++26"
+#  endif
+#  if __cpp_lib_common_reference != 202302L
+#    error "__cpp_lib_common_reference should have the value 202302L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_has_unique_object_representations
 #    error "__cpp_lib_has_unique_object_representations should be defined in c++26"
 #  endif