[PATCH 2/2] libstdc++: Implement P2655R3 common_reference of reference_wrapper
Patrick Palka
ppalka@redhat.com
Tue Oct 7 03:09:08 GMT 2025
Tested on x86_64-pc-linux-gnu, does this look OK for trunk? I wonder
about backports, for either the whole paper or just the common_reference
change in <type_traits> for avoiding dangling references.
-- >8 --
This patch implements this paper as a DR against C++20 (as do MSVC and
libc++).
PR libstdc++/120446
libstdc++-v3/ChangeLog:
* include/bits/refwrap.h (__detail::__is_ref_wrapper):
Define as per P2655R3 for C++20.
(__detail::__ref_wrap_common_reference_exists_with):
Likewise.
(basic_common_reference): Define partial specializations using
the above as per P2655R3 for C++20.
* include/bits/version.def (common_reference): New.
(common_reference_wrapper): New.
* include/bits/version.h: Regenerate.
* include/std/functional (__glibcxx_want_common_reference_wrapper):
Define.
* include/std/type_traits (__glibcxx_want_common_reference):
Define.
(__common_reference_impl<T1, T2, 1>): Add pointer convertibility
constraints as per P2655R3.
* libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc: New
test.
* libstdc++-v3/testsuite/20_util/reference_wrapper/p2655r3.cc: New
test.
---
libstdc++-v3/include/bits/refwrap.h | 34 +++++++++++++++++++
libstdc++-v3/include/bits/version.def | 16 +++++++++
libstdc++-v3/include/bits/version.h | 20 +++++++++++
libstdc++-v3/include/std/functional | 1 +
libstdc++-v3/include/std/type_traits | 7 ++++
.../20_util/common_reference/p2655r3.cc | 15 ++++++++
.../20_util/reference_wrapper/p2655r3.cc | 33 ++++++++++++++++++
7 files changed, 126 insertions(+)
create mode 100644 libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc
create mode 100644 libstdc++-v3/testsuite/20_util/reference_wrapper/p2655r3.cc
diff --git a/libstdc++-v3/include/bits/refwrap.h b/libstdc++-v3/include/bits/refwrap.h
index 612715e9b7bd..5d9f8c8f49f9 100644
--- a/libstdc++-v3/include/bits/refwrap.h
+++ b/libstdc++-v3/include/bits/refwrap.h
@@ -457,6 +457,40 @@ _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
/// @}
+#if __glibcxx_common_reference_wrapper // C++ >= 20
+ namespace __detail
+ {
+ template<typename _Tp>
+ constexpr bool __is_ref_wrapper = false;
+
+ template<typename _Tp>
+ constexpr bool __is_ref_wrapper<reference_wrapper<_Tp>> = true;
+
+ template<typename _Rp, typename _Tp, typename _RQual, typename _TQual>
+ concept __ref_wrap_common_reference_exists_with = __is_ref_wrapper<_Rp>
+ && requires { typename common_reference_t<typename _Rp::type&, _TQual>; }
+ && convertible_to<_RQual, common_reference_t<typename _Rp::type&, _TQual>>;
+ } // namespace __detail
+
+ template<typename _Rp, typename _Tp,
+ template<typename> class _RQual, template<typename> class _TQual>
+ requires __detail::__ref_wrap_common_reference_exists_with<_Rp, _Tp,
+ _RQual<_Rp>, _TQual<_Tp>>
+ && (!__detail::__ref_wrap_common_reference_exists_with<_Tp, _Rp,
+ _TQual<_Tp>, _RQual<_Rp>>)
+ struct basic_common_reference<_Rp, _Tp, _RQual, _TQual>
+ { using type = common_reference_t<typename _Rp::type&, _TQual<_Tp>>; };
+
+ template<typename _Tp, typename _Rp,
+ template<typename> class _TQual, template<typename> class _RQual>
+ requires __detail::__ref_wrap_common_reference_exists_with<_Rp, _Tp,
+ _RQual<_Rp>, _TQual<_Tp>>
+ && (!__detail::__ref_wrap_common_reference_exists_with<_Tp, _Rp,
+ _TQual<_Tp>, _RQual<_Rp>>)
+ struct basic_common_reference<_Tp, _Rp, _TQual, _RQual>
+ { using type = common_reference_t<typename _Rp::type&, _TQual<_Tp>>; };
+#endif
+
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 3a26234f87ed..49d8d17e6677 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1774,6 +1774,22 @@ ftms = {
};
};
+ftms = {
+ name = common_reference;
+ values = {
+ v = 202302;
+ cxxmin = 20; // We treat P2655R3 as a DR against C++20.
+ };
+};
+
+ftms = {
+ name = common_reference_wrapper;
+ values = {
+ v = 202302;
+ cxxmin = 20; // We treat P2655R3 as a DR against C++20.
+ };
+};
+
ftms = {
name = formatters;
values = {
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 46e4c1121e7a..3176a80f32e3 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -1983,6 +1983,26 @@
#endif /* !defined(__cpp_lib_flat_set) && defined(__glibcxx_want_flat_set) */
#undef __glibcxx_want_flat_set
+#if !defined(__cpp_lib_common_reference)
+# if (__cplusplus >= 202002L)
+# define __glibcxx_common_reference 202302L
+# if defined(__glibcxx_want_all) || defined(__glibcxx_want_common_reference)
+# define __cpp_lib_common_reference 202302L
+# endif
+# endif
+#endif /* !defined(__cpp_lib_common_reference) && defined(__glibcxx_want_common_reference) */
+#undef __glibcxx_want_common_reference
+
+#if !defined(__cpp_lib_common_reference_wrapper)
+# if (__cplusplus >= 202002L)
+# define __glibcxx_common_reference_wrapper 202302L
+# if defined(__glibcxx_want_all) || defined(__glibcxx_want_common_reference_wrapper)
+# define __cpp_lib_common_reference_wrapper 202302L
+# endif
+# endif
+#endif /* !defined(__cpp_lib_common_reference_wrapper) && defined(__glibcxx_want_common_reference_wrapper) */
+#undef __glibcxx_want_common_reference_wrapper
+
#if !defined(__cpp_lib_formatters)
# if (__cplusplus >= 202100L) && _GLIBCXX_HOSTED
# define __glibcxx_formatters 202302L
diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index 8ad73b343bda..6f7f2f627a2a 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -64,6 +64,7 @@
#define __glibcxx_want_not_fn
#define __glibcxx_want_ranges
#define __glibcxx_want_reference_wrapper
+#define __glibcxx_want_common_reference_wrapper
#define __glibcxx_want_transparent_operators
#include <bits/version.h>
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 8b5110464e50..47cbf21b4c1d 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -41,6 +41,7 @@
#define __glibcxx_want_bool_constant
#define __glibcxx_want_bounded_array_traits
+#define __glibcxx_want_common_reference
#define __glibcxx_want_constant_wrapper
#define __glibcxx_want_has_unique_object_representations
#define __glibcxx_want_integral_constant_callable
@@ -4223,6 +4224,12 @@ template<typename _Ret, typename _Fn, typename... _Args>
template<typename _Tp1, typename _Tp2>
requires is_reference_v<_Tp1> && is_reference_v<_Tp2>
&& requires { typename __common_ref<_Tp1, _Tp2>; }
+#if __cpp_lib_common_reference // C++ >= 20
+ && is_convertible_v<add_pointer_t<_Tp1>,
+ add_pointer_t<__common_ref<_Tp1, _Tp2>>>
+ && is_convertible_v<add_pointer_t<_Tp2>,
+ add_pointer_t<__common_ref<_Tp1, _Tp2>>>
+#endif
struct __common_reference_impl<_Tp1, _Tp2, 1>
{ using type = __common_ref<_Tp1, _Tp2>; };
diff --git a/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc
new file mode 100644
index 000000000000..bc0372f79686
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc
@@ -0,0 +1,15 @@
+// P2655R3 - common_reference_t of reference_wrapper Should Be a Reference Type
+// Implemented as a DR against C++20
+// { dg-do compile { target c++20 } }
+
+#include <type_traits>
+
+#if __cpp_lib_common_reference != 202302L
+# error "Feature-test macro __cpp_lib_common_reference has wrong value in <type_traits>"
+#endif
+
+struct A { };
+struct B { operator A&() const; };
+
+static_assert(std::is_same_v<std::common_reference_t<A&, const B&>, A&>);
+static_assert(std::is_same_v<std::common_reference_t<const B&, A&>, A&>);
diff --git a/libstdc++-v3/testsuite/20_util/reference_wrapper/p2655r3.cc b/libstdc++-v3/testsuite/20_util/reference_wrapper/p2655r3.cc
new file mode 100644
index 000000000000..f66c2821554a
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/reference_wrapper/p2655r3.cc
@@ -0,0 +1,33 @@
+// P2655R3 - common_reference_t of reference_wrapper Should Be a Reference Type
+// Implemented as a DR against C++20
+// { dg-do compile { target c++20 } }
+
+#include <functional>
+
+#if __cpp_lib_common_reference_wrapper != 202302L
+# error "Feature-test macro __cpp_lib_common_reference_wrapper has wrong value in <functional>"
+#endif
+
+using std::is_same_v;
+using std::common_reference_t;
+using std::reference_wrapper;
+
+static_assert(is_same_v<common_reference_t<const reference_wrapper<int>&, int&>, int&>);
+static_assert(is_same_v<common_reference_t<int&, const reference_wrapper<int>&>, int&>);
+
+static_assert(is_same_v<common_reference_t<reference_wrapper<int>, int&>,
+ common_reference_t<int&, int&>>);
+static_assert(is_same_v<common_reference_t<reference_wrapper<int>, const int&>,
+ common_reference_t<int&, const int&>>);
+static_assert(is_same_v<common_reference_t<reference_wrapper<const int>, int&>,
+ common_reference_t<const int&, int&>>);
+
+static_assert(is_same_v<common_reference_t<int&, reference_wrapper<int>>,
+ common_reference_t<int&, int&>>);
+static_assert(is_same_v<common_reference_t<const int&, reference_wrapper<int>>,
+ common_reference_t<int&, const int&>>);
+static_assert(is_same_v<common_reference_t<int&, reference_wrapper<const int>>,
+ common_reference_t<const int&, int&>>);
+
+static_assert(is_same_v<common_reference_t<reference_wrapper<int>&, reference_wrapper<int>&>,
+ reference_wrapper<int>&>);
--
2.51.0.433.g45547b60ac
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