From ea73cc5f1a5da65e9bf98a074b1d0220f177af0e Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Mon, 26 May 2025 17:16:33 +0800
Subject: [PATCH 1/4] [libc++] Disallow specializing `common_reference`

`common_reference` isn't an exception for [meta.rqmts]/4, so it's better
to disallow users to specialize it.

`indirectly_readable.compile.pass.cpp` was a bit problematic. It
attempted to opt-out common reference type in some wrong ways. Also, the
standard effectively forbids opting-out common reference type for `T&`
and `T&&`. This patch removes and adjusts some problematic cases.
---
 .../include/__type_traits/common_reference.h  | 15 ++++++---
 .../type_traits/no_specializations.verify.cpp |  7 +++++
 .../indirectly_readable.compile.pass.cpp      | 31 +++----------------
 3 files changed, 22 insertions(+), 31 deletions(-)

diff --git a/libcxx/include/__type_traits/common_reference.h b/libcxx/include/__type_traits/common_reference.h
index c27da5251b9b7..a2e360405ecf6 100644
--- a/libcxx/include/__type_traits/common_reference.h
+++ b/libcxx/include/__type_traits/common_reference.h
@@ -109,11 +109,18 @@ struct __common_ref {};
 // Note C: For the common_reference trait applied to a parameter pack [...]
 
 template <class...>
-struct common_reference;
+struct _LIBCPP_NO_SPECIALIZATIONS common_reference;
 
 template <class... _Types>
 using common_reference_t = typename common_reference<_Types...>::type;
 
+template <class, class, template <class> class, template <class> class>
+struct basic_common_reference {};
+
+_LIBCPP_DIAGNOSTIC_PUSH
+#  if __has_warning("-Winvalid-specialization")
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-specialization")
+#  endif
 // bullet 1 - sizeof...(T) == 0
 template <>
 struct common_reference<> {};
@@ -145,9 +152,6 @@ struct __common_reference_sub_bullet1<_Tp, _Up> {
 
 // sub-bullet 2 - Otherwise, if basic_common_reference<remove_cvref_t<T1>, remove_cvref_t<T2>, XREF(T1), XREF(T2)>::type
 // is well-formed, then the member typedef `type` denotes that type.
-template <class, class, template <class> class, template <class> class>
-struct basic_common_reference {};
-
 template <class _Tp, class _Up>
 using __basic_common_reference_t _LIBCPP_NODEBUG =
     typename basic_common_reference<remove_cvref_t<_Tp>,
@@ -180,10 +184,11 @@ struct __common_reference_sub_bullet3 : common_type<_Tp, _Up> {};
 template <class _Tp, class _Up, class _Vp, class... _Rest>
   requires requires { typename common_reference_t<_Tp, _Up>; }
 struct common_reference<_Tp, _Up, _Vp, _Rest...> : common_reference<common_reference_t<_Tp, _Up>, _Vp, _Rest...> {};
+_LIBCPP_DIAGNOSTIC_POP
 
 // bullet 5 - Otherwise, there shall be no member `type`.
 template <class...>
-struct common_reference {};
+struct _LIBCPP_NO_SPECIALIZATIONS common_reference{};
 
 #endif // _LIBCPP_STD_VER >= 20
 
diff --git a/libcxx/test/libcxx/type_traits/no_specializations.verify.cpp b/libcxx/test/libcxx/type_traits/no_specializations.verify.cpp
index 38560161f162e..897ae89365014 100644
--- a/libcxx/test/libcxx/type_traits/no_specializations.verify.cpp
+++ b/libcxx/test/libcxx/type_traits/no_specializations.verify.cpp
@@ -186,5 +186,12 @@ struct std::enable_if<true, S>; // expected-error {{cannot be specialized}}
 #  if TEST_STD_VER >= 20
 template <>
 struct std::integral_constant<S, {}>; // expected-error {{cannot be specialized}}
+
+template <>
+struct std::common_reference<S>; // expected-error {{cannot be specialized}}
+template <>
+struct std::common_reference<S, S>; // expected-error {{cannot be specialized}}
+template <>
+struct std::common_reference<S, S, S>; // expected-error {{cannot be specialized}}
 #  endif
 #endif
diff --git a/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.readable/indirectly_readable.compile.pass.cpp b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.readable/indirectly_readable.compile.pass.cpp
index 5fc6ffd37caf1..086e79b799c36 100644
--- a/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.readable/indirectly_readable.compile.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.readable/indirectly_readable.compile.pass.cpp
@@ -78,16 +78,10 @@ struct missing_iter_value_t {
 };
 static_assert(!check_indirectly_readable<missing_iter_value_t>());
 
-struct unrelated_lvalue_ref_and_rvalue_ref {};
-
-struct iter_ref1 {};
-template <>
-struct std::common_reference<iter_ref1&, iter_ref1&&> {};
-
-template <>
-struct std::common_reference<iter_ref1&&, iter_ref1&> {};
-
-static_assert(!std::common_reference_with<iter_ref1&, iter_ref1&&>);
+struct iter_ref1 {
+  iter_ref1(const iter_ref1&) = delete;
+  iter_ref1(iter_ref1&&)      = delete;
+};
 
 struct bad_iter_reference_t {
   using value_type = int;
@@ -128,24 +122,9 @@ struct different_reference_types_with_common_reference {
 static_assert(check_indirectly_readable<different_reference_types_with_common_reference>());
 
 struct iter_ref4 {
-  operator iter_rvalue_ref() const;
+  operator iter_rvalue_ref();
 };
 
-template <template <class> class XQual, template <class> class YQual>
-struct std::basic_common_reference<iter_ref4, iter_rvalue_ref, XQual, YQual> {
-  using type = iter_rvalue_ref;
-};
-template <template <class> class XQual, template <class> class YQual>
-struct std::basic_common_reference<iter_rvalue_ref, iter_ref4, XQual, YQual> {
-  using type = iter_rvalue_ref;
-};
-
-// FIXME: This is UB according to [meta.rqmts], and there is no exception for common_reference.
-template <>
-struct std::common_reference<iter_ref4 const&, iter_rvalue_ref&&> {};
-template <>
-struct std::common_reference<iter_rvalue_ref&&, iter_ref4 const&> {};
-
 static_assert(std::common_reference_with<iter_ref4&&, iter_rvalue_ref&&>);
 static_assert(!std::common_reference_with<iter_ref4 const&, iter_rvalue_ref&&>);
 

From 3157c36d1c5554308211a81fad05e1401d050ba9 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Thu, 29 May 2025 09:54:38 +0800
Subject: [PATCH 2/4] Restore formatting

---
 libcxx/include/__type_traits/common_reference.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/include/__type_traits/common_reference.h b/libcxx/include/__type_traits/common_reference.h
index a2e360405ecf6..7df55f7fecfd5 100644
--- a/libcxx/include/__type_traits/common_reference.h
+++ b/libcxx/include/__type_traits/common_reference.h
@@ -188,7 +188,7 @@ _LIBCPP_DIAGNOSTIC_POP
 
 // bullet 5 - Otherwise, there shall be no member `type`.
 template <class...>
-struct _LIBCPP_NO_SPECIALIZATIONS common_reference{};
+struct _LIBCPP_NO_SPECIALIZATIONS common_reference {};
 
 #endif // _LIBCPP_STD_VER >= 20
 

From b2f241d45b0abffd425990214f2db142bf3cc361 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Wed, 4 Jun 2025 09:28:22 +0800
Subject: [PATCH 3/4] Add release note

---
 libcxx/docs/ReleaseNotes/21.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index d0383a705190d..64b5cc1e86710 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/libcxx/docs/ReleaseNotes/21.rst
@@ -97,6 +97,9 @@ Potentially breaking changes
 - The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
   output when printing pointers.
 
+- User-defined specializations of ``std::common_reference`` are disallowed with the ``[[clang::no_specializations]]``
+  attribute. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
+
 Announcements About Future Releases
 -----------------------------------
 

From 349d1411ba0e0f1a46c896c4084e96f73564960e Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Wed, 4 Jun 2025 16:09:04 +0800
Subject: [PATCH 4/4] Adopt @philnik777's suggestion for release note

Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>
---
 libcxx/docs/ReleaseNotes/21.rst | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index 64b5cc1e86710..6cbc0baf29487 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/libcxx/docs/ReleaseNotes/21.rst
@@ -97,8 +97,7 @@ Potentially breaking changes
 - The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
   output when printing pointers.
 
-- User-defined specializations of ``std::common_reference`` are disallowed with the ``[[clang::no_specializations]]``
-  attribute. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
+- User-defined specializations of ``std::common_reference`` are diagnosed now. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
 
 Announcements About Future Releases
 -----------------------------------