[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.

Author: frederick-vs-ja

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
 

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

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&&>);