[libc++] Implement LWG3990 for Clang (#128834)

This patch adds `[[_Clang::__no_specializations__]]` to `tuple`, with
warning/error suppressed for `tuple<>`.

libcxx/docs/Status/Cxx2cIssues.csv

@@ -37,7 +37,7 @@
 "`LWG3973 <https://wg21.link/LWG3973>`__","Monadic operations should be ADL-proof","2023-11 (Kona)","","",""
 "`LWG3974 <https://wg21.link/LWG3974>`__","``mdspan::operator[]`` should not copy ``OtherIndexTypes``","2023-11 (Kona)","","",""
 "`LWG3987 <https://wg21.link/LWG3987>`__","Including ``<flat_foo>`` doesn't provide ``std::begin``/``end``","2023-11 (Kona)","","",""
-"`LWG3990 <https://wg21.link/LWG3990>`__","Program-defined specializations of ``std::tuple`` and ``std::variant`` can't be properly supported","2023-11 (Kona)","","",""
+"`LWG3990 <https://wg21.link/LWG3990>`__","Program-defined specializations of ``std::tuple`` and ``std::variant`` can't be properly supported","2023-11 (Kona)","|Complete|","21",""
 "`LWG4001 <https://wg21.link/LWG4001>`__","``iota_view`` should provide ``empty``","2023-11 (Kona)","|Complete|","19",""
 "","","","","",""
 "`LWG3767 <https://wg21.link/LWG3767>`__","``codecvt<charN_t, char8_t, mbstate_t>`` incorrectly added to locale","2024-03 (Tokyo)","","",""

libcxx/include/tuple

@@ -535,7 +535,7 @@ __memberwise_forward_assign(_Dest& __dest, _Source&& __source, __tuple_types<_Up
 }
 
 template <class... _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS tuple {
   typedef __tuple_impl<typename __make_tuple_indices<sizeof...(_Tp)>::type, _Tp...> _BaseT;
 
   _BaseT __base_;
@@ -1005,6 +1005,10 @@ public:
 #    endif // _LIBCPP_STD_VER >= 23
 };
 
+_LIBCPP_DIAGNOSTIC_PUSH
+#    if __has_warning("-Winvalid-specialization")
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-specialization")
+#    endif
 template <>
 class _LIBCPP_TEMPLATE_VIS tuple<> {
 public:
@@ -1022,6 +1026,7 @@ public:
   _LIBCPP_HIDE_FROM_ABI constexpr void swap(const tuple&) const noexcept {}
 #    endif
 };
+_LIBCPP_DIAGNOSTIC_POP
 
 #    if _LIBCPP_STD_VER >= 23
 template <class... _TTypes, class... _UTypes, template <class> class _TQual, template <class> class _UQual>

libcxx/test/libcxx/utilities/tuple/no_specializations.verify.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++11
+
+// Check that user-specializations are diagnosed
+// See [tuple.tuple.general]/1
+
+#include <tuple>
+
+#if !__has_warning("-Winvalid-specialization")
+// expected-no-diagnostics
+#else
+struct S {};
+
+template <>
+class std::tuple<S>; // expected-error {{cannot be specialized}}
+#endif