[libc++][type_traits] reference_{constructs|converts}_from_temporary -Winvalid-specialization tests

Addresses comment:
https://github.com/llvm/llvm-project/pull/128649/files#r2022341035

Drive-by fix: libcxx/test/std/language.support/support.initlist/support.initlist.syn/specialization.verify.cpp

https://clang.llvm.org/docs/DiagnosticsReference.html#winvalid-specialization

Author: H-G-Hristov

libcxx/test/std/language.support/support.initlist/support.initlist.syn/specialization.verify.cpp

@@ -15,7 +15,7 @@
 
 #include <initializer_list>
 
-#if !__has_warning("-Winvalid-specializations")
+#if !__has_warning("-Winvalid-specialization")
 // expected-no-diagnostics
 #else
 

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.verify.cpp

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++23
+
+// These compilers don't support std::reference_converts_from_temporary yet.
+// UNSUPPORTED: android, apple-clang-15, apple-clang-16, clang-19.1
+
+// <type_traits>
+
+// template<class T, class U> struct reference_constructs_from_temporary;
+
+// template<class T, class U>
+// constexpr bool reference_constructs_from_temporary_v
+//   = reference_constructs_from_temporary<T, U>::value;
+
+#include <type_traits>
+
+#if !__has_warning("-Winvalid-specialization")
+// expected-no-diagnostics
+#else
+
+// expected-error@+2 {{'reference_constructs_from_temporary' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <>
+struct std::reference_constructs_from_temporary<int, int> {
+}; //expected-error 0-1 {{explicit specialization of 'std::reference_constructs_from_temporary<int, int>' after instantiation}}
+
+// expected-error@+2 {{'reference_constructs_from_temporary' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <typename T, typename U>
+struct std::reference_constructs_from_temporary<T&, U&> {};
+
+// expected-error@+2 {{'reference_constructs_from_temporary_v' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <>
+inline constexpr bool std::reference_constructs_from_temporary_v<int, int> =
+    false; // expected-error 0-1 {{explicit specialization of 'std::reference_constructs_from_temporary_v<int, int>' after instantiation}}
+
+// expected-error@+2 {{'reference_constructs_from_temporary_v' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <class T, class U>
+inline constexpr bool std::reference_constructs_from_temporary_v<T&, U&> = false;
+
+#endif

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.verify.cpp

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++23
+
+// These compilers don't support std::reference_converts_from_temporary yet.
+// UNSUPPORTED: android, apple-clang-15, apple-clang-16, clang-18, clang-19.1
+
+// <type_traits>
+
+// template<class T, class U> struct reference_converts_from_temporary;
+
+// template<class T, class U>
+// constexpr bool reference_converts_from_temporary_v
+//   = reference_converts_from_temporary<T, U>::value;
+
+#include <type_traits>
+
+#if !__has_warning("-Winvalid-specialization")
+// expected-no-diagnostics
+#else
+
+// expected-error@+2 {{'reference_converts_from_temporary' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <>
+struct std::reference_converts_from_temporary<int, int> {
+}; //expected-error 0-1 {{explicit specialization of 'std::reference_converts_from_temporary<int, int>' after instantiation}}
+
+// expected-error@+2 {{'reference_converts_from_temporary' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <typename T, typename U>
+struct std::reference_converts_from_temporary<T&, U&> {};
+
+// expected-error@+2 {{'reference_converts_from_temporary_v' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <>
+inline constexpr bool std::reference_converts_from_temporary_v<int, int> =
+    false; // expected-error 0-1 {{explicit specialization of 'std::reference_converts_from_temporary_v<int, int>' after instantiation}}
+
+// expected-error@+2 {{'reference_converts_from_temporary_v' cannot be specialized: Users are not allowed to specialize this standard library entity}}
+template <class T, class U>
+inline constexpr bool std::reference_converts_from_temporary_v<T&, U&> = false;
+
+#endif