libstdc++: Add std::is_scoped_enum for C++23

Implement this C++23 feature, as proposed by P1048R1.

This implementation assumes that a C++23 compiler supports concepts
already. I don't see any point in using preprocessor hacks to detect
compilers which define __cplusplus to a post-C++20 value but don't
support concepts yet.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_scoped_enum): Define.
	* include/std/version (__cpp_lib_is_scoped_enum): Define.
	* testsuite/20_util/is_scoped_enum/value.cc: New test.
	* testsuite/20_util/is_scoped_enum/version.cc: New test.

libstdc++-v3/include/std/type_traits

@@ -3279,6 +3279,23 @@ template <typename _From, typename _To>
     inline constexpr bool is_unbounded_array_v
       = is_unbounded_array<_Tp>::value;
 
+#if __cplusplus > 202002L
+#define __cpp_lib_is_scoped_enum 202011L
+
+  template<typename _Tp>
+    struct is_scoped_enum
+    : false_type
+    { };
+
+  template<typename _Tp> requires __is_enum(_Tp)
+    struct is_scoped_enum<_Tp>
+    : __not_<is_convertible<_Tp, __underlying_type(_Tp)>>::type
+    { };
+
+  template<typename _Tp>
+    inline constexpr bool is_scoped_enum_v = is_scoped_enum<_Tp>::value;
+#endif // C++23
+
 #ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
 
 #define __cpp_lib_is_constant_evaluated 201811L

libstdc++-v3/include/std/version

@@ -259,6 +259,7 @@
 
 #if __cplusplus > 202002L
 // c++2b
+#define __cpp_lib_is_scoped_enum 202011L
 #define __cpp_lib_string_contains 202011L
 #define __cpp_lib_to_underlying 202102L
 #endif // C++2b

libstdc++-v3/testsuite/20_util/is_scoped_enum/value.cc

@@ -0,0 +1,62 @@
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <type_traits>
+
+#ifndef __cpp_lib_is_scoped_enum
+# error "Feature test macro for is_scoped_enum is missing in <type_traits>"
+#elif __cpp_lib_is_scoped_enum < 202011L
+# error "Feature test macro for is_scoped_enum has wrong value in <type_traits>"
+#endif
+
+#include <testsuite_tr1.h>
+
+template<typename T>
+  concept Is_scoped_enum
+    = __gnu_test::test_category<std::is_scoped_enum, T>(true);
+
+void
+test01()
+{
+  enum class E { e1, e2 };
+  static_assert( Is_scoped_enum<E> );
+  enum class Ec : char { e1, e2 };
+  static_assert( Is_scoped_enum<Ec> );
+
+  // negative tests
+  enum U { u1, u2 };
+  static_assert( ! Is_scoped_enum<U> );
+  enum F : int { f1, f2 };
+  static_assert( ! Is_scoped_enum<F> );
+  struct S { };
+  static_assert( ! Is_scoped_enum<S> );
+
+  static_assert( ! Is_scoped_enum<int> );
+  static_assert( ! Is_scoped_enum<int[]> );
+  static_assert( ! Is_scoped_enum<int[2]> );
+  static_assert( ! Is_scoped_enum<int[][2]> );
+  static_assert( ! Is_scoped_enum<int[2][3]> );
+  static_assert( ! Is_scoped_enum<int*> );
+  static_assert( ! Is_scoped_enum<int&> );
+  static_assert( ! Is_scoped_enum<int*&> );
+  static_assert( ! Is_scoped_enum<int()> );
+  static_assert( ! Is_scoped_enum<int(*)()> );
+  static_assert( ! Is_scoped_enum<int(&)()> );
+}

libstdc++-v3/testsuite/20_util/is_scoped_enum/version.cc

@@ -0,0 +1,27 @@
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <version>
+
+#ifndef __cpp_lib_is_scoped_enum
+# error "Feature test macro for is_scoped_enum is missing in <version>"
+#elif __cpp_lib_is_scoped_enum < 202011L
+# error "Feature test macro for is_scoped_enum has wrong value in <version>"
+#endif