libstdc++: Add __cpp_lib_unwrap_ref feature test macro

We already defined the traits in <type_traits> as now required by LWG
3348, but the macro was missing. This adds it.

	* include/std/type_traits (__cpp_lib_unwrap_ref): Define (LWG 3348).
	* include/std/version (__cpp_lib_unwrap_ref): Likewise.
	* testsuite/20_util/unwrap_reference/1.cc: Check macro.
	* testsuite/20_util/unwrap_reference/3.cc: New test.

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/type_traits (__cpp_lib_unwrap_ref): Define (LWG 3348).
+	* include/std/version (__cpp_lib_unwrap_ref): Likewise.
+	* testsuite/20_util/unwrap_reference/1.cc: Check macro.
+	* testsuite/20_util/unwrap_reference/3.cc: New test.
+
 	* include/std/numeric (midpoint(T8, T*)): Do not check for complete
 	type during overload resolution, use static assert instead (LWG 3200).
 	* testsuite/26_numerics/midpoint/pointer.cc: Do not test with

libstdc++-v3/include/std/type_traits

@@ -3242,6 +3242,8 @@ template <typename _From, typename _To>
   template<typename _Tp>
     using type_identity_t = typename type_identity<_Tp>::type;
 
+#define __cpp_lib_unwrap_ref 201811L
+
   /// Unwrap a reference_wrapper
   template<typename _Tp>
     struct unwrap_reference { using type = _Tp; };

libstdc++-v3/include/std/version

@@ -178,6 +178,7 @@
 #ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
 # define __cpp_lib_is_constant_evaluated 201811L
 #endif
+#define __cpp_lib_unwrap_ref 201811L
 
 #if _GLIBCXX_HOSTED
 #define __cpp_lib_bind_front 201907L

libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc

@@ -20,6 +20,12 @@
 
 #include <type_traits>
 
+#ifndef __cpp_lib_unwrap_ref
+# error "Feature-test macro for unwrap_reference missing in <type_traits>"
+#elif __cpp_lib_unwrap_ref != 201811L
+# error "Feature-test macro for unwrap_reference has wrong value in <type_traits>"
+#endif
+
 template<typename T, typename U> struct expect_same;
 template<typename T> struct expect_same<T, T> : std::true_type { };
 

libstdc++-v3/testsuite/20_util/unwrap_reference/3.cc

+// Copyright (C) 2020 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++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <version>
+
+#ifndef __cpp_lib_unwrap_ref
+# error "Feature-test macro for unwrap_reference missing in <version>"
+#elif __cpp_lib_unwrap_ref != 201811L
+# error "Feature-test macro for unwrap_reference has wrong value in <version>"
+#endif