Add a new header for diagnosing the use of C++17 facilities in pre-C++17 modes.

From-SVN: r238058

libstdc++-v3/ChangeLog

@@ -1,3 +1,9 @@
+2016-07-06  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	Add a new header for diagnosing the use of C++17 facilities
+	in pre-C++17 modes.
+	* include/bits/c++17_warning.h:	New.
+
 2016-07-06  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
 	Implement LWG 2451, optional<T> should 'forward' T's

libstdc++-v3/include/bits/c++17_warning.h

@@ -0,0 +1,37 @@
+// Copyright (C) 2016 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.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/c++17_warning.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{iosfwd}
+ */
+
+#ifndef _CXX17_WARNING_H
+#define _CXX17_WARNING_H 1
+
+#if __cplusplus <= 201402L
+#error This file requires compiler and library support \
+for the ISO C++ 2017 standard. This support must be enabled \
+with the -std=c++17 or -std=gnu++17 compiler options.
+#endif
+
+#endif