Define std::as_const

	* include/std/utility (as_const): Define.
	* testsuite/20_util/as_const/1.cc: New test.
	* testsuite/20_util/as_const/rvalue_neg.cc: New test.

From-SVN: r239090

libstdc++-v3/ChangeLog

 2016-08-03  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/utility (as_const): Define.
+	* testsuite/20_util/as_const/1.cc: New test.
+	* testsuite/20_util/as_const/rvalue_neg.cc: New test.
+
 	* include/bits/shared_ptr.h (owner_less): Add default template
 	argument.
 	* include/bits/shared_ptr_base.h (_Sp_owner_less<void, void>): Define

libstdc++-v3/include/std/utility

@@ -356,6 +356,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template <size_t _Idx>
     in_place_tag in_place(__in_place_index<_Idx>*) {terminate();}
 
+#define  __cpp_lib_as_const 201510
+  template<typename _Tp>
+    constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
+
+  template<typename _Tp>
+    void as_const(const _Tp&&) = delete;
+
 #endif
 
 _GLIBCXX_END_NAMESPACE_VERSION

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

+// 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.
+
+// 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++17" }
+// { dg-do compile }
+
+#include <utility>
+
+#if __cpp_lib_as_const != 201510
+# error "__cpp_lib_as_const != 201510"
+#endif
+
+int i;
+constexpr auto& ci = std::as_const(i);
+static_assert( &i == &ci );
+static_assert( std::is_same_v<decltype(std::as_const(i)), const int&> );

libstdc++-v3/testsuite/20_util/as_const/rvalue_neg.cc

+// 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.
+
+// 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++17" }
+// { dg-do compile }
+
+#include <utility>
+
+void test01()
+{
+  int i = 0;
+  std::as_const(std::move(i));	// { dg-error "deleted function" }
+  std::as_const(0);		// { dg-error "deleted function" }
+}