[PATCH] Define std::as_const

[Jonathan Wakely]

Another C++17 feature.

        * 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.

Tested powerpc64-linux, committed to trunk.


commit c6d91a7d6d5ec64f8c4e84cd79aadde72f01a4f4
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Aug 3 18:43:20 2016 +0100

    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.

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 106ba4d..0c03644 100644
--- a/libstdc++-v3/include/std/utility
+++ b/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
diff --git a/libstdc++-v3/testsuite/20_util/as_const/1.cc 
b/libstdc++-v3/testsuite/20_util/as_const/1.cc
new file mode 100644
index 0000000..2f257b4
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/as_const/1.cc
@@ -0,0 +1,30 @@
+// 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&> );
diff --git a/libstdc++-v3/testsuite/20_util/as_const/rvalue_neg.cc 
b/libstdc++-v3/testsuite/20_util/as_const/rvalue_neg.cc
new file mode 100644
index 0000000..3fe9e18
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/as_const/rvalue_neg.cc
@@ -0,0 +1,28 @@
+// 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" }
+}