Implement P0007: Constant View: A proposal for a std::as_const helper function template.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@253274 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/utility/as_const/as_const.fail.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// template <class T> constexpr add_const<T>& as_const(T& t) noexcept;      // C++17
+// template <class T>           add_const<T>& as_const(const T&&) = delete; // C++17
+
+#include <utility>
+
+struct S {int i;};
+
+int main()
+{
+	std::as_const(S{});
+}