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.pass.cpp

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+#include <cassert>
+
+struct S {int i;};
+bool operator==(const S& x, const S& y) { return x.i == y.i; }
+bool operator==(const volatile S& x, const volatile S& y) { return x.i == y.i; }
+
+template<typename T>
+void test(T& t)
+{
+    static_assert(std::is_const<typename std::remove_reference<decltype(std::as_const                  (t))>::type>::value, "");
+    static_assert(std::is_const<typename std::remove_reference<decltype(std::as_const<               T>(t))>::type>::value, "");
+    static_assert(std::is_const<typename std::remove_reference<decltype(std::as_const<const          T>(t))>::type>::value, "");
+    static_assert(std::is_const<typename std::remove_reference<decltype(std::as_const<volatile       T>(t))>::type>::value, "");
+    static_assert(std::is_const<typename std::remove_reference<decltype(std::as_const<const volatile T>(t))>::type>::value, "");
+    
+    assert(std::as_const(t) == t);
+    assert(std::as_const<               T>(t) == t);
+    assert(std::as_const<const          T>(t) == t);
+    assert(std::as_const<volatile       T>(t) == t);
+    assert(std::as_const<const volatile T>(t) == t);
+}
+
+int main()
+{
+	int i = 3;
+	double d = 4.0;
+	S s{2};
+	test(i);
+	test(d);
+	test(s);
+}