Implement N3672, optional<T>.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189772 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/optional/optional.object/optional.object.observe/value_const.fail.cpp

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <optional>
+
+// constexpr const T& optional<T>::value() const;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#if _LIBCPP_STD_VER > 11
+
+struct X
+{
+    constexpr int test() const {return 3;}
+    int test() {return 4;}
+};
+
+#endif  // _LIBCPP_STD_VER > 11
+
+int main()
+{
+#if _LIBCPP_STD_VER > 11
+    {
+        constexpr std::optional<X> opt;
+        static_assert(opt.value().test() == 3, "");
+    }
+#else
+#error
+#endif  // _LIBCPP_STD_VER > 11
+}