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.nullopt/nullopt_t.pass.cpp

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// struct nullopt_t{see below};
+// constexpr nullopt_t nullopt(unspecified);
+
+#include <optional>
+#include <type_traits>
+
+#if _LIBCPP_STD_VER > 11
+
+constexpr
+int
+test(const std::nullopt_t&)
+{
+    return 3;
+}
+
+#endif
+
+int main()
+{
+#if _LIBCPP_STD_VER > 11
+	static_assert((std::is_class<std::nullopt_t>::value), "");
+	static_assert((std::is_empty<std::nullopt_t>::value), "");
+	static_assert((std::is_literal_type<std::nullopt_t>::value), "");
+	static_assert((!std::is_default_constructible<std::nullopt_t>::value), "");
+    
+    static_assert(test(std::nullopt) == 3, "");
+#endif
+}