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_or_const.pass.cpp

@@ -0,0 +1,80 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// template <class U> constexpr T optional<T>::value_or(U&& v) const&;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#if _LIBCPP_STD_VER > 11
+
+struct Y
+{
+    int i_;
+
+    constexpr Y(int i) : i_(i) {}
+};
+
+struct X
+{
+    int i_;
+
+    constexpr X(int i) : i_(i) {}
+    constexpr X(const Y& y) : i_(y.i_) {}
+    constexpr X(Y&& y) : i_(y.i_+1) {}
+    friend constexpr bool operator==(const X& x, const X& y)
+        {return x.i_ == y.i_;}
+};
+
+#endif  // _LIBCPP_STD_VER > 11
+
+int main()
+{
+#if _LIBCPP_STD_VER > 11
+    {
+        constexpr std::optional<X> opt(2);
+        constexpr Y y(3);
+        static_assert(opt.value_or(y) == 2, "");
+    }
+    {
+        constexpr std::optional<X> opt(2);
+        static_assert(opt.value_or(Y(3)) == 2, "");
+    }
+    {
+        constexpr std::optional<X> opt;
+        constexpr Y y(3);
+        static_assert(opt.value_or(y) == 3, "");
+    }
+    {
+        constexpr std::optional<X> opt;
+        static_assert(opt.value_or(Y(3)) == 4, "");
+    }
+    {
+        const std::optional<X> opt(2);
+        const Y y(3);
+        assert(opt.value_or(y) == 2);
+    }
+    {
+        const std::optional<X> opt(2);
+        assert(opt.value_or(Y(3)) == 2);
+    }
+    {
+        const std::optional<X> opt;
+        const Y y(3);
+        assert(opt.value_or(y) == 3);
+    }
+    {
+        const std::optional<X> opt;
+        assert(opt.value_or(Y(3)) == 4);
+    }
+#endif  // _LIBCPP_STD_VER > 11
+}