The Optional implementation doesn't handle r-value references and hence can't be used with move-only types, like std::unique_ptr.

Adding a move constructor solves this problem, http://ideone.com/PpByK5

sidenote: the rule of 5 might be applicable http://en.cppreference.com/w/cpp/language/rule_of_three