Method or_else

dlib/optional.h:784–787

          std::enable_if_t<std::is_same<U, dlib::optional<T>>::value, bool> = true
        >
        constexpr optional or_else( F&& f ) const&
        {
            return *this ? *this : std::forward<F>(f)();
        }

        template < 
          class F,