Function swap

dlib/test/invoke.cpp:31–35

        struct some_other_struct {int i{0};};

        void swap(some_other_struct& a, some_other_struct& b)
        {
            std::swap(a.i, b.i);
            throw std::runtime_error("toy example");
        }

        static_assert(dlib::is_swappable<some_other_struct>::value, "oops");
        static_assert(!dlib::is_nothrow_swappable<some_other_struct>::value, "oops");