Method operator()

test/visit.cpp:278–278

        {
            kind k;
            bool operator()(std::nullptr_t) && { return k == kind::null; }
            bool operator()(bool) && { return k == kind::bool_; }
            bool operator()(std::int64_t) && { return k == kind::int64; }
            bool operator()(std::uint64_t) && { return k == kind::uint64; }