template <typename E, typename F> auto then(E && executor, F && f) && (1) template <typename E, typename F> auto then(E && executor, F && f) const & (2) template <typename F> auto then(F && f) && (3) template <typename F> auto then(F && f) const & (4) (1) template <typename E, typename F> auto then(E && executor, F && f) && MISSING Parameters E && executor F && f (2) template <typename E, typename F> auto then(E && executor, F && f) const & MISSING Parameters E && executor F && f (3) template <typename F> auto then(F && f) && MISSING Parameters F && f (4) template <typename F> auto then(F && f) const & MISSING Parameters F && f