2.99.2 (unstable):

* slots and signal: These should now be declared with the
  sigc::slot<R(Args...)>
  and
  sigc::signal<R(Args...)>
  syntax, like std::function<>.
  Version 2.9/2.10 of libsigc++-2.0 supports both the old and new syntax.
  (Murray Cumming)
* sigc::men_fun(): Take the instance by reference only.