'Compute the actual implementation sig for the given formal argument types.'

def _resolve_impl_sig(self, typ, attr, sig_args, sig_kws):

    disp = self._get_dispatcher(self.context, typ, attr, sig_args, sig_kws)
    if (disp is None):
        return None
    disp_type = types.Dispatcher(disp)
    sig = disp_type.get_call_type(self.context, sig_args, sig_kws)
    return sig
