somewhere->register_call<MagicCode>( std::bind( std::bind( &Y::call_it
                                                         , this
                                                         , function )
                                              , std::ref(x_) );

auto fobj = std::bind( &Y::call_it, this, function );
fobj(); // Produces int

somewhere->register_call<MagicCode>( std::bind(fobj, std::ref(x_) );
// Trying to call int with X&?