What if back pay isn't guaranteed this time?

Economists traditionally do not worry too much about the growth impact of US government shutdowns, as the short-term loss of economic output tends to be paid back with an economic bounce when government reopens. It is still a net negative—contractors are not compensated for lost activity in a shutdown—but government workers get their back pay paid back. US President Trump has suggested that may not happen this time. If that were legally enforced, it would reduce the bounce of the bounce back.