Incompatibility with PHP 7.1

I've just spend the day trying to understand why Thematic return me following error :
Error thrown

Call to undefined function childtheme_override_init_navmenu()

I didn't get it first because I had the same theme on another website that was working great.

After a while, reverting from PHP 7.1 to 7.0 did the trick.

Still don't know why but kind of bored now and the solution found fits my needs so I won't further investigate.

Just to let you know if you're stuck too with this error.

Cheers.