https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67282 --- Comment #3 from Arseny Solokha <asolokha at gmx dot com> --- Are gcc 5 and 6 in your setup linked against different versions of ISL? In my case, it was 0.15 for all installed gcc versions back in December and 0.16, for all of them as well, as for now.