:= inside || () is forbidden and broken. Please fix those deps. Common options: 1. introduce virtuals -- if packages have sane ABI compat, 2. introduce USE flags, 3. (discouraged) drop :=.
commit 4061d02628704ead1625fd9034ddbaafbf5a547b Author: Andrew Savchenko <bircoph@gentoo.org> Date: Sun Jun 19 11:52:38 2016 +0300 sci-physics/root: fix bug 586326 While bug itself is of questionable validity (see parent bug 586238), affected dependencies have neither subslots nor multiple slots, thus := can be safely dropped.