The comment in the ebuild and the checks in configure.ac suggest that ares and threads cannot both be enabled, but the REQUIRED_USE additionally forces one of them to be on. If I change "^^ ( ares threads )" to "ares? ( !threads )" curl will successfully build with neither flag set. The ebuild should either allow USE="-ares -threads" or the comments should be changed to correspond with the code. Reproducible: Always
This was already changed almost 6 hours ago. Please sync your portage tree again and reopen the bug if the issue is still there.