Reading ebuild.5 under the section REQUIRED_USE, there could be a possible error in the expression syntax. It seems a little strange there is two lines with duplicate behaviour but differing expression, I'm not sure of the intended functionality so please see the attached patch. Reproducible: Always
Created attachment 297701 [details, diff] portage-ebuild.5-REQUIRED_USE-expression.patch
Thanks, this is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=45ad6be3512e98791c3c0d28c6ec55c6298cfb9d
This is fixed in 2.1.10.45 and 2.2.0_alpha85.