While working on #9109: How to replace udev with eudev?, I added -systemd -udev USE flags in make.conf, but got:
$ emerge -uDNat @world
These are the packages that would be merged, in reverse order:
Calculating dependencies... done!
!!! The ebuild selected to satisfy ">=kde-base/kdelibs-4.14.22:4[aqua=]" has unmet requirements.