The existing default emerge --backtrack=10 setting can lead to lots of wasted cpu time in cases where it will ultimately fail to find a valid solution anyway. So, I recommend that we reduce the default to --backtrack=3 or so.
I have a patch in the following branch: https://github.com/zmedico/portage/tree/bug_536926 I've posted it for review here: http://thread.gmane.org/gmane.linux.gentoo.portage.devel/5098
This is in the master branch now: https://github.com/gentoo/portage/commit/1891388ea0ae0dd58903a71a3adc779731523601
Released in portage-2.2.16