Gentoo's Bugzilla –
Attachment 418844 Details for
Bug 567686backtracking aborted after 1 tries, but succeeds after package.masking: (sys-libs/ncurses-5.9-r5:0/5::gentoo, ebuild scheduled for merge) does not satify all version requirements. (sys-libs/ncurses-6.0-r1:0/6::gentoo, installed) has pending USE changes.