This version will fail to emerge unless you manually eselect a v.3.x python version
Please provide build.log if you want to proceed with this bug.
Don't have build.log anymore, gone after successfully emerged build fails if your e-selected python is first in list v.2.7 and builds fine if you eselect a v.3.x once built you can eselect back v.2.7
There's not one trace of python3 in my build.log, and for the record: # eselect python list Available Python interpreters, in order of preference: [1] python2.7 [2] python3.4 [3] python3.5 (fallback) You could at a minimum at least provide your list of USE flags.
This was supposedly fixed with 2.18.6-r1, please re-open if it is still a problem on your systems.