When there are unsatisfied conditional USE dependencies, emerge will report an unsatisfied dependency and suggest a USE change as follows
emerge: there are no ebuilds built with USE flags to satisfy "dev-python/mock[python_targets_python2_7(-)?,python_targets_python3_3(-)?,python_targets_python3_4(-)?,python_targets_jython2_7(-)
!!! One of the following packages is required to complete your request:
- dev-python/foo::gentoo (Change USE: -python_targets_jython2_7)
It would be nicer if emerge converted the "Change USE" suggestion into an autounmask change which can be automatically written to package.use when --autounmask-write is enabled.
There's a patch in the following branch:
I've posted it for review here:
This is in the master branch:
Released in 2.2.26.