To make it support sys-foo/foo[useflags] as pkgcore do see
Now you get some thing like this
laptop1 ~ # emerge -pv udev[test]
These are the packages that would be merged, in order:
Calculating dependencies ... done!
emerge: there are no ebuilds built with USE flags to satisfy "udev[test]".
!!! One of the following packages is required to complete your request:
- sys-fs/udev-164-r1 (Change USE: +test)
I think we can consider this as a subset of bug 258371, since if we add an option to automatically satisfy USE dependencies, it should apply to atoms given on the command line.
This already works with emerge --autounmask, which is enabled by default since portage-2.1.10. However, it requires that you run the command a second time after making the required config changes (for which you can use --autounmask-write to have the config changes written automatically).