It would be very nice to be able to specify that some USE flags are only meaningful in the presence of another USE flag. For example dev-vcs/git has IUSE=perforce which requires Python. Python support is not needed otherwise. By default some PYTHON_SINGLE_TARGET is enabled, and when the default target changes (e.g. in the profile) emerge --newuse will trigger a rebuild of dev-vcs/git even if USE=-perforce and thus Python isn't used. Just as a simple strawman, I'd propose some new syntax like IUSE="... perforce? ( python_single_target_python3_{7..9} ) ..." And emerge --verbose output would perhaps indicate that the flags are dependent: [ebuild R ] dev-vcs/git-2.26.3::gentoo USE="blksha1 curl gnome-keyring gpg iconv nls pcre pcre-jit perl subversion threads webdav -cgi -cvs -doc -emacs -highlight -mediawiki -mediawiki-experimental -perforce (-ppcsha1) -test -tk -xinetd" perforce? ( PYTHON_SINGLE_TARGET="-python3_7 -python3_8" ) Probably a better way to display that. Suggestions welcome.
A REQUIRED_USE extension might fit better, since that already specifies relationships between flags.
