REQUIRED_USE allows specifying that one USE flag depends on another; however, that only allows hard dependencies. I'd like a way to have one USE flag enable or disable another by default, while still allowing an explicitly specified value to override that default. For instance (strawman syntax proposal): IUSE="flag1 +flag2" DEFAULT_USE="flag1? ( -flag2 )" By default, this package would build with -flag1 +flag2. If the user explicitly enables flag1, but does not specify a value for flag2, the package would build with +flag1 -flag2. However, the user can still explicitly enable both flag1 and flag2 if they want, or for that matter disable both. This would come in handy for some ebuilds in Chrome OS. In particular, some global USE flags are used for specific target environments (such as embedded, or target graphics stacks), and those global USE flags should affect the defaults for certain package-specific USE flags, without preventing the user from overriding those defaults.
