pkg_pretend USE validation and VALID_USE alternative
>> | Occasionally, ebuilds will have conflicting USE flags for
>> | functionality. Checking for them and returning an error is not a
>> | viable solution. Instead, you must pick one of the USE flags in
>> | conflict to favour.
>>  <http://devmanual.gentoo.org/general-concepts/use-flags/>
> I honestly consider the ebuild silently making decisions on the user's
> behalf *worse*. Â*Consider if openoffice silently made decisions like
> that- 4 hours later it'll wind up choosing the option you didn't
> really want and you'll be in a foul mood.
I'm pretty sure it says that because there was no way to fail early
before. And failing in the middle of 300 packages upgrade because some
useflags are in conflict wasn't reasonable.