Policy: USE

Concern: When do USE conditionals fire, and how do bracketed USE deps propagate?

PMS / Portage: flag? ( deps ) / !flag? ( deps ) gate dependency lists. Bracketed deps (pkg[flag], [flag=], [flag?], …) constrain the child’s effective USE. Multiple parents may require different flags on a shared child; the effective set is a meet / merge.

Literals:

• use_conditional_group(positive|negative, Use, R://E, Deps):Action?{Ctx}
• Proof-term context: build_with_use/1, required_use/1, suggestion(use_change, …)

Owns: Rules/Resolving/use.pl, Rules/Resolving/featureterm.pl, resolving.pl USE section, feature unification for context meet.

Invariants:

• Positive conditional includes deps only when Use is enabled in the effective set for that ebuild/path; negative is the dual.
• Bracketed USE on a child becomes build_with_use in the child’s proof-term context — not a silent global PROFILE change.
• Shared CN: USE requirements from distinct parents must unify; irreconcilable meets become domain assumptions / conflicts.
• Soft defaults (flag(+)) lose to hard REQUIRED_USE / explicit forbid.

Examples: test14, test33, test35, test49, test52.
See also: REQUIRED_USE, Install.