Portage USE flag precedence and package.use override behavior

USE Flag Precedence

Yes, entries in /etc/portage/package.use take absolute precedence over global flags defined in /etc/portage/make.conf, regardless of the dependency chain depth. The hierarchy of resolution follows this order (from lowest to highest priority):

1. Profile Defaults: The base flags provided by the selected Gentoo profile.
2. Global Flags: Settings defined in make.conf.
3. Package-Specific Overrides: Settings defined in package.use.

Handling Shared Dependencies

Portage applies USE flags to a package as a single entity, not per-dependency path. A package is compiled once with one specific set of flags. If two parent packages depend on the same shared dependency but require different features (flags), Portage does not "merge" these requirements automatically based on the parent.

Instead, the final flag state is the union of the profile, global settings, and package.use. If the resulting configuration fails to satisfy the requirements of any parent package, the SAT solver will trigger a dependency conflict error during the emerge process.

Resolution Strategy

When a conflict occurs between parent requirements for a shared dependency, you must manually resolve the state in package.use to satisfy all parents simultaneously. For example, if Parent A requires dependency[ssl] and Parent B requires dependency[pam], you must explicitly enable both:

# /etc/portage/package.use/dependency
category/dependency ssl pam

Verification

To verify the calculated flags before committing to an installation, use the pretend flag:

emerge -pv <package_name>

For packages already installed, you can inspect the active flags using equery (from app-portage/gentoolkit):

equery uses <package_name>

Note: To provide a more specific resolution, please specify if you are using a custom profile or a specific version of the Portage SAT solver, as this can impact how conflicts are reported.