4.5 Optional Dependencies
An optional dependency is never included in a plan automatically.
peipkg does not carry optional dependencies into the resolver at all: they are not part of the candidate model, so there is no path by which one could be installed without being asked for. An operator who wants one names it as a goal.
A package whose optional dependencies are absent functions correctly with reduced capability. A package that does not function without an "optional" dependency has mis-categorised it — that is a required dependency wearing the wrong label.
Optional dependencies still participate in claims. A claim path declared on an optional-dependency entry is an optional claim consumer (§9.6): the dependency need not be satisfied for the declaring package to install, and the path is materialised only if and when some eligible provider holds the role.