enumConstraint.swift

protocol Actionable {
    func action()
}

enum Planets: Actionable {
    case Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
}

protocol PlanetsKind {}
extension Planets: PlanetsKind {}
extension Actionable where Self: PlanetsKind {
    func action() {
        print ("Planets!")
    }
}

let m = Planets.Mercury
m.action()

This example is contrived. It makes much more sense to just implement Actionable at the point it is declared.