Adding a setter is purely additive—like going from KeyPath to WritableKeyPath. It doesn't require the mental gymnastics that everything else that compiles in this thread does. You also can't derive "in the wrong direction".
protocol GetSet {
var property: Void { get set }
}
protocol Get: GetSet {
var property: Void { get } // Cannot override mutable property with read-only property 'property'
}
After thinking more about this, and looking into the failure of Rethrowing protocol conformances, I wouldn't know what issue to bring up—I feel pretty disheartened about the inconsistencies, and lack of ability to represent truth. But while I can't bring myself to see e.g. nonmutating being a subtype of mutating, I can at least see how it's the same as being able to shadow a member at various levels of a protocol hierarchy.
protocol GetVoid {
var property: Void { get }
}
protocol GetBool: GetVoid {
var property: Bool { get }
}
protocol GetInt: GetBool {
var property: Int { get }
}
protocol GetString: GetInt {
var property: String { get }
}
extension GetString {
var properties: (Void, Bool, Int, String) {
let selfs: (some GetVoid, some GetBool, some GetInt) = (self, self, self)
return (
selfs.0.property,
selfs.1.property,
selfs.2.property,
property
)
}
}