Goofing off

generic_base_class.swift

/// A protocol with an associated type requirement
protocol SomeProtocol {
    associatedtype SomePAT
    func someFunction(param: SomePAT)
}

/// A generic class which satisfies the associated type with its generic parameter
class Base<T>: SomeProtocol {
    func someFunction(param: T) {
        print("Executed Base")
    }
}

/// A dummy type so we have a concrete type to play with
struct Thing {
    let property: Any
}

/// When extending the generic, if we provide a concrete type parameter for the parent
/// then the child does not also have to be generic.
class Child: Base<Thing> {
    override func someFunction(param: Thing) {
        super.someFunction(param)
        print("Executed Child")
    }
}

/// A function that uses out PAT as a generic contraint;
/// We can pass an instance of Child to it (though we can't use it for much!)
func example<T: SomeProtocol, U where U == T.SomePAT>(param: T, _ other: U) {
    param.someFunction(other)
}

example(Child(), Thing(property: 2))