LSPDemo.swift

class Father {
	public func doSomething() {
		print("Did the father thing")
	}
}

class Child: Father {
	@available (*, unavailable)
	override func doSomething() {
		print("did the child thing")
	}
	
	fileprivate func internal_doSomething() {
		super.doSomething()
	}
}
class Unrelated {
	func operateOnAThing(_ someThing:Father) {
		someThing.doSomething()
	}
}

let u=Unrelated()
let f = Father()
let c = Child()
u.operateOnAThing(f)
u.operateOnAThing(c)