class SuperClass {}

protocol Crashable where Self: SuperClass {
     func crashMe(foo: String?)
}

class MyClass: SuperClass, Crashable {
    private var someProperty: String?
    
    func crashMe(foo: String?) {
        self.someProperty = foo // EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    }
}

func triggerACrash(using crashable: Crashable) {
    crashable.crashMe(foo: "foo")
}

triggerACrash(using: MyClass())