gistfile1.swift

// original class

class A {
    var x: String
    init(x: String) {
        self.x = x
    }
}

// new class

class A {
    var x: String
    var y: String
    init(x: String, y: String = "bar") {
        self.x = x
        self.y = y
    }
}

// that way, this still works:

let a = A(x: "foo")