Why can't Swift initializers call convenience initializers on their superclass?

This is Rule 1 of the "Initializer Chaining" rules as specified in the Swift Programming Guide, which reads:

Rule 1: Designated initializers must call a designated initializer from their immediate superclass.

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html

Emphasis mine. Designated initializers cannot call convenience initializers.

There is a diagram that goes along with the rules to demonstrate what initializer "directions" are allowed:

Consider

class A{    var a: Int    var b: Int    init (a: Int, b: Int) {        print("Entering A.init(a,b)")        self.a = a; self.b = b    }    convenience init(a: Int) {        print("Entering A.init(a)")        self.init(a: a, b: 0)    }    convenience init() {        print("Entering A.init()")        self.init(a:0)    }}class B : A{    var c: Int    override init(a: Int, b: Int)    {        print("Entering B.init(a,b)")        self.c = 0; super.init(a: a, b: b)    }}var b = B()

Because all designated initializers of class A are overridden, class B will inherit all convenience initializers of A. So executing this will output

Entering A.init()Entering A.init(a:)Entering B.init(a:,b:)Entering A.init(a:,b:)

Now, if the designated initializer B.init(a:b:) would be allowed to call the base class convenience initializer A.init(a:), this would result in a recursive call to B.init(a:,b:).

It's because you can end up with an infinite recursion. Consider:

class SuperClass {    init() {    }    convenience init(value: Int) {        // calls init() of the current class        // so init() for SubClass if the instance        // is a SubClass        self.init()    }}class SubClass : SuperClass {    override init() {        super.init(value: 10)    }}

and look at:

let a = SubClass()

which will call SubClass.init() which will call SuperClass.init(value:) which will call SubClass.init().

The designated/convenience init rules are designed that a class initialisation will always be correct.