Why can't we add designated initialisers in extensions in swift?

Question

The only difference I can see between designated and convenience initialisers is that the former necessarily calls a super class init (if available).

I don't understand then why I can't add a designated init to a class in an extension, while adding a convenience one is OK.

Why is it so bad to have an init from an extension to possibly call a super class initialiser?

Answer 1

Let's recall what a designated initializer is.

A designated initializer fully initializes all properties introduced by that class and calls an appropriate superclass initializer to continue the initialization process up the superclass chain.

^{Excerpt From: Apple Inc. “The Swift Programming Language.”}

class ClassA {
        private let propertyA: Int

        init(propertyA: Int) {
                self.propertyA = propertyA
        }
}

class ClassB: ClassA {
        private let propertyB: Int

        init(propertyA: Int, propertyB: Int) {
                self.propertyB = propertyB
                super.init(propertyA: propertyA)
        }
}

extension ClassB {
        // If this was a designated initializer, you need to initialize propertyB before calling a superclass initializer.
        // But propertyB is a private property that you can't access.
        // If you don't have the source code of ClassB, you will not even know there is a property called propertyB.
        // This is why we can't use extensions to add designated initializers.
        init(propertyC: Int) {
                ...
        }
}