Why does this init gets dispatched to an internal extension init?

I'm having a hard time understanding this intermodule communication. My app consumes 3 modules.

We will call them Module A, Module B and Module C.

Both Module B and Module C consume Module A.

The dependency looks something like this:

Let's say I have this BaseViewController declared in Module A.

open class BaseViewController: UIViewController { }

In module B, I have the following declaration:

extension BaseViewController {
    internal convenience init() {
        self.init(nibName: String(describing: type(of: self)), bundle: .moduleA)
    }
}

Now from my knowledge, this declaration should only be visible at a module level, not outside the module.

In module C, I have a BaseViewController subclass which uses programmatic layout.

class MySuperViewController: BaseViewController { }

In the same module C, I have a coordinator which initialises this view controller like this:

let mySuperVC = MySuperViewController()

I call this coordinator method that instantiates this mySuperVC from my iOS app where I import all A, B and C modules.

I was expecting that MySuperViewController() to call the UIViewController's initialiser, since the convenience init of the BaseViewController is declared inside Module B and it is internal, however, the initialiser that is being called is the internal one from Module B, thus making my app crash since it's not finding the nib.

I tried to write an identical convenience init inside Module C, but it calls the same internal init from Module B. Why is this happening?

From Extensions:

>Extensions can add new functionality to a type, but they can’t override existing functionality.

Your extension is invalid and it is undefined what method will be called. init() is an existing method on UIViewController, and so you cannot override it in an extension.

As a practical matter of why this happens, BaseViewController is an Objective-C type. When the module is loaded, this init method is attached to it (there isn't one currently; there's just one on its superclass). When a later caller sends the selector "init" to the BaseViewController instance, it will find this method while walking up the hierarchy. At runtime, the system does not know where the method came from. It just gets attached when the module is loaded. This is non-deterministic, which is why overriding methods in extensions is undefined behavior.