How to require that a protocol can only be adopted by a specific class

protocol AddsMoreCommands: class {
    // Code
}

extension AddsMoreCommands where Self: UIViewController {
    // Code
}

Because of an issue in the previous answer I ended up with this declaration:

protocol AddsMoreCommands where Self : UIViewController { 
    // protocol stuff here  
}

no warnings in Xcode 9.1

This can also be achieved without an extension:

protocol AddsMoreCommands: UIViewController {
    // code
}

Which is exactly the same as:

protocol AddsMoreCommands where Self: UIViewController {
   // code
}

I usually use the first option, IIRC I read a while ago on the Swift docs that that is the recomended way when the constraint is Self, if is other constraint such as associate types is when where can be used.

Notice that since UIViewController is a class too, this protocol can be implemented for weak properties such as delegates.

EDITED 2021/01/05: Previous posted solution had a warning, I have removed it and use this one which does not produce any warning.