where Self: UIViewcontroller -> Compiler thinks I am dealing with a non-AnyObject instance

Question

This is my code

public protocol MyProtocol where Self: UIViewController {
    var money: Int { get set }
}

public extension MyProtocol {
    func giveMoney() { // <-- ERROR: Left side of mutating operator isn't mutable: 'self' is immutable
        money += 1
    }
}

This error shouldn't be thrown right? Every conforming instance of this protocol is a UIViewController, therefore a subclass of AnyObject. The compiler validates this, because when I add : AnyObject to my protocol, it compiles. BUT: Now I see an ugly error:

Redundant constraint 'Self' : 'AnyObject'

This is the compiling code (so with the compiler warning):

public protocol MyProtocol: AnyObject where Self: UIViewController {
    var money: Int { get set }
}

public extension MyProtocol {
    func giveMoney() {
        money += 1
    }
}

Is this some bug? I am using Xcode 10 and Swift 4.2.

Answer 1

The problem is that this syntax is not supported:

public protocol MyProtocol where Self: UIViewController {

It compiles, sort of, but that fact is a bug. The correct syntax is to attach the where condition to the extension:

public protocol MyProtocol  {
    var money: Int { get set }
}
public extension MyProtocol where Self: UIViewController {
    func giveMoney() { 
        money += 1
    }
}