cc @swiftix

I discussed it with @jckarter and he explained that the optimized version is correct and the non-optimized version is wrong. This is because "init" gets a C value boxed into an existential of type P. Therefore, the type of this boxed value is P.Protocol and not C. The runtime produces a wrong result in the non-optimized case.

I'll create a radar for it.

That would be P.Type, not P.Protocol. But type(of:) doesn't give you a static type; it gives you a dynamic type, always. That's never a protocol type.

Yes, what Jordan wrote was also my understanding:

Comment by 100mango (JIRA)

cc@swiftix
I came to the same situation。But it's not in the init. In my project (https://github.com/100mango/SwiftNotificationCenter)

protocol:

protocol UpdateTitle: class {
    
    func updateWithNewTitle(title: String)
    
}

in view controller

class FirstViewController: UIViewController, UpdateTitle {
    
    override func viewDidLoad() {
        super.viewDidLoad()
         // Crash here
        Broadcaster.register(UpdateTitle.self, observer: self)
    }
}

and with optimization on:

public class Broadcaster {
   
    public static func register<T>(_ protocolType: T.Type, observer: T) {
        let key = "\(protocolType)"
        guard type(of: observer) is AnyClass else {
            //with optimization on.  it will always goes here
            fatalError("expecting reference type but found value type: \(observer)")
            return
        }
        //..........
    }
}

The inconsistency was addressed in SR-3304.

I don't think this should be closed…? As of Xcode 9 beta 5, it now always gives “P / false”.

That is the wrong answer, per Jordan above:

But type(of🙂 doesn't give you a static type; it gives you a dynamic type, always. That's never a protocol type.

It's correct as the language is defined today, as discussed in detail in SR-3304.