Comment by Abel Sanchez (JIRA)

Here I attach a "working" code I copied from Playground.

I'm not seeing this in Xcode 9 beta 4. What version are you testing with?

Comment by Abel Sanchez (JIRA)

XCode Version 8.3.3 (8E3004b)
More details if 'value' is struct o enum (As its in sample code 'MyEnum') its lose protocol witness information but if is class its cast properly.

Ah, that sounds like SR-3871. But the difference between NSObject and a Swift class seems surprising to me.

Aha, right, the NSObject version is going through Objective-C dispatch, and the Swift-only version is not. And I didn't see it because in Swift 4 you have to explicitly write @objc if you want an extension method on an Objective-C class to be exposed to Objective-C. Okay, this is SR-3871 again.

Comment by Abel Sanchez (JIRA)

Thanks @belkadan It looks like SR-3871.

Here's a trimmed-down reproducer:

import Foundation

protocol MyProtocol { }
enum MyEnum: Int, MyProtocol {
    case b
}

// ObjC method can't cast the value out of the Any?
extension NSObject {
    @objc func entry(value: Any?) -> Bool {
        value is MyProtocol
    }
}

// Native class for comparison; this works correctly.
class SomeClass {
    func entry(value: Any?) -> Bool {
        value is MyProtocol
    }
}

let v = MyEnum.b
print(NSObject().entry(value: v)) // Prints "false", should be "true"
print(SomeClass().entry(value: v)) // Prints "true", should be "true"