Swift – The use of Swift 3 @objc inference in Swift 4 mode is deprecated

You need to declare the type of the protocol as AnyObject.

protocol ProtocolNameDelegate: AnyObject {
    // Protocol stuff goes here
}

class SomeClass {
    weak var delegate: ProtocolNameDelegate?
}

Using AnyObject you say that only classes can conform to this protocol, whereas structs or enums can't.

The fix-it is correct – there's nothing about the selector you can change in order to make the method it refers to exposed to Objective-C.

The whole reason for this warning in the first place is the result of SE-0160. Prior to Swift 4, internal or higher Objective-C compatible members of NSObject inheriting classes were inferred to be @objc and therefore exposed to Objective-C, therefore allowing them to be called using selectors (as the Obj-C runtime is required in order to lookup the method implementation for a given selector).

However in Swift 4, this is no longer the case. Only very specific declarations are now inferred to be @objc, for example, overrides of @objc methods, implementations of @objc protocol requirements and declarations with attributes that imply @objc, such as @IBOutlet.

The motivation behind this, as detailed in the above linked proposal, is firstly to prevent method overloads in NSObject inheriting classes from colliding with each other due to having identical selectors. Secondly, it helps reduce the binary size by not having to generate thunks for members that don't need to be exposed to Obj-C, and thirdly improves the speed of dynamic linking.

If you want to expose a member to Obj-C, you need to mark it as @objc, for example:

class ViewController: UIViewController {

    @IBOutlet weak var button: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
        button.addTarget(self, action: #selector(foo), for: .touchUpInside)
    }

    @objc func foo() {
       // ... 
    }
}

(the migrator should do this automatically for you with selectors when running with the "minimise inference" option selected)

To expose a group of members to Obj-C, you can use an @objc extension:

@objc extension ViewController {

    // both exposed to Obj-C
    func foo() {}
    func bar() {}
}

This will expose all the members defined in it to Obj-C, and give an error on any members that cannot be exposed to Obj-C (unless explicitly marked as @nonobjc).

If you have a class where you need all Obj-C compatible members to be exposed to Obj-C, you can mark the class as @objcMembers:

@objcMembers
class ViewController: UIViewController {
   // ...
}

Now, all members that can be inferred to be @objc will be. However, I wouldn't advise doing this unless you really need all members exposed to Obj-C, given the above mentioned downsides of having members unnecessarily exposed.