Non-'@objc' method does not satisfy optional requirement of '@objc' protocol

While I think I can answer your question, it's not an answer you will like.

TL;DR: @objc functions may not currently be in protocol extensions. You could create a base class instead, though that's not an ideal solution.

Protocol Extensions and Objective-C

First, this question/answer (Can Swift Method Defined on Extensions on Protocols Accessed in Objective-c) seems to suggest that because of the way protocol extensions are dispatched under the hood, methods declared in protocol extensions are not visible to the objc_msgSend() function, and therefore are not visible to Objective-C code. Since the method you are trying to define in your extension needs to be visible to Objective-C (so UIKit can use it), it yells at you for not including @objc, but once you do include it, it yells at you because @objc is not allowed in protocol extensions. This is probably because protocol extensions are not currently able to be visible to Objective-C.

We can also see that the error message once we add @objc states "@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes." This is not a class; an extension to an @objc protocol is not the same as being in the protocol definition itself (i.e. in requirements), and the word "concrete" would suggest that a protocol extension does not count as a concrete class extension.

Workaround

Unfortunately, this pretty much completely prevents you from using protocol extensions when the default implementations must be visible to Objective-C frameworks. At first, I thought perhaps @objc was not allowed in your protocol extension because the Swift Compiler could not guarantee that conforming types would be classes (even though you have specifically specified UIViewController). So I put a class requirement on P1. This did not work.

Perhaps the only workaround is to simply use a base class instead of a protocol here, but this is obviously not completely ideal because a class may only have a single base class but conform to multiple protocols.

If you choose to go this route, please take this question (Swift 3 ObjC Optional Protocol Method Not Called in Subclass) into account. It appears that another current issue in Swift 3 is that subclasses do not automatically inherit the optional protocol requirement implementations of their superclass. The answer to that questions uses a special adaption of @objc to get around it.

Reporting the Issue

I think this is being discussed already among those working on the Swift open source projects, but you could be sure they are aware by either using Apple's Bug Reporter, which would likely eventually make its way to the Swift Core Team, or Swift's bug reporter. Either of these may find your bug too broad or already known, however. The Swift team may also consider what you are looking for to be a new language feature, in which case you should first check out the mailing lists.

Update

In December 2016, this issue was reported to the Swift community. The issue is still marked as open with a medium priority, but the following comment was added:

This is intended. There is no way to add the implementation of the method to every adopter, since the extension could be added after the conformance to the protocol. I suppose we could allow it if the extension is in the same module as the protocol, though.

Since your protocol is in the same module as your extension, however, you may be able to do this in a future version of Swift.

Update 2

In February 2017, this issue was officially closed as "Won't Do" by one of the Swift Core Team members with the following message:

This is intentional: protocol extensions cannot introduce @objc entry points due to limitations of the Objective-C runtime. If you want to add @objc entry points to NSObject, extend NSObject.

Extending NSObject or even UIViewController will not accomplish exactly what you want, but it unfortunately does not look like it will become possible.

In the (very) long-term future, we may be able to eliminate reliance on @objc methods entirely, but that time will likely not come anytime soon since Cocoa frameworks are not currently written in Swift (and cannot be until it has a stable ABI).

Update 3

As of Fall 2019, this is becoming less of a problem because more and more Apple frameworks are being written in Swift. For example, if you use SwiftUI instead of UIKit, you sidestep the problem entirely because @objc would never be necessary when referring to a SwiftUI method.

Apple frameworks written in Swift include:

• SwiftUI
• RealityKit
• Combine
• CryptoKit

One would expect this pattern to continue over time now that Swift is officially ABI and module stable as of Swift 5.0 and 5.1, respectively.

I just ran into this after enabling 'module stability' (turning on 'Build libraries for distribution') in a swift framework I use.

What I had was something like this:

class AwesomeClass: LessAwesomeClass {
...
}

extension AwesomeClass: GreatDelegate {
  func niceDelegateFunc() {
  }
}

The function in the extension had these errors:

• '@objc' instance method in extension of subclass of 'LessAwesomeClass' requires iOS 13.0.0

• Non-'@objc' method 'niceDelegateFunc' does not satisfy requirement of '@objc' protocol 'GreatDelegate'

Moving the functions into the class rather than in an extension resolved the issue.