[SR-7301] Bad fixit suggested for protocol extension implementation for inherited objc protocol

[swift-ci]

Previous ID	SR-7301
Radar	None
Original Reporter	kdawgwilk (JIRA User)
Type	Bug

Attachment: Download

Environment

Xcode 9.2

Swift 4

macOS 10.13.3

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, StarterBug
Assignee	None
Priority	Medium

md5: f483fe309b1876278257725a1dc2b1b5

relates to:

• SR-3349 @objc methods cannot be contained in protocol extensions

Issue Description:

I have the following code that has a compiler error

@objc protocol IntroAnimationController: UIViewControllerAnimatedTransitioning {
    func frame(for view: UIView, fromView: UIView) -> CGRect
    func animations(fromView: UIView, toView: UIView) -> () -> Void
}

extension IntroAnimationController {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.20
    }
}

The error says:


and includes a fixit that that turns my code into this:

extension IntroAnimationController {
    @objc(transitionDuration:) func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.20
    }
}

And then gives me these two wonderful new errors:


None of the fixits are helpful and the error message does not give me any clue why I can't do this

[swift-ci]

Comment by Kaden Wilkinson (JIRA)

Also get the same fixits using Xcode 9.3 beta 4

[belkadan]

Yep, this just isn't possible, and we shouldn't offer the fix-it. (It's not possible because Objective-C doesn't have a way to add a method to every adopter of a protocol.)

[swift-ci]

Comment by Tapan Thaker (JIRA)

Taking this up

[swift-ci]

Comment by Tapan Thaker (JIRA)

The above code compiles fine on `Apple Swift version 4.1` from Xcode 9.3. Not sure what changed the behaviour.

[belkadan]

I think Swift 4.1 just ignored the extension method, rather than complaining that it was a near-match. The intended change here is that the note and fix-it are not offered if the method is defined on a protocol extension.

[swift-ci]

Comment by Tapan Thaker (JIRA)

The compilation also fails on Swift 4.1 as soon as we make a ViewController conform to a protocol.
Like the code below:

class CustomViewController: UIViewController, IntroAnimationController {
func frame(for view: UIView, fromView: UIView) -> CGRect { return CGRect(x: 0, y: 0, width: 0, height: 0) }

func animations(fromView: UIView, toView: UIView) -> () -> Void {
let x : () -> Void = {
}
return x
}

public func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {}
}

I think all we need to do is delete the following:
https://github.com/apple/swift/blob/master/lib/AST/ASTContext.cpp#L2297-L2328
Also, couldn't find any tests for this.

[belkadan]

In that case the fix-it is correct, no?

[swift-ci]

Comment by Tapan Thaker (JIRA)

No, the fix-it doesn't seem to be correct. Once you apply the fix-it, you get into the same problem of 2 new errors
I think the problem is just that @objc protocol can never have extensions. So if we emit a diagnostic for that it would make more sense.

A simpler code snippet for reproducing the issue:

import Foundation

@objc protocol Print {
    func log(message string: NSString)
}

extension Print {
    func log(message string: NSString) {
        print(string)
    }
}

class CustomClass: NSObject, Print { }

[belkadan]

It's totally fine for an @objc protocol to have extensions; what's not valid is to use those extensions to satisfy the requirements. You get the same bad fix-it if the requirement is declared in one protocol but the default implementation comes from another.

[swift-ci]

Comment by Suyash Shekhar (JIRA)

tt (JIRA User) Are you still working on this? If not I would like to give it a shot 🙂

[belkadan]

Resetting assignee for all Starter Bugs not modified since 2018.