[SR-15789] MainActor isolation is not ensured

[swift-ci]

Previous ID	SR-15789
Radar	rdar://problem/88285600
Original Reporter	PoltoraIvana (JIRA User)
Type	Bug
Status	Closed
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@kavon
Priority	Medium

md5: 2e59f16869dc5cd2056f09f72289da88

Issue Description:

Having the code:

import UIKit


class ViewController: UIViewController {
  
  private let presenter: Presenter
  
  required init?(coder: NSCoder) {
    let presenter = Presenter()
    self.presenter = presenter
    super.init(coder: coder)
    presenter.view = self
  }
  
  override func viewDidLoad() {
    super.viewDidLoad()
   
    print("did load")
    presenter.doSmth()
  }


}


extension ViewController {
  
  @MainActor
  func doSmthOnMain() {
    dispatchPrecondition(condition: .onQueue(.main))
  }
  
}


final class Presenter {
  
  @MainActor weak var view: ViewController?
  
  func doSmth() {
    Task {
      await view?.doSmthOnMain()
    }
  }
  
}

application crashes on the line:

dispatchPrecondition(condition: .onQueue(.main)) 

If you run the project attached, you will see that @mainactor isolation is not ensured and this method is called from cooperative queue.

Interestingly, if I delete @mainactor from the declaration:

  @MainActor weak var view: ViewInternal? 

Method is ensured to be run on main, so no crash.

[swift-ci]

Comment by Ivan Sobolevskiy (JIRA)

I further simplified the case and attached a new one.

Interestingly, if we remove 'final' from 'Presenter' declaration, everything works fine. So it seems because of some optimisation Main actor isolation is not ensured

[DougGregor]

@swift-ci create

[kavon]

Fixed in Swift 5.6 #41176

[kavon]

Thank you for the report.