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[SR-9000] of deinitializers, property observers, and defer #51503

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mattneub opened this issue Oct 15, 2018 · 4 comments
Open

[SR-9000] of deinitializers, property observers, and defer #51503

mattneub opened this issue Oct 15, 2018 · 4 comments
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compiler The Swift compiler in itself improvement

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@mattneub
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Previous ID SR-9000
Radar None
Original Reporter @mattneub
Type Improvement
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Improvement
Assignee None
Priority Medium

md5: 659d8175f6537ca6db993b3627e234f6

is duplicated by:

  • SR-10207 Immediate execution of defer warning shouldn't be given in a direct-to-storage context

relates to:

  • SR-1437 defer block in init triggers property observers

Issue Description:

The property observer here never fires:

class A {}
class C {
    var a: A? {
        didSet { print("didset") }
    }
    deinit {
        print("deinit")
        self.a = nil
    }
}
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        var c : C? = C()
        c = nil
        _ = c
    }
}

Okay, I can accept that; it isn't documented AFAIK, but it makes sense that we wouldn't want to run property observers during deinit, lest there be side effects. But now hold my beer and watch this:

class A {}
class C {
    var a: A? {
        didSet { print("didset") }
    }
    deinit {
        print("deinit")
        defer {
            self.a = nil
        }
    }
}
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        var c : C? = C()
        c = nil
        _ = c
    }
}

The observer is called! Okay, that is incoherent. If it's wrong to call property observers during deinit, it is really wrong to call property observers after deinit.
@hamishknight
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Related SR for defer blocks in initialisers not doing direct accesses: https://bugs.swift.org/browse/SR-1437

@mattneub
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@hamishknight Excellent, thanks; I'd say that's related but not identical.

@hamishknight
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@mattneub Sure – though in the compiler it's the same logic that determines whether or not to perform a direct access, so ideally a fix for one would also be a fix for the other (that is, depending on compatibility implications – I believe the defer-within-init to trigger observers pattern is used by quite a few people).

@belkadan
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Right. defer probably shouldn't have this behavior, but of course any method you call during deinit will result in the observers running, so it's not like it's not allowed at all.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@belkadan @mattneub @hamishknight