Explanation Wanted: using 'super' in a closure where 'self' is explicitly captured is not yet supported

Using Swift
1

I've ran into an error message while writing some combine code.

"Using 'super' in a closure where 'self' is explicitly captured is not yet supported"

Here's reproduced code snippet image:

captured
captured2770×2098 350 KB

import UIKit
import Combine

class SomeParentObject {
    func openFunction() {
        print("parentFunction")
    }
}

final class SomeChildObject: SomeParentObject {
    var cancelBag = Set<AnyCancellable>()

    override func openFunction() {
        print("childFunction")
    }

    func bind() {
        Just("hello")
            .sink(receiveValue: { [weak self] _ in
                super.openFunction() // ERROR: "Using 'super' in a closure where 'self' is explicitly captured is not yet supported"

            })
            .store(in: &cancelBag)
    }
}

I tried to find articles about this error, but I couldn't find one explaining why.
I wonder why such restriction was introduced in Swift.
Could anyone give me an explanation?

Thanks in advance.

2

You're getting the error because you capture self weakly. Remove [weak self] and it will compile and give the expected results.

I do not know why capturing weakly breaks this.

3

If you need to do this particular combination of things, this should suffice for a workaround:

final class SomeChildObject: SomeParentObject {
    var cancelBag = Set<AnyCancellable>()

    override func openFunction() {
        print("childFunction")
    }

    private func superOpenFunction() {
        super.openFunction()
    }

    func bind() {
        Just("hello")
            .sink(receiveValue: { [weak self] _ in
                self?.superOpenFunction()
            })
            .store(in: &cancelBag)
    }
}
4

There's no deep fundamental reason why it has to be prohibited; It's an implementation limitation. Fixing it is a matter of ensuring that SuperExpr can represent this in the AST, and then possibly adjusting SILGen's lowering of super calls to take this possibility into account. It's doable without a huge effort; it's just one of those edge cases that hasn't been a priority yet.

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