[SR-3805] Creating a strong reference to self in a closure should warn if self is never accessed "strongly"

[swift-ci]

Previous ID	SR-3805
Radar	None
Original Reporter	alanf (JIRA User)
Type	Improvement

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 4 | |Component/s | Compiler | |Labels | Improvement, LanguageFeatureRequest | |Assignee | None | |Priority | Medium | md5: 749bbb1e7e9a82cb5f26ca678d24746e

is duplicated by:

• SR-4893 [weak self] leaks

Issue Description:

This was discussed on the mailing list over a year ago but no proposal was made:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001197.html

class ViewControllerBase: UIViewController {
    let timer:DispatchSourceTimer = DispatchSource.makeTimerSource(flags: [], queue:  DispatchQueue(label: "q.q"))

    deinit {
        NSLog("deinit of \(NSStringFromClass(type(of: self)))")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        timer.scheduleRepeating(deadline: .now(), interval: .seconds(1))

        timer.setEventHandler {
            UIView.animate(withDuration: 0.2) { [weak self] in
                self?.view.backgroundColor = UIColor.green
            }
        }
}

This leaks memory in a subtle way...there is a retain loop even though self is only used in a "weak" manner. This is because the nested closure captures a strong reference for use in the closure that follows.

[belkadan]

Full test case in the dup.

[swift-ci]

Comment by Kostiantyn Herasimov (JIRA)

This also hides errors when captured instance (like a UIViewController) is deallocating inside wrong thread (like not main). It could be hard to find that cases because where is no any warning from compiler