[SR-6895] Timer property with initializer and `self` target causes crash at runtime

[mattneub]

Previous ID	SR-6895
Radar	None
Original Reporter	@mattneub
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

Swift 4.1, Xcode 9.3 beta 1

Additional Detail from JIRA

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

md5: b788bbb198c95ec64db19b32b0652bd1

duplicates:

• [SR-4559] Method called 'self' can be confused with regular 'self' #47136

Issue Description:

I thought I had raised this issue before, but now I see that I never made it explicit, so I'd like this code on the record:

class ViewController: UIViewController {
    var timer : Timer? = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(timerFired), userInfo: nil, repeats: true)
    @objc func timerFired(_ : Timer) {
        print("fired")
    }
}

That code is legal (the compiler doesn't complain) but causes a crash at runtime. The reason seems to be that the self referred to as the target is not the view controller (presumably because our object is not fully formed the time we are initializing properties).

This is a very common trap for beginners to fall into. The compiler should disallow this use of self.

This may be a duplicate of #47442 but I was not so explicit there about the Timer problem, which is the common use case that bites everyone.

[belkadan]

Yeah, I don't think we get enough benefit from tracking this separately.

[belkadan]

I feel like it's only a "very common trap" if you actually define the timer in a property's initial value. Is that really such a common thing to do?

[mattneub]

@belkadan It's plenty common. I earn some healthy Stack Overflow rep. from telling people not to do it and why they are crashing when they do.