I'm concerned that changing this would be a language change at this point, since the initial value expression may have side effects.

Those side effects are exactly what concern me. If the first access is to set the variable, that is (or should be) the initialization. The (possibly expensive) initializer isn't needed.

Also there's a weird difference here; setting a `lazy var` instance variable doesn't run its initial value expression. To see that, run this variant of the above code:

var laz : String = {
    print("lazy initializing global")
    return "Howdy"
}()

class ViewController: UIViewController {
    lazy var lazyProperty : String = {
        print("lazy initializing property")
        return "Howdy"
    }()
    override func viewDidLoad() {
        super.viewDidLoad()
        laz = "Greetings"
        print(laz)
        lazyProperty = "Greetings"
        print(lazyProperty)
    }
}

So why should setting a global do so? You surely wouldn't argue that the language should be changed the other way, to make them both run their initializers when set?

So I've long regarded this behavior as a bug. I can see that fixing it might require passing through a decision-making process, but I'm not in any doubt that the current behavior is intuitively not what one would expect or want.