is duplicated by:

• SR-10051 Impossible to override insetsLayoutMarginsFromSafeArea on a project whose IPHONEOS_DEPLOYMENT_TARGET = 10.0

Issue Description:

Trying to override a property with a custom didSet logic, it's that's available only in 10.11. The following fails with <unknown>:0: error: 'highlightState' is only available on OS X 10.11 or newer.

@available(OSX 10.11, *) override public var highlightState: NSCollectionViewItemHighlightState {
    didSet {
        Swift.print(oldValue)
    }
}

Trying to add extra checks interpreted as redundant, however, the below code works.

@available(OSX 10.11, *) override public var highlightState: NSCollectionViewItemHighlightState {
    get {
        return super.highlightState
    }
    set {
        super.highlightState = newValue
    }
}