Cannot convert value of type 'ViewController?' to expected argument type '_OptionalNilComparisonType'

I'm implementing MapKit into my app I have been able to display users current location. Now I'm trying to display current street name and street address onto a label which I have on my storyboard but i get this error::

Cannot convert value of type 'ViewController?' to expected argument type '_OptionalNilComparisonType' on this line of code

guard let self = self else { return }

Below is my sample code:

extension ViewController: MKMapViewDelegate {
func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) {
    let center = getCenterLocation(for: mapView)
    let geoCoder = CLGeocoder()

    guard let previousLocation = self.previousLocation else { return }
    guard center.distance(from: previousLocation) > 50 else { return }
    self.previousLocation = center

    geoCoder.reverseGeocodeLocation(center) { [weak self] (placemarks, error) in
        guard let self = self else { return }

        if let _ = error {
            //TODO show alert informing the user
            return
        }

        guard let placemark = placemarks?.first else {
            //TODO Show alert informing the user
            return
        }

        let streetNumber = placemark.subThoroughfare ?? ""
        let streetName = placemark.thoroughfare ?? ""

        DispatchQueue.main.async {
            self?.addressLabel.text = "\(streetNumber) \(streetName)"
        }
    }
}
}

Update to Xcode 10 and Swift 4.2. That is the earliest version of Swift where the expression

guard let self = self else { return }

is legal.

If you can’t do that, use some other variable name on the left side of the equal sign. For example:

guard let strongself = self else { return }