How to write 'instancetype'-like code in Swift

swift_instancetype_problem

import Foundation
import UIKit

extension UIViewController {
    
    class func fromMainStoryboard () -> UIViewController {
        var storyboardID = NSStringFromClass(self)
        let dotRangeOptional = storyboardID.rangeOfString(".", options: NSStringCompareOptions.allZeros)
        
        if let dotRange = dotRangeOptional {
            storyboardID = storyboardID.substringFromIndex(dotRange.startIndex.successor())
        }
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        return storyboard.instantiateViewControllerWithIdentifier(storyboardID) as UIViewController
    }
    
}

class SomeViewController: UIViewController {}

let someVC = SomeViewController.fromMainStoryboard() // returns UIViewController ... I want it to be SomeViewController