Issue Description:

For supporting new dark mode for iOS 13, using UIColor extension for the bifurcation of colors for other OS version as below.

import UIKit

extension UIColor {
    static var groupedBackground: UIColor {
        if #available(iOS 13, *) {
            return UIColor.systemGroupedBackground
        } else {
            return UIColor.groupedBackground
        }
    }
}

Here I get a typo for OS version not iOS 13, and will result in crash at run-time.

While if we try to write as below:

extension UIColor {
    static var groupedBackground: UIColor {
        return UIColor.groupedBackground  
    }
}

Here we get compile-time warning saying All paths through this function will call itself.

Expected Result:

• We should present with the compile time warning for the other part in case of #available checks.