title: Overriding non-@objc declarations from extensions is not supported
body: <p>Seemingly out of nowhere I started getting hundreds of errors associated with two extension functions that I use frequently.</p>
  
  <p>I tried commenting out all the new code that preceding this error showing up. I also tried cleaning my build folder. How do I get ride of the error found in the title of this post? Why did it randomly appear when I've been using these extensions successfully for months?</p>
  
  <pre><code>extension UITableViewCell {
  
  public func getSize(large: CGFloat, medium: CGFloat, small: CGFloat) -&gt; CGFloat {
      var size = CGFloat()
      let screenHeight = Int(UIScreen.main.bounds.height)
      if screenHeight &gt;= 800 {
          size = large
      } else if screenHeight &gt;= 600 {
          size = medium
      } else {
          size = small
      }
      return size
  }
  
  public func formatPrice(_ price: Int) -&gt; String {
      let lastDigit = price % 10
      var stringPrice = ""
      if lastDigit == 0 {
          stringPrice = "$\(Double(price) / 100)0"
      } else {
          stringPrice = "$\(Double(price) / 100)"
      }
      return stringPrice
  }
  
  }
  </code></pre>