title: Can't use #available to check iOS version with ViewBuilder
body: <p>I'm creating a SwiftUI app that is using mostly iOS 13 code, with a bit of iOS 14 code. I'm trying to use <code>if #available(iOS 14.0, *) {}</code> in the body of my views when I use iOS 14 code. When I try, I get the following error.</p>
  <blockquote>
  <p>Closure containing control flow statement cannot be used with function builder 'ViewBuilder'</p>
  </blockquote>
  <p>Why am I getting this error, and what can I do to solve it?</p>