title: predefined type System.Range is not defined or imported
body: <p>I'm using C# 8.0 (beta) in my UWP app with Visual Studio 2019 v16.0.2.
  I was trying to use the new range operator of C# 8 : <code>str[start..index]</code> and then two errors showed up:</p>
  <blockquote>
  <p>Predefined type <code>System.Range</code> is not defined or imported</p>
  </blockquote>
  <blockquote>
  <p>Predefined type <code>System.Index</code> is not defined or imported</p>
  </blockquote>
  <p>But there is no such reference that I could use.</p>
  <p>Visual Studio itself suggested me to use this feature. Is that the feature hasn't made available yet or what?</p>