title: Predefined type 'System.ValueTuple´2´ is not defined or imported
body: <p>I've installed Visual Studio 15 Preview 3 and tried to use the new tuple feature</p>
  
  <pre><code>static void Main(string[] args)
  {
      var x = DoSomething();
      Console.WriteLine(x.x);
  }
  
  static (int x, int y) DoSomething()
  {
      return (1, 2);
  }
  </code></pre>
  
  <p>When I compile I get the error:</p>
  
  <blockquote>
    <p>Predefined type 'System.ValueTuple´2´ is not defined or imported</p>
  </blockquote>
  
  <p>According to <a href="https://blogs.msdn.microsoft.com/dotnet/2016/07/13/visual-studio-15-preview-3-for-c-and-visual-basic/" rel="noreferrer">the blog post</a>, this features should be "on" by default. </p>
  
  <p>What did I do wrong?</p>