title: Compilation fails when using System.ValueTuple
body: <ol>
  <li>VS 2017 version 15.3.2 </li>
  <li>project targets framework 4.6.2</li>
  <li>Installed nuget package System.ValueTuple  latest stable 4.4.0</li>
  <li>Use value tuple, no error shown in editor</li>
  <li>Run compile step, no errors in error window, only output window shows compilation failed and actual error messages</li>
  </ol>
  
  <blockquote>
    <p>Invalid token '(' in class, struct, or interface member declaration</p>
  </blockquote>
  
  <p>What is going on?</p>
  
  <p>I created a branch in git from this one, and everything works fine. I merge changes back, compilation fails. I tried uninstalling package and re-installing, same thing.</p>