title: How do I fix an unfounded 'Use of undeclared type' error in Xcode 8 / Swift 3?
body: <p>The other day, after I did a full text search for a string in Xcode, Xcode started to report 'Use of undeclared type' for two of my classes.</p>
  
  <p>One class I had not touched in months, the other is a fundamental class, but I know they both exist and everything is named correctly.</p>
  
  <p>If I build, it will succeed, but I will still get a little red error warnings all over my code as if it failed every where one of the classes in question are used, and they will not go away even if I perform a clean build.</p>
  
  <p>However, if I quit Xcode, reopen, and rebuild the problem might go away for a little bit.  But it will usually return I make any changes to code anywhere.</p>
  
  <p>Has anyone else had this problem, and how do I make it stop?</p>
  
  <p>Thanks</p>
  
  <p>Stan</p>