title: Adding an include guard breaks the build
body: <p>I added #ifndef..#define..#endif to a file of my project and the compiler fails.  As soon as I remove it or put any other name in the define it compiles fine.  What could be the problem? </p>
  
  <p>Sounds like the file is already declared, but I do not know where. I'm fine just removing it, but I really want to know why this is happening.</p>
  
  <pre><code>error: expected class-name before ‘{’ token
  error: ‘QDesignerFormEditorInterface’ has not been declared
  </code></pre>
  
  <p>And a couple of other errors.</p>
  
  <p>I am actually using an example from Qt, "Custom Widget Plugin Example".</p>
  
  <p>The difference is I am using my own class for the custom widget (.h, .cpp and .ui file).</p>
  
  <p>It might have to do with the file that has 2 includes, though that is how the example did it.</p>