title: How to trigger C-preprocessor error about undefined symbol at #if? (LLVM/CLang/Xcode)
body: <p>How to trigger C-preprocessor error about missing definition at #if?
  I'm using LLVM/Clang/Xcode.</p>
  
  <p>This code works.</p>
  
  <pre><code>#define AAAAA 1
  #if AAAAA
  #endif
  </code></pre>
  
  <p>And I expected this code will be an error for undefined symbol.</p>
  
  <pre><code>//#define AAAAA 1     Removed definition.
  #if AAAAA
  #endif
  </code></pre>
  
  <p>But it did not. Is this standard/regular behavior? And is there other way to triggering preprocessor error?</p>