title: Unknown type name 'namespace' in XCode, other solutions do not work
body: <p>I'm running OS 10.11, and my XCode version is 7.2.</p>
  
  <p>All of the sudden my project fails to compile in XCode. XCode seems to be confused about the file type -- I'm getting errors that indicate my code is being compiled as C++ even tho all the files in my project are labeled as type Objective-C code. Here are some of the errors that occur in deeply-nested system header files:</p>
  
  <p><code>Unknown type name 'namespace'
  'utility' file not found (from #include_next &lt;utility&gt;)
  </code></p>
  
  <p>I've verified that all the files are listed as Objective-C. I've even tried changing them to Objective-C++, by 'Identity and Type', and by also changing the extensions to .mm, but this introduces an even bigger set of mysterious compile errors deep in system-level header files, such as:</p>
  
  <p><code>Use of undeclared identifier 'nullptr_t'; did you mean 'nullptr'?
  No member named 'memchr' in the global namespace
  </code></p>
  
  <p>I've tried reinstalling XCode a couple times. Any suggestions?</p>
  
  <p>Further mystifying is that I have a different Mac with the same version of OS X, same version of XCode, and with the exact same code (verified thru git), and it compiles fine.</p>