title: Compatibility of C++17 of apple clang++ that xcode 10 beta bundle
body: <p>clang that bundled with Xcode 10 beta is support c++17?</p>
  
  <p>In Xcode 9, some of C++17 features are not support.<br>
  For example, lack of <code>&lt;filesystem&gt;</code>, std::optional has bug, and does not support std::variant and so on. </p>
  
  <p>These features already supported opensource clang.</p>
  
  <p>Are they improved at Xcode10 beta?</p>