title: how to use std::filesystem on gcc 8?
body: <p>I have updated version of gcc, gcc --version produces the following output</p>
  
  <pre><code>    gcc (Ubuntu 8.1.0-5ubuntu1~16.04) 8.1.0
  Copyright (C) 2018 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  </code></pre>
  
  <p>i can include filesystem in header file without any error</p>
  
  <pre><code>#include&lt;filesystem&gt;
  </code></pre>
  
  <p>But when i try to access the namespace filesystem like below then i get the error</p>
  
  <pre><code>namespace fs = std::filesystem;
  </code></pre>
  
  <p><strong>Error message</strong></p>
  
  <pre><code>error: ‘filesystem’ is not a namespace-name
   namespace fs = std::filesystem;
  </code></pre>
  
  <p>This seems to be weird since the gcc 8 has support for std::filesystem and it is not available in namespace, am i doing anything wrong in accessing std::filesystem?</p>
  
  <p>and yes i built with -std=c++17</p>