title: structured binding with existing vars not possible?
body: <p>Is it possible to use already existing vars as target for return values in connection with <code>structured bindings</code>?</p>
  
  <pre><code>auto f()
  {
      return std::make_tuple(1,2.2);
  }
  
  int main()
  {
      int x;
      double z;
  
      [ x, z ] = f();  // Did not work in gcc 7.1
  
      // structured bindings only work with "new" vars?
      auto [a, b] = f();  // works fine
  }
  </code></pre>