title: array_merge for Maps and Vectors at hacklang
body: <p>Given the following code</p>
  
  <pre><code>&lt;?hh
  $a = Vector {"qux"};
  $b = Vector {"foo","bar"};
  
  $a = array_merge($a, $b);
  </code></pre>
  
  <p>The following error is raised:</p>
  
  <blockquote>
    <p>Warning: Invalid operand type was used: array_merge expects array(s)</p>
  </blockquote>
  
  <p>Is there any workaround for <code>array_merge</code> for vectors and maps, without making a foreach by hand, of the vector?</p>