title: What does static_cast mean when it's followed by two pairs of parentheses?
body: <p>What does <a href="https://github.com/skarupke/flat_hash_map/blob/2c4687431f978f02a3780e24b8b701d22aa32d9c/bytell_hash_map.hpp#L1004" rel="noreferrer">this</a> say:</p>
  <pre><code>return static_cast&lt;Hasher &amp;&gt;(*this)(key);
  </code></pre>
  <p>?</p>
  <p>I can't tell whether <code>*this</code> or <code>key</code> is passed to <code>static_cast</code>. I looked around and found <a href="https://stackoverflow.com/a/40024058/4397162">this answer</a>, but unlike what I'm stuck on, there's nothing inside the first pair of parentheses.</p>