  <span class="comment">// Handler is NOT type-erased</span>
<span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">async_completion</span><span class="special">&lt;</span>  <span class="comment">// Return value customization</span>
    <span class="identifier">ReadHandler</span><span class="special">,</span>            <span class="comment">// supports futures and coroutines</span>
    <span class="keyword">void</span><span class="special">(</span><span class="iden