      An iterator type (can be unused_type)</span>

    <span class="identifier">Derived</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">derived</span><span class="special">()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="special">*</span><span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">Derived</span> <span class="keywor