span class="special">::</span><span class="identifier">forward_iterator_tag</span></code>. Otherwise <code class="computeroutput"><span class="identifier">iterator_category</span></code> is convertible to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">input_iterator_tag</span></code>.
      </p>
<h4>
<a name="iterator.specialized.filter.h3"></a>
        <span class="phrase"><a name="iterator.specialized.filter.requirements"></a></span>