> <span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <a class="link" href="../boost/histogram/axis/traits/width.html" title="Function template width"><span class="identifier">width</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Axis</span> <span class="special">&amp;</span><span class="special">,</span> <span class="identifier">index_type</span><span class="special">)</span><s