special">,</span> <span class="identifier">DefaultT</span><span class="special">,</span> <span class="identifier">DescriptorT</span> <span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> 
  <span class="identifier">extract_or_default</span><span class="special">(</span><span class="identifier">expressions</span><span class="special">::</span><span class="identifier">attribute_keyword</span><span class="special">&lt;</span> <span class="identifier">DescriptorT</