n a pointer-to-member (here, having a type of <code class="computeroutput"><span class="identifier">an_int</span>
      <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span>
      <span class="identifier">an_int</span><span class="special">,</span>
      <span class="number">0</span><span class="special">&gt;</span> <span class="special">::</span> <span class="special">*</span></code>) instead
      of a pointer 