span><span class="special">::</span><span class="identifier">error_condition</span></code> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">optional</span></code>,
          <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">variant</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">mono