the default <code class="computeroutput"><span class="identifier">dont_care</span></code>
        type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>.
        Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means
        that the return value of the operator can be used as argument to a function
        expecting <code class="computeroutput"><span class="identifier">Ret</span></code>:
</p>
<pre 