pport for <code class="computeroutput"><span class="identifier">constexpr</span></code>
        in the library, currently the <code class="computeroutput"><span class="identifier">number</span></code>
        front end supports <code class="computeroutput"><span class="identifier">constexpr</span></code>
        on default construction and all forwarding constructors, but not on any of
        the non-member operators. So if some type <code class="computeroutput"><span class="identifier">B</span></code>
   