Consider the following class used to calculate at compile time the factorial of a number:<div><br></div><div>    template&lt;unsigned n&gt;</div><div>    struct Factorial {</div><div>        static constexpr unsigned value = n * Factorial&lt;n-1u&gt;::value;</div><div>    };</div><div><br></div><div>    template&lt;&gt;</div><div>    struct Factorial&lt;0&gt; {</div><div>        static constexpr unsigned value = 1u;</div><div>    };</div><div><br></div><div>Now let&#39;s try to double it:</div><div><br></div><div>    consteval unsigned DoubleFactorial(unsigned const arg)</div><div>    {</div><div>        return 2u * Factorial&lt;arg&gt;::value;</div><div>    }</div><div><br></div><div>This won&#39;t compile because &#39;arg&#39; isn&#39;t a constant expression . . .even though it&#39;s inside a consteval function and its value is known at compile time.</div><div><br></div><div>It might be a lot of work to rewrite the standard to specify that the arguments to a function are constant expressions if the function is consteval, but maybe it would be a lot less work to add a new feature that allows us to explicitly specify that a value is a constant expression, perhaps something like:</div><div><br></div><div>    consteval unsigned DoubleFactorial(unsigned const arg)</div><div>    {</div><div>        [constexpr arg : carg];</div><div>        return 2u * Factorial&lt;carg&gt;::value;</div><div>    }</div><div><br></div><div>The line:</div><div><br></div><div>    [constexpr arg : carg];</div><div><br></div><div>takes the value of &#39;arg&#39; which is known at compile time, and turns it into a constant expression called &#39;carg&#39; which can be used as a template parameter.</div><div><br></div><div>A more complex use would be something like:</div><div><br></div><div>    constexpr unsigned DoubleFactorial(unsigned arg)</div><div>    {</div><div>        if consteval</div><div>        {</div><div>            [constexpr arg : carg];</div><div>            return 2u * Factorial&lt;carg&gt;::value;</div><div>        }</div><div>        else</div><div>        {</div><div>            unsigned retval = 2u;</div><div>            while ( arg ) retval *= arg--;</div><div>            return arg;</div><div>        }</div><div>    }</div><div><br></div><div>I realise that this feature isn&#39;t very pretty and is more of an improvisation to get around pre-existing language limitations, but it&#39;s nowhere near as bad as forwarding references.</div>