write a function template specialisation that tests if an argument is known at compile time

[Cecil Ward]

On Sunday, 12 August 2018 at 12:27:59 UTC, Alex wrote:
> On Saturday, 11 August 2018 at 05:17:51 UTC, Cecil Ward wrote:
>> T myfunc(T)( T x, uint mask )
>>    if ( mask == 3 )
>>    {
>>    return fast_func( x, mask );
>>    }
>>
>> [...]
>
> Is it the volcano pattern you are looking for?
> https://p0nce.github.io/d-idioms/#Is-this-available-at-compile-time-or-runtime?

Wow, now that _is_ clever. I think that is definitely a big part 
of it.

Now somehow after having used a static if to select the 
known-at-compile-time case I then have to test the argument for 
particular values.

So how to get the next step along the way?