Once we have support for compile time functions (the current term for them is “pure functions”) you can add that yourself 
:-) OTOH I'd suggest that that specific case might be worth examining whenever anybody complains that his desired initialisation can't be coded... I'm not suggesting that some implementation be added prematurely, only that it might turn out to be a common "pattern".
No, it's not worth examining, because then the next one will come with the next function and so on. Better focus this into a more general approach.
Also if all you need is the size in bits then you can always use
BitSizeOf.