@ctfeonly

[bauss]

On Thursday, 7 December 2017 at 04:45:15 UTC, Jonathan M Davis 
wrote:
As I understand it, with the way CTFE works,
> it pretty much can't know whether a function can be called at 
> compile time until it tries
>
> - Jonathan M Davis

I think that's the point of the attribute. You tell the compiler 
that this function can only be called at compile-time and any 
attempt to call it during run-time would be an error.