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?