why compiler try use my sqrt not std.math?

[KnightMare]

so compiler thinks and try convert my code upper to
ulong sqrt1( ulong n ) {
   return cast( ulong )ceil<float OR real?>( @ret(ulong)my_sqrt( 
cast( ulong )real( n )));
}
why that? I didnt ask that.

yes, I know that I can add inside sqrt1
import std.math : sqrt;
but.. WHY?