I want to do something trivial like

#include <math.h>

double x_sqrt = sqrt(16);

gives me a compiler error because I am blocked by

#include "src/common_cpp/Math/Complex.hh" 

MAUS::Complex sqrt(const double x)

I think the standard routine should trump here - i.e. if the user wants a complex in return should either give a Complex with Im(z) = 0 as argument or convert the return double to a Complex.