non_complex_t.cpp

#include <complex>
#include <iostream>

template <typename T> using real_t = T;

template < typename precision, template <typename> class input_t, template <typename> class output_t >
void do_stuff(input_t<precision> a, output_t<precision> b) {
  std::cout << a + b << std::endl;
}

int main() {
  real_t<double> a = 1.;
  real_t<double> b = 2.;
  std::complex<double> c {0., 10.};
  std::complex<double> d {0., 33.};

  do_stuff<double, real_t, real_t>(a, b);
  do_stuff<double, real_t, std::complex>(a, c);
  do_stuff<double, std::complex, std::complex>(c, d);
  do_stuff<double, std::complex, real_t>(c, b);

  /* prints:
     3
     (1,10)
     (0,43)
     (2,10) */
  
  do_stuff(a, b);
  do_stuff(a, c);
  do_stuff(c, d);
  do_stuff(c, b);

  /* does not compile:
     note:   template argument deduction/substitution failed:
     note:   can't deduce a template for 'input_t<precision>' from non-template type 'double' */
}