Real/complex polymorphism with a vanilla Fortran compiler

test.F90

#ifndef INCLUDED

module mymod

implicit none

interface add_one
    module procedure add_one_real
    module procedure add_one_complex
end interface

contains

#endif
#ifndef TYPE
#define TYPE 0
#endif

#if TYPE == 0
subroutine add_one_real(x)
    real(8), intent(inout) :: x
#elif TYPE == 1
subroutine add_one_complex(x)
    complex(8), intent(inout) :: x
#endif

    x = x + 1
end subroutine

#ifndef INCLUDED
#define INCLUDED
#undef TYPE
#define TYPE 1
#include "test.F90"

end module

program main

use mymod

implicit none

real(8) :: x
complex(8) :: z

call add_one(x)
call add_one(z)

end program

#endif