This toy code has a problem. The program needs an explicit interface for the subroutine print_i.

module types

type plain
  integer :: i
end type plain

type, extends(plain) :: fancy
  integer :: i2
end type fancy

end module



program poly

use types

!  UH-OH!  Missing interface!

type(plain) :: pl
type(fancy) :: fan

pl%i = 10
fan%i = 20

call print_i(pl)
call print_i(fan)

end program


subroutine print_i(instr)
use types
class(plain) :: instr
print '("i is ",i5)', instr%i
end subroutine

When I try to compile with GNU Fortran, it won’t compile, and gfortran complains about the missing interface. What is strange to my mind is that Intel Fortran compiles an executable without complaining, but then it dies with a seg fault in print_i. When I put the interface in …

interface
  subroutine print_i(instr)
    use types
    class(plain) :: instr
  end subroutine
end interface

both compilers compile without error and their executables run fine.

The Intel behavior seems strange to me. Why not flag something that can’t run? Any ideas?

(BTW, I understand that I can put print_i into the module and have simpler code. But this came up because of something I am trying to test.)