module my_type

  interface my_cfunc
     module procedure my_cfunc1, my_cfunc2, my_cfunc3
  end interface my_cfunc

  ! comment interface above and uncomment interface below to get working code

!!$  interface my_cfunc
!!$     module procedure my_cfunc1, my_cfunc3
!!$  end interface my_cfunc

contains

  function my_cfunc1(s, i, sb, sa)
    integer(kind = 4) :: i
    character(len = :), allocatable :: my_cfunc1
    integer(kind = 4), optional :: sb, sa
    character(len = *) :: s
    ! works: character length of result is not specified
    ! ... do something with arguments here
    my_cfunc1 = 'first'
  end function my_cfunc1

  function my_cfunc2(s, i, n, sb, sa)
    integer(kind = 4) :: i, n
    character(len = n) :: my_cfunc2
    integer(kind = 4), optional :: sb, sa
    character(len = *) :: s
    ! does not work: not distinguishable from my_cfunc1 although character length is a required arguments
    ! ... do something with arguments here
    my_cfunc2 = 'second'
  end function my_cfunc2

  function my_cfunc3(n, s, i, sb, sa)
    integer(kind = 4) :: i, n
    character(len = n) :: my_cfunc3
    integer(kind = 4), optional :: sb, sa
    character(len = *) :: s
    ! works: passed character length n is now the first argument
    ! ... do something with arguments here
    my_cfunc3 = 'third'
  end function my_cfunc3

end module my_type

program main
  use my_type  
  implicit none
  print *, my_cfunc('test', 1)
  read *
end program main