(j3.2006) (SC22WG5.4520) Callback functions with polymorphic arguments

[Bader, Reinhold]

Hello all, 

consider the following module:

module mod_callback_up_01
  implicit none
  abstract interface
    subroutine callback(data)
      class(*), dimension(:) :: data    ! (A)
    end subroutine
  end interface
contains
  subroutine foo(data, proc)
    class(*), dimension(:) :: data    ! (B)
    procedure(callback) :: proc
    call proc(data)
  end subroutine
end module

and then the following code which uses this module: 

module mod_impl
  implicit none
  type :: bar
    integer :: i
  end type
contains
  subroutine callback1(data)
    class(bar) :: data(:)         ! (C)
    integer :: i
    do i=1,size(data)
      data(i)%i = i
    end do
  end subroutine
end module
program prog
  use mod_callback_up_01
  use mod_impl
  integer, parameter :: dim=5
  type(bar) :: x(dim)
  call foo(x, callback1)
end program

Questions:

(1) Is the above code standard conforming?

(2) Assume that at (C), the declaration is replaced by 
       type(bar) :: data(:)
     Is the resulting code standard conforming?
         
(3) Assume that at (A) and (B), class(*) is replaced by
     class(base), where base is some known type, and
    bar is an extension of base.
     Is the resulting code standard conforming?

(4) Apply both assumptions in (2) and (3) - is the
     resulting code then standard conforming?

The background for this is the issue of handling callbacks in MPI-3. 
For backward compatibility it is considered desirable to have 
abstract interfaces for callbacks which use arguments declared
as TYPE(*), DIMENSION(*). The dummy arguments of the callbacks 
of course will have some definite type in existing code. (The 
programmer may still need to add BIND(C), but this is considered 
sufficiently low-intrusive). Depending on the answers to above questions, 
something may need to be done about this in the TR. 

(For the above code, different compilers give widely varying results
with respect  to both compilation and run time behaviour). 

Regards
Reinhold