To: J3                                                     10-208
From: R. Bader
Subject: Executing a type-bound procedure on a coindexed object
Date: 2010 September 22
Reference: 08-308r1, 08-301r1

NUMBER: F08/xxxx
TITLE:  Executing a type-bound procedure on a coindexed object
KEYWORDS: coarrays, polymorphism
DEFECT TYPE: Request for interpretation
STATUS: J3 consideration in progress

Consider the following code:

module m
  type :: foo
    integer :: i = 0
  contains
    procedure, pass :: op
  end type
contains
  subroutine op(this, i)
    class(foo) :: this
    this%i = i
  end subroutine
end module m
program p
  use m
  class(foo), allocatable :: o_foo[:]
  allocate(foo :: o_foo[*])
  integer :: me

  me = this_image()
  if (me == 1) call o_foo[2]%op(1)
  sync all
  if (me == 2) write(*,*) o_foo%i
end program p


DISCUSSION:

According to section 12.5.2.4, para 2,

"[...]. If the actual argument is a polymorphic coindexed object,
the dummy argument shall not be polymorphic."

Hence the above code is not standard-conforming. (If enclosed in
a TYPE IS clause of a SELECT TYPE block, the call would be fine,
but this appears strange since the point of having TBPs is that
they perform type resolution dynamically).

Upon consulting the paper 08-308r1 which introduced the restriction
in 12.5.2.4, it appears that the focus was on disallowing coindexed
entities with polymorphic subcomponents, inasmuch as

call o[2]%x%foo()

(where x is a polymorphic type component) cannot be resolved
at run time with locally available information about the dynamic
type of the invoking subobject.

The rationale given for the second edit in 08-308r1 is a bit unclear.
In particular, the edit seems to exceed the intended scope as described
in the rationale, since the above program appears to be harmless.

QUESTION:

Was it not rather intended that the edit should read
"If the actual argument is a polymorphic subobject of a coindexed
 object, the dummy argument shall not be polymorphic."?

08-308r1 also mentions that C1229a from 08-301r1 is subsumed by
the restriction; however in the DIS text it is still there (as C1229).


SUBMITTED BY: R. Bader

HISTORY: First draft August 3, 2010
         Fixed wrong declaration in example August 4, 2010
         Submitted after further changes on September 22, 2010