To: J3                                                     J3/23-219
From: John Reid & Reinhold Bader
Subject: Interp on correspondence of unallocated coarrays
Date: 2023-October-02

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NUMBER: Fxx/xxx
TITLE: Correspondence of unallocated coarrays
KEYWORDS: coarray, allocatable
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress
REFERENCES: 23-007r1

QUESTION:

One of the restrictions on the allocation of a coarray
(see [147:32-33] 9.7.1.2, para 4, sentence 3) is
"If the coarray is a dummy argument, the ultimate arguments
(15.5.2.4) on those images shall be corresponding coarrays."
and another (see [148:29-30-33] 9.7.1.3, bullet 1, sentence 2)
is that the coarray be unallocated . However, there appears to be no
definition of correspondence for unallocated coarrays.

For example, does this program conform to the standard?

program tsplit
  type :: inner
     real, allocatable :: c[:]
  end type inner
  type :: outer
     type(inner) :: left, right
  end type outer
  type(outer) :: split
  if (this_image() <= num_images()/2) then
     call create(split%left%c)
  else
     call create(split%right%c)
  end if
  write(*,*) this_image(), allocated(split%left%c), &
                           allocated(split%right%c)
contains
  subroutine create(this)
    real, allocatable :: this[:]
    allocate(this[*])
  end subroutine create
end program tsplit

ANSWER:

A definition is needed. The above program is not intended to conform to
the standard because it causes the coarray split%left%c to be allocated
only on images 1 to num_images()/2 and the coarrary split%right%c to be
allocated only on the other images.

5.4.7 is intended to define correspondence only for nonallocatable or
allocated allocatable coarrays. An edit makes this clear. Since the
concept of correspondence of unallocated coarrays is needed only for
the allocate statement, we provide a defintion in 9.7.1.2.

Defining correspondence for unallocated coarray subobjects requires a
definition of correspondence for their parent objects. We provide text
that defines correspondence for any object, coarray or not. Once this
is in place, we need the requirement that that the allocate statement
specifies corresponding coarrays on every active image in the current
team. This makes all but the first two sentences of 9.7.1.2 para 4
redundant.


EDITS:

[48:26-27] In 5.4.7 para 3, sentence 1, change both occurences of
"coarray" to "nonallocatable or allocated allocatable coarray" so that
the sentence reads
"For each nonallocatable or allocated allocatable coarray on an image,
there is a corresponding nonallocatable or allocated allocatable coarray
with the same type, type parameters, and bounds on every other image of
a team in which it is established (5.4.8)."

[147:30-38] In 9.7.1.2 Execution of an ALLOCATE statement, replace
para 4 by

"For each named object that is not a dummy argument there is a
corresponding object with the same name on every other image of the
current team; if the object is an unsaved local variable of a recursive
procedure, its corresponding objects are the ones at the same depth of
recursion of that procedure on each image. Each element of an array
corresponds to the element of each corresponding array that is in the
same position in array element order. Each component of a scalar named
object corresponds to the same component of a corresponding object.
Each dummy argument corresponds to the objects that correspond to its
actual argument.

If an allocation specifies a coarray, it shall specify corresponding
coarrays on every active image in the current team; the dynamic type
and the values of corresponding type parameters shall be the same for
all these coarrays. The values of corresponding bounds and
corresponding cobounds shall be the same for all these coarrays."