To: J3                                                     J3/22-100
From: John Reid
Subject: F18/031 revised
Date: 2021-December-06
References: 18-007r1, 21-173, 21-184r1

Discussion
---------

Interp F18/031 failed the letter ballot (see 21-184r1). The reason for
my NO vote was faulty, see the comments of /interp in 21-184r1.
After further discussion by /HPC and /interp, I wish to propose two
alternatives:

A. The response in 21-173, which does not permit the argument A to be
polymorphic.  An additional small edit is proposed.

B. The suggestion in my ballot. The argument A is not permitted to be
   nonallocatable polymorpic.

For alternative A, the additional edit I suggest is

[355:19] 16.9.46 CO_BROADCAST, p3 Arguments, argument A, sentence 1,
delete "dynamic" to make the sentence read
    "A shall have the same shape, type, and type parameter values, in
    corresponding references."

I think it is misleading to the reader to talk of "dynamic type" when
A cannot be polymorphic.

For alternative B, note that intrinsic assignment (18-007r1, 10.2.1.2)
has the requirement
    (1) if the variable is polymorphic it shall be allocatable and not
        a coarray,
which means that intrinsic assignment for a noncorray polymorphic
variable does not conform with the standard unless the variable is
allocatable. As the interp. points out, it seems strange to allow
polymorphic broadcast across images, but not to allow polymorphic
assignment of a single variable within an image. The proposed edits
align CO_BROADCAST with intrinsic assignment.

Note also that CO_BROADCAST has the requirement
    A shall have the same shape, dynamic type, and type parameter
    values, in corresponding references.
which implies that A is permitted to be polymorphic. If it is
allocatable, is not a coarray, and the above conditions hold,
A becomes defined on each image as if by intrinsic assignment.
Because no reallocation is needed or intended, the "not a coarray"
condition is not needed here. An edit to specify this is included.

I prefer alternative B. I see alternative A as an unnecessary
restriction on the use of polymorphic variables.

----------------------------------------------------------------------
NUMBER: F18/031
TITLE: CO_BROADCAST with polymorphic argument
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider the code fragment

    Subroutine asgn_for_image(a,b,srcimg)
      Class(*) a,b
      Integer,Intent(In) :: srcimg
      ! The assignment A = B is not allowed, so...
      If (This_Image()==srcimg) Then
        Call asgn(b)
      Else
        Call asgn(a)
      End If
    Contains
      Subroutine asgn(x)
        Class(*) x
        Call co_broadcast(x,Source_Image=srcimg)
      End Subroutine
    End Subroutine

There is no requirement forbidding the A argument of CO_BROADCAST
from being polymorphic, so on the face of it, this appears to get
around the prohibition against nonallocatable polymorphic
assignment.

However, CO_BROADCAST states
    "A becomes defined, as if by intrinsic assignment"
but intrinsic assignment is not defined when the variable is a
nonallocatable polymorphic. It can be convincingly argued that
the standard therefore does not establish an interpretation, and
thus the call to CO_BROADCAST is not valid.

Philosophically, it would seem to be strange to allow polymorphic
broadcast across images, but not to allow polymorphic assignment
of a single variable within an image let alone across images.

Is the call to CO_BROADCAST in the example standard-conforming?

(And if so, what are the actual semantics?)

ANSWER:

Alternative A

No, this was not intended to be conforming. Edits are supplied to
correct this.

EDITS to 18-007r1:

[355:19] 16.9.46 CO_BROADCAST, p3 Arguments, argument A, sentence 1,
delete "dynamic" to make the sentence read
    "A shall have the same shape, type, and type parameter values, in
    corresponding references."

[355:20] 16.9.46 CO_BROADCAST, p3 Arguments, argument A, sentence 2,
after "It shall not be" insert "polymorphic or" making the sentence read
    "It shall not be polymorphic or a coindexed object."

Alternative B

No, this was not intended to be conforming. Edits are supplied to
correct this.

EDITS to 18-007r1:

[355:20] 16.9.46 CO_BROADCAST, p3 Arguments, argument A, sentence 2,
after "It shall not be" insert "nonallocatable polymorphic or"
making that sentence read
    "It shall not be nonallocatable polymorphic or a coindexed object."

[355:21] 16.9.46 CO_BROADCAST, p3 Arguments, argument A, final sentence,
    after "intrinsic assignment", add
    "for a noncoarray variable" making that sentence read
    "If no error condition occurs, A becomes defined, as if by intrinsic
    assignment for a noncoarray variable, on all images in the current
    team with the value of A on image SOURCE_IMAGE."

SUBMITTED BY: Malcolm Cohen

HISTORY: 21-151   m224  Submitted
         21-151r1 m224  Eliminated alternative answer,
                        passed by J3 meeting 224.
         21-184r1 m225  Failed J3 letter ballot.