To: J3                                                     J3/21-151r1
From: Malcolm Cohen
Subject: Interp F18/031 CO_BROADCAST with polymorphic argument
Date: 2021-July-01

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NUMBER: F18/031
TITLE: CO_BROADCAST with polymorphic argument
DEFECT TYPE: Erratum
STATUS: Passed by J3 meeting

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:

No, this was not intended to be conforming.
An edit is supplied with the missing requirement.

EDIT to 18-007r1:

[355:20] 16.9.46 CO_BROADCAST, p3 Arguments, argument A,
         After "It shall not be"
         Insert "polymorphic or"
         Making that sentence read
    "It shall not be polymorphic or a coindexed object."

SUBMITTED BY: Malcolm Cohen

HISTORY: 21-151   m224  Submitted
         21-151r1 m224  Eliminated alternative answer.

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