To: J3                                                     J3/21-139
From: Brad Richardson
Subject: Interpretation of CO_BROADCAST of object with allocatable
         component
Date: 2021-May-18

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NUMBER: F18/027
TITLE: CO_BROADCAST with allocatable component
KEYWORDS: CO_BROADCAST allocatable component
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider the program

program example
  type :: my_string_t
    character(len=:), allocatable :: contents
  end type
  type(my_string_t) :: string
  if (this_image() == 1) string%contents = "Hello, World!"
  call co_broadcast(string, source_image=1)
  print *, string%contents
end program

Is this program valid?
If so, what is the status of string%contents on images other than 1?

The description for argument A in the standard (Section 16.9.46)
states that A:
  "shall have the same shape, dynamic type, and type parameter value,
  in corresponding references."

and given the example that follows, would indicate that allocatable
array arguments must be allocated to the same shape on all images
prior to the call to CO_BROADCAST. However, it also states that A:
  "becomes defined, as if by intrinsic assignment, on all images"

which would indicate that objects with allocatable components would
have those components (re)allocated.

Is it intended for CO_BROADCAST to be usable by objects with
allocatable components, and have those components be (re)allocated on
the receiving images? Furthermore, can those components be polymorphic,
(i.e. be declared with CLASS instead of TYPE)?

ANSWER:

The program is conforming because argument A satisfies the
requirements:
  "shall have the same shape, dynamic type, and type parameter value,
  in corresponding references."

and string%contents should be allocated with value "Hello, World!"
because string shall:
  "becomes defined, as if by intrinsic assignment, on all images"

EDITS to 21-007:

[369:2] 16.9.54.3 Argument A, last sentence
    At the end of the last sentence
    add ", including (re)allocation of any allocatable ultimate
         components, and setting the dynamic type of any components
         that are polymorphic".

Somewhat redundant because "as if by intrinsic assignment" should
already indicate it, but clearly the clarification is needed, as some
compilers have not interpreted it as such.

ALTERNATE ANSWER:

The program is not conforming because collective subroutines are not
intended to allow automatic (re)allocation. This is an oversight in the
current specification. The following edits will correct this.

ALTERNATE EDITS to 21-007:

[368:37] 16.9.54.3 Argument A, first sentence
    Before "in corresponding references"
    add "including all of its ultimate components, "

[369:2] 16.9.54.3 Argument A, last sentence
    After "as if by intrinsic assignment"
    add ", except that no (re)allocation of ultimate components takes
         place, nor setting the dynamic type of any polymorphic components"

This severely limits the usefulness of CO_BROADCAST, but eliminates
the possibility of performance issues in dealing with potential
(re)allocation on remote images.

SUBMITTED BY: Brad Richardson

HISTORY: 21-139   m224  Submitted

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