J3/11-198
To: J3
From: Malcolm Cohen
Subject: Procedure pointer interp.
Date: 2011 June 13

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NUMBER:
TITLE: Procedure pointer assignment with an EXTERNAL target.
KEYWORDS: Procedure pointer, pointer assignment, EXTERNAL attribute.
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider

  Program m5
    Print *,f()
    Call s
  Contains
    Subroutine s
      Procedure(Real),Pointer :: p
      Print *,g()
      p => f                           ! (1)
      Print *,p()
      p => g                           ! (2)
      Print *,p()
    End Subroutine
  End Program
  Function f()
    f = 1
  End Function
  Function g()
    g = 2
  End Function

is this program standard-conforming?

On the face of it, both (1) and (2) violate constraint C729, which
only allows an external procedure
  "that is accessed by use or host association and is referenced in
   the scoping unit as a procedure or that has the EXTERNAL
   attribute".
The function f is accessed by host association but is not referenced
in the scoping unit as a procedure, whereas the function g is
referenced in the scoping unit as a procedure but is not accessed by
use or host association.

Furthermore, consider

  Program m5a
    Real,External :: f,p
    Pointer p
    p => f                       ! (3)
    Print *,p()
  End Program
  ! function f as above.

Is this conforming?  The constraint has a single clause for external
procedures with no commas but with a disjunction and a conjunction,
therefore it is ambiguous whether it means
   "accessed by use or host association" AND
    ("is referenced in the scoping unit as a procedure" OR
     "has the external attribute")
or whether it means
   ("accessed by use or host association" AND
    "is referenced in the scoping unit as a procedure") OR
     "has the external attribute")

Since the standard does not unambiguously give an interpretation of
this program it seems that statement (3) is also not conforming.

Which of these three procedure pointer assignment statements were
intended to be conforming?

DISCUSSION:

The wording of this constraint was determined by Interp F03/0138,
which intended to change it to

  "C727 (R742) A <procedure-name> shall be the name of a module or
        dummy procedure, a specific intrinsic function listed in 13.6
        and not marked with a bullet ($\bullet$), a procedure pointer,
        or an external procedure that is accessed by use or host
        association, referenced in the scoping unit as a procedure, or
        that has the EXTERNAL attribute."

but a last-minute "editorial" change at the WG5 ballot stage made it

  "C727 (R742) A <procedure-name> shall be the name of a module or
        dummy procedure, a specific intrinsic function listed in 13.6
        and not marked with a bullet ($\bullet$), a procedure pointer,
        or an external procedure that is accessed by use or host
        association and is referenced in the scoping unit as a
        procedure, or that has the EXTERNAL attribute."

instead, and the comma got lost in between there and Fortran 2008.
Despite supposedly being an editorial change, the WG5 wording has
different technical results.

According to the original wording of the interp, all three examples
above were intended to be allowed.

ANSWER:

All three pointer assignment statements were intended to be valid.
An edit is supplied to correct the error in the standard.

EDIT:

[158:33-159:2] In 7.2.2.2, C729,
  replace "an external ... bullet ($\bullet$)"
  with
    "a specific intrinsic function listed in 13.6 and not marked with
     a bullet ($\bullet$), or an external procedure that is accessed
     by use or host association, referenced in the scoping unit as a
     procedure, or that has the EXTERNAL attribute".
{NB: $\bullet$ is the LaTeX command that produces a bullet.}

SUBMITTED BY: Malcolm Cohen

HISTORY: 11-nnn   m195 Submitted

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