To: J3                                                     J3/21-138r1
From: Van Snyder & Malcolm Cohen
Subject: Interp F18/nnn Access Internal Procedure in Interface Body
Reference: 18-007r1
Date: 2021-June-21


The r0 of this paper asked the question
    Was it intended that it be impossible to use an internal subprogram to
    specify the interface of a dummy procedure in an interface body?

The constraint in question says:
      Within an interface body, an entity that is accessed by host
      association shall be accessible by host or use association within
      the host scoping unit, or explicitly declared prior to the
      interface body.

Actually, it is not quite impossible to use an internal procedure name in
IMPORT, it merely cannot be done before the declaration (definition) of the
internal procedure. So, not in the specification part of the containing
main program or subprogram, but could be in a later internal procedure.

There is no doubt that IMPORT was intended to be capable only of importing
things that were previously declared. As evidence, the text in the standard
which did not appear accidentally. This text was part of the IMPORT feature
from its beginning in Fortran 2003.

(One of the authors of the feature confirms that this was intended to avoid
errors such as circular definition, and to reduce impact on implementation,
i.e. they don't have to have robust dependency detection or be able to sort
declarations by dependency. Whether one agrees with the technical reasons
or not, there is no doubt as to the intention.)

As such, the question is answered directly without need for interpretation,
by the existing text in the standard.

It has been long-standing policy of /INTERP that if a question can be
answered directly by reference to the text of the standard, without need
for complicated reasoning or inference, the question is not accepted as a
request for interpretation but simply answered. (This helps to reduce our
workload, as interp processing requires multiple rounds of discussion and
voting even when everyone agrees.)

Therefore, /INTERP declines to accept this request. On examination, it
seems to be a new feature, which should be considered on its technical
merits.

Therefore, /INTERP further recommends that this new feature be considered
in the Fortran 202y timeframe.

Original text of the r0 follows:

----------------------------------------------------------------------

NUMBER: F18/nnn
TITLE: Access Internal Procedure in Interface Body
DEFECT TYPE: TBD
STATUS: TBD

EXAMPLE:

Consider:

      program Test_Internal

        interface
          subroutine S1 ( S )
            import S2
            procedure ( S2 ) :: S
          end subroutine S1
        end interface

      contains

        subroutine S2
        end subroutine S2

      end program Test_Internal

C897  (R867) Each <import-name> shall be the name of an entity in the
      host scoping unit.

S2 satisfies this constraint. It is in the host scoping unit.

C8101 Within an interface body, an entity that is accessed by host
      association shall be accessible by host or use association within
      the host scoping unit, or explicitly declared prior to the
      interface body.

S2 violates this constraint. It is not, and cannot be, declared before
the interface block.

Some processors accept the program, and others do not. Interestingly, if a
procedure declaration (with or without the POINTER attribute) that uses
the internal subroutine to specify its interface appears before the
interface block, e.g.,

      procedure ( s2 ), pointer :: P

the processor that rejects the program then accepts it. This is not a
declaration of S2 that precedes the interface block.

QUESTION:

Unlike Pascal, Fortran does not have a FORWARD declaration. It is
therefore impossible to declare an internal subprogram before an
interface block in the host scoping unit.

Was it intended that it be impossible to use an internal subprogram to
specify the interface of a dummy procedure in an interface body?

ANSWER 1:

This was intended.

ANSWER 2:

This was an oversight.

EDITS 1:

No edits are necessary.

EDITS 2:

Revise constraint C8101

C8101 Within an interface body, an entity that is accessed by host
      association shall be accessible by host or use association within
      the host scoping unit, explicitly declared prior to the
      interface body, or an internal procedure of the host scoping unit.

SUBMITTED BY: Van Snyder

HISTORY: 21-138   m224  Submitted

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