11-135r1
To: J3
From: Malcolm Cohen
Subject: Explicit interface requirement interpretation
Date: 2011 February 17

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NUMBER: F08/0054
TITLE: Requirements for needing an explicit interface
KEYWORDS: Explicit interface
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Consider the program

  SUBROUTINE Fred() BIND(C,NAME='Nurke')
    PRINT *,'ok'
  END SUBROUTINE
  SUBROUTINE S(dummy)
    INTERFACE
      SUBROUTINE dummy() BIND(C)
      END SUBROUTINE
    END INTERFACE
    CALL dummy
  END SUBROUTINE
  PROGRAM example1
    EXTERNAL Fred,S
    CALL S(Fred)
  END PROGRAM

According to 12.4.2.2, an explicit interface is only ever required in
a scope where a procedure is referenced.  The main program does not
reference Fred, so this appears to be conforming.  It appears that
no processors accept this example...

However, the very similar program minus the BIND(C) specifications

  SUBROUTINE Freddy(x)
    LOGICAL,OPTIONAL :: x
    IF (PRESENT(x)) PRINT *,'ok'
  END SUBROUTINE
  SUBROUTINE SUB(dummy)
    INTERFACE
      SUBROUTINE dummy(x)
        LOGICAL,OPTIONAL :: x
      END SUBROUTINE
    END INTERFACE
    CALL dummy(.TRUE.)
  END SUBROUTINE
  PROGRAM example2
    EXTERNAL Freddy,SUB
    CALL SUB(Freddy)
  END PROGRAM

but with a different reason for requiring an explicit interface, is
also apparently valid in Fortran 2003/2008 but is not valid in
Fortran 90 or Fortran 95, because they require the explicit interface
without the condition of the procedure being referenced in that
scope.

This feature (viz only require the explicit interface where it is
referenced) was added by paper 02-144, which makes no mention of it
being a new feature.

Was this a deliberate new feature?

ANSWER:

No, this was a mistake in 02-144 and thus in Fortran 2003 and 2008.
An edit is provided to correct the mistake.

EDITS to 10-007r1:

In 12.4.2.2p1, delete "it is referenced and" [279:19].

SUBMITTED BY: Malcolm Cohen

HISTORY: 11-135r1   m194 Submitted

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