J3/04-322

Date:    13 May 2004
To:      J3
From:    Richard Maine
Subject: Referencing deferred bindings

NUMBER:
TITLE: Referencing deferred bindings
KEYWORDS: Type-bound procedure, deferred binding
DEFECT TYPE:
STATUS: J3 consideration in progress

QUESTION: I thought that the intent was that it would be
impossible to reference a deferred binding.  However, it
doesn't appear to me that this intent was achieved.
Consider the following program (Sorry, but I don't have
any compilers up to syntax-checking this).

  program defer

    type, abstract :: t
    contains
      procedure (sub), deferred :: deferred_proc
    end type t
    type, extends(t) :: t2
    contains
      procedure :: deferred_proc => sub2
    end type t2

    class(t), pointer :: x

    nullify(x)
    call x%deferred_proc

  contains
    subroutine sub
      write (*,*) 'Hello.'
    end subroutine sub
    subroutine sub2
      write (*,*) 'Goodbye.'
    end subroutine sub2
  end program defer

Is this a valid program?  If not, what restriction of the
standard does it violate?

Note that x%deferred_proc does not require the value of x (4.5.7)
and thus is not a reference to x (2.5.6).  Therefore, [83:23-24]
does not prohibit this.  Nor is it clear that there is an intent
to prohibit invocation of type-bound procedures for disassociated
pointer objects; except in the case of deferred bindings, this
seems well-defined and potentially useful.

Because x is disassociated, its dynamic type is the same
as its declared type, thus making the interpretation of
x%nondeferred_proc reasonably clear.

ANSWER:

No, this is not a valid program.  The standard does not provide
an interpretation of this program, so [2:10] applies.  (This
deserves a better answer, but that's the only one I can get from
the standard.)

EDITS:

SUBMITTED BY: Richard Maine

HISTORY: J3/04-322   m169 Submitted