J3/15-216
To:        J3 Members
From:      Steve Lionel
Subject:   F2008 interp: Can initial-data-target be coindexed?
Date: 2015 August 14

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NUMBER: F08/00145
TITLE: Can initial-data-target be coindexed?
KEYWORDS: initialization, pointers, coarrays
DEFECT TYPE: ! /interp assigns
STATUS: J3 consideration in progress

QUESTION:

In pointer assignment, the standard is quite clear that data-target
cannot be coindexed:

C725 (R737) A data-target shall not be a coindexed object.

And there are a couple of notes related to this:

NOTE 7.45
A data pointer and its target are always on the same image. A coarray
may be of a derived type with pointer or allocatable subcomponents. For
example, if PTR is a pointer component, Z[P]%PTR is a reference to the
target of component PTR of Z on image P. This target is on image P and
its association with Z[P]%PTR must have been established by the
execution of an ALLOCATE statement or a pointer assignment on image P.

NOTE 7.46
A pointer assignment statement is not permitted to involve a coindexed
pointer or target, see C723 and C725. This prevents a pointer
assignment statement from associating a pointer with a target on
another image. If such an association would otherwise be implied, the
association status of the pointer becomes undefined. For example, a
derived-type intrinsic assignment where the variable and expr are on
different images and the variable has an ultimate pointer component.

Fortran 2008 added the ability to specify an initial-data-target in
pointer initialization:

R505 initialization is = constant-expr
                    or => null-init
                    or => initial-data-target
R506 null-init      is function-reference

C510 (R503) If => appears in initialization, the entity shall have the
POINTER attribute. If = appears in initialization, the entity shall not
have the POINTER attribute.

C511 (R503) If initial-data-target appears, object-name shall be
data-pointer-initialization compatible with it (4.5.4.6).

Initial-data-target is defined in 4.5.4.6 and the only relevant
constraint for it is:

C461 (R443) The designator shall designate a nonallocatable variable
that has the TARGET and SAVE attributes and does not have a vector
subscript. Every subscript, section subscript, substring starting
point, and substring ending point in designator shall be a constant
expression.

The definition of "data-pointer-initialization compatible" is:

"A pointer variable or component is data-pointer-initialization
compatible with a target if the pointer is type compatible with the
target, they have the same rank, all nondeferred type parameters of the
pointer have the same values as the corresponding type parameters of
the target, and the target is contiguous if the pointer has the
CONTIGUOUS attribute."

Given this, is initializing a pointer to a coindexed object permitted?

For example:

program test
integer, save, target :: C[*]
integer, pointer :: P => C[3] ! Permitted?
end

While ordinary pointer assignment to a coindexed object is prohibited
by C725, there is no corresponding constraint prohibiting such
association by way of data pointer initialization.

Note that 16.5.2.5p1(2) says that pointer-assigning to a target on a
different image causes the pointer to become undefined. Since the
initialization occurs on all images, and it's not possible to restrict
the target to only the same image, the effect would be to make a
program that does this nonconforming.

ANSWER:

No, this was not intended to be permitted. A clarifying edit is
provided.

EDITS to 10-007r1:

[70:3] 4.5.4.6

In the first sentence of C461, insert ", noncoindexed" after
"nonallocatable" so that the sentence reads:

C461 (R443) The designator shall designate a nonallocatable,
noncoindexed variable that has the TARGET and SAVE attributes and
does not have a vector subscript.

SUBMITTED BY: Steve Lionel

HISTORY: yy-nnn    m208  F08/nnnn submitted


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