To: J3                                                     J3/26-142
From: Brad Richardson & Generics
Subject: Response to UTI 012
Date: 2026-May-26

Reference: 26-007r1

UTI 012 states:

    "Not interoperating with coarrays is a big restriction.
    Interoperating with OpenMP and MPI, but not with the Fortran
    standard way of expressing parallelism, is not a good look.

    Perhaps you want a CODIMENSIONABLE or COINDEXABLE
    deferred-type-attr?

    I further note that as written, there can be at most one
    deferred-type-attr and thus no need for a list or the constraints...
    that would, of course, change if we did add
    CODIMENSIONABLE/COINDEXABLE.

To the extent that templates are not interoperating with coarrays
they are also not interoperating with MPI. The reasons are subtly
different however. Upon further inspection, we have missed an
important edit. For example, consider the following templates
differing only in their use of coarray vs MPI.

    TEMPLATE TMPL_MPI{T}
      DEFERRED TYPE :: T
    CONTAINS
      SUBROUTINE DO_THING()
        USE MPI_F08
        TYPE(T), ALLOCATABLE :: BUFF(:)
        ...
        CALL MPI_ISEND(BUFF, ...)
      END SUBROUTINE
    END TEMPLATE

    TEMPLATE TMPL_COARR{T}
      DEFERRED TYPE :: T
    CONTAINS
      SUBROUTINE DO_THING()
        TYPE(T), ALLOCATABLE :: BUFF(:)[:]
        ...
        BUFF[...] = ...
      END SUBROUTINE
    END TEMPLATE

In the former example it is invalid to pass a deferred type as an
actual argument to an assumed-type dummy argument. In the later
example it is invalid to perform coindexed access to a data
element of deferred type.

It is worth noting that in cases that it would be valid to use MPI
it is also valid to use a corray. Consider the following examples.

    TEMPLATE TMPL_MPI{T}
      DEFERRED TYPE :: T
    CONTAINS
      SUBROUTINE DO_THING()
        USE MPI_F08
        REAL, ALLOCATABLE :: BUFF(:)
        ...
        CALL MPI_ISEND(BUFF, ...)
      END SUBROUTINE
    END TEMPLATE

    TEMPLATE TMPL_COARR{T}
      DEFERRED TYPE :: T
    CONTAINS
      SUBROUTINE DO_THING()
        REAL, ALLOCATABLE :: BUFF(:)[:]
        ...
        BUFF[...] = ...
      END SUBROUTINE
    END TEMPLATE

While subgroup agrees that integration of templates and coarrays
is desirable, resource constraints make it unlikely we will be able
to sufficiently design such an integration in time for F202Y. The
COINDEXABLE attribute may very well be an appropriate method, but
we feel it needs further exploration before incorporating. We do not
believe the prohibition on coarrays of deferred type, nor the
prohibition on derived types with coarray potential subojbect
components as instantiation arguments have precluded any future
designs.

The point about the deferred-type-attr is well taken.

Edits as follows:

[367:19-21] after "argument is" add " of a deferred type, or"

            such that the sentence now reads:

    "If the actual argument is of a deferred type, or of a derived
    type that has type parameters, type-bound procedures, or final
    subroutines, the dummy argument shall not be assumed-type."

[402:1] delete "-list" from the syntax "deferred-type-attr-list"
        such that the rule becomes:

    "deferred-type-declaration-stmt is DEFERRED TYPE
        [, deferred-type-attr ] :: deferred-arg-name-list"

[402:7-8] delete constraints C1614 and C1615

[402:8+] delete UTI 012

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