Fortran 2023: An array or allocatable object may have a coarray component

[Federchen]

We are currently discussing the following new Fortran 2023 feature:

See this quote from Modern Fortran explained, Fortran 2023 edition, chapter 23.6.1, p.455-456:

“…, Fortran 2018 requires that a variable or component that is of a type with a coarray ultimate component be a non-pointer non-allocatable non-coarray scalar. Fortran 2023 permits such a variable or component to be an array and/or allocatable, … This can happen at any depth of component selection,… ”

I did interpret that this would allow a coding style as shown with the simple test case below:

program main
end program main


module test
implicit none
private

type, public :: a
  private
  integer, public, allocatable, dimension(:), codimension[:] :: coarray
end type a

type, public :: b
  private
  type(a), public, allocatable, dimension(:) :: channels_1 ! compile time error 
  ! with ifx 2025.0.0:
  ! error #8474: A data component whose type has a coarray potential subobject 
  ! component must be a nonpointer nonallocatable scalar.
end type b

type(a), public, allocatable, dimension(:) :: channels_2 ! compiles and runs 
  ! perfectly with ifx 2025.0.0

end module test

But the compiler engineers do validly point to the Fortran 2023 standards text:

“C753 A data component whose type has a coarray potential subobject component shall be a nonpointer non-allocatable scalar and shall not be a coarray.”

Any idea what is wrong here? Did I mistake Modern Fortran explained? Or is there an error in the Fortran 2023 standard text, or is Modern Fortran explained wrong?

Regards

[AniruddhaDas]

This has been addressed in the paper 25-100 with the number F23/009 titled coarray sub-object of component.

[Federchen]

@AniruddhaDas

This has been addressed in the paper 25-100 with the number F23/009 titled coarray sub-object of component.

Thanks very much for pointing to this. This already solves the issue:

NUMBER: F23/009
TITLE: Coarray subobject of component
KEYWORDS: coarray, allocatable, array, component
DEFECT TYPE: Erratum
STATUS: Passed by J3 meeting

QUESTION:

The Introduction of Fortran 2023 says
“A data object with a coarray component can be an array or
allocatable.”

This appears to be true for named variables, but there is a constraint
that makes it impossible for an array component or an allocatable
component:
“C753 A data component whose type has a coarray potential subobject
component shall be a nonpointer nonallocatable scalar and
shall not be a coarray.”

That means that given the type

Type real_coarray
  Real,Allocatable :: c[:]
End Type

the statements

Type(real_coarray) x(100)
Type(real_coarray),Allocatable :: y

are acceptable as type declaration statements, but unacceptable as
component definition statements.

Is this irregularity deliberate?

ANSWER:

No, this constraint was accidentally overlooked when extending types
with coarray components to be subobjects of arrays and allocatables.

An edit is provided to correct this mistake.

EDIT to N2218 (Fortran 2023 FDIS):

[79:constraint C753] In subclause
Delete this constraint, which begins
“C753 A data component whose type has a coarray”

[80:After NOTE 1] Insert new NOTE
“NOTE 1.5
A data component whose type has a coarray potential subobject
component cannot be a coarray or a pointer, see constraint C825.”
{C825 says “An entity whose type has a coarray potential subobject…”
and components are certainly entities. We specifically wrote “entity”
instead of “named variable” to cover the component case.
I prefer to say it once and refer to it.}

SUBMITTED BY: John Reid & Reinhold Bader

HISTORY: 23-210 m231 Submitted
23-210r1 m231 Revised
23-210r2 m231 Passed as amended by J3 meeting 231.

Interestingly, I was already aware of C825 using the word “entity” and thought hopefully this could be for exactly that case.

Regards