Re: [Patch, Fortran] PR 36947/40039: Better error messages for dummy procedures and check for OPTIONAL

[Tobias Burnus <burnus at net-b dot de>]

Janus,

> this follow-up adds a check for OPTIONAL and improves the
> error messages

Thanks for improving the error messages - they are very useful!
Thanks also for the OPTIONAL check.

I will look though the patch itself later (ditto for your other
patch).

> "If the interface of the dummy argument is explicit, the
>  characteristics listed in 12.2 shall be the same for the
>  associated actual argument and the corresponding dummy argument, ..."

I think passing an actual argument with an implicit interface to
an explicit-interface dummy is also allowed. If the explicit
argument were required, the standard had written it explicitly.

(If an explicit interface is required (allocatable, optional, assumed-shape
argument etc.; both dummy/actual and proc-pointer/proc-target have to
have an explicit interface. That could actually be checked for :-)

It just states that they need to match, which means that it might not be
checkable if the procedure definition is not in host/use-associated scope
(or has an INTERFACE, which in turn shall match the actually existing
procedure).

 * * *

I think some other checks should still be added, e.g.

a) PUREness check (see example below); passing/assigning
   a pure to a non-pure dummy/proc-pointer is OK; doing vice versa
   is not.

See "12.4.1.3" (dummy-actual arguments)
 "If the interface of the dummy argument is explicit,
  the characteristics listed in 12.2 shall be the same for the
  associated actual argument and the corresponding dummy argument,
  except that a pure actual argument may be associated with a dummy
  argument that is not pure and an elemental intrinsic actual
  procedure may be associated with a dummy procedure (which is
  prohibited from being elemental)."
And also "7.4.2.2" (proc-pointer assignment):
 "If proc-pointer-object has an explicit interface, its
  characteristics shall be the same as proc-target except that
  proc-target may be pure even if proc-pointer-object is not pure
  and proc-target may be an elemental intrinsic procedure even
  if proc-pointer-object is not elemental."

b) Similarly for ELEMENTAL. For proc-pointer assignments, use the
   first example with PURE changed to ELEMENTAL. That non-intrinsic
   elementals are not allowed as actual argument, is already checked
   for (cf. C1228). Except of the remark in parentheses I could not
   find in F2003/F2008 anything which prohibits ELEMENTAL for the
   dummy argument; however, the parentheses is normative. Maybe one
   should re-check the standard before adding an error check (see
   example below).

c) One needs to go recursively over the arguments as the second
   example below shows.


Tobias


PROGRAM PURENESS
implicit none
interface
  subroutine one(a,b,c,d,e,f,g,h,i)
    implicit none
    integer,intent(in) :: a,b,c,d,e,f,g,h,i
  end subroutine one
  pure subroutine two(a,b,c,d,e,f,g,h,i)
    implicit none
    integer,intent(in) :: a,b,c,d,e,f,g,h,i
  end subroutine two
end interface
procedure(two), pointer :: ptr
ptr => one  ! Invalid: (pure) => (unpure)
end program pureness


program RecursiveInterface
  interface
    subroutine a(x)
      real :: x
    end subroutine a
    subroutine b(a)
      integer :: a
    end subroutine b
    subroutine c(f)
      procedure(a) :: f
    end subroutine c
    subroutine d(f)
      procedure(b) :: f
    end subroutine d
    subroutine e(f)
     procedure(c) :: f
    end subroutine e
  end interface
  call e(d) ! Argument (dummy subroutine) d has an integer argument
            ! but e's f expects a real argument
end program RecursiveInterface


interface
  elemental subroutine a()  ! Expected: Warning: ELEMENTAL procedure
                            ! without arguments
  ! (Having ELEMENTAL does not make much sense without arguments, but
  !  it is valid)
  end subroutine a
  subroutine sub(f)
    interface
      elemental subroutine f(a)
        integer,intent(IN) :: a
      end subroutine f
    end interface
! Invalid per 12.4.1.3?
! "an elemental intrinsic actual procedure may be associated with
!  a dummy procedure (which is prohibited from being elemental)."
!                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
! Todo: Find it elsewhere in the standard - or in the corrigenda;
!       other compilers accept it. However, the part above is normative
  end subroutine sub
end interface
end program elementalCheck