[Bug fortran/78737] [OOP] linking error with deferred, undefined user-defined derived-type I/O

["pault at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78737

--- Comment #15 from Paul Thomas <pault at gcc dot gnu.org> ---
(In reply to Damian Rouson from comment #14)
> Hi Paul,
> 
> Based on comment #12, I assume you no longer believe that type guarding is
> required.  If I misinterpreted comment #12, please let me know. Otherwise,
> please let me know what in the standard indicates that type guarding is
> required.  I too would be surprised.

Dear Damian and Janus,

What I meant is that we did not implement dynamic dispatch of the dtio
procedures. In the corrected testcase of comments #12 and #13, where the first
argument of write_formatted has declared type 'object', it is necessary to use
type guarding within write_formatted to obtain the appropriate IO.

I have to confess that this "interpretation" is arrived at by the absence of
any mention of the need for dynamic dispatch and by the implementation by other
vendors.

See this further example, which works with the last version of the patch.

module object_interface
  type, abstract :: object
  contains
    procedure(write_formatted_interface), deferred :: write_formatted
    generic :: write(formatted) => write_formatted
  end type
  abstract interface
    subroutine write_formatted_interface(this,unit,iotype,vlist,iostat,iomsg)
      import object
      class(object), intent(in) :: this
      integer, intent(in) :: unit
      character (len=*), intent(in) :: iotype
      integer, intent(in) :: vlist(:)
      integer, intent(out) :: iostat
      character (len=*), intent(inout) :: iomsg
    end subroutine
  end interface
  type, extends(object) :: non_abstract_child
    integer :: i
  contains
    procedure :: write_formatted => write_formatted2
  end type
contains
  subroutine write_formatted(this,unit,iotype,vlist,iostat,iomsg)
    class(object), intent(in) :: this
    integer, intent(in) :: unit
    character (len=*), intent(in) :: iotype
    integer, intent(in) :: vlist(:)
    integer, intent(out) :: iostat
    character (len=*), intent(inout) :: iomsg
    select type (this)
      class is (non_abstract_child)
        print '(a,i4/)', "write_formatted => ", this%i
        print *, this
      class default
        print *, "Error"
    end select
  end subroutine
  subroutine write_formatted2(this,unit,iotype,vlist,iostat,iomsg)
    class(non_abstract_child), intent(in) :: this
    integer, intent(in) :: unit
    character (len=*), intent(in) :: iotype
    integer, intent(in) :: vlist(:)
    integer, intent(out) :: iostat
    character (len=*), intent(inout) :: iomsg
    print '(a,i4/)', "write_formatted2 => ", this%i
  end subroutine
  subroutine assert(a)
    class(object):: a
    write(*,*) a
  end subroutine
end module

  use object_interface
  class (object), allocatable :: z
  allocate (z, source = non_abstract_child (99))
  call assert (z)
end

Is this now OK?

Paul