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

[pault at gcc dot gnu.org]

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

--- Comment #16 from Paul Thomas <pault at gcc dot gnu.org> ---
(In reply to Paul Thomas from comment #15)
> (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

On thinking about it, is the specific instance of write_formatted allowed? it
is easily forbidden, if not.

Cheers

Paul