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

["damian at sourceryinstitute dot org" <gcc-bugzilla at gcc dot gnu dot org>]

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

            Bug ID: 78737
           Summary: linking error with deferred, undefined user-defined
                    derived-type I/O
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: damian at sourceryinstitute dot org
  Target Milestone: ---

A linking error results when an abstract type defers a user-defined derived
type output binding that is not implemented anywhere in the program:

$ cat undefined-ud-dtio.f90 
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
contains
  subroutine assert(a)
    class(object):: a
    write(*,*) a 
  end subroutine
end module

end

$ gfortran undefined-ud-dtio.f90 
Undefined symbols for architecture x86_64:
  "_write_formatted_interface_", referenced from:
      ___object_interface_MOD_assert in ccMQHXbf.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
rouson@localhost:~/xfr$ gfortran --version
GNU Fortran (MacPorts gcc7 7-20161127_0) 7.0.0 20161127 (experimental)

Adding a type that defines the binding eliminates the linking error.