[Bug fortran/77657] New: link error with implementation of user-defined derived type input/output (UD-DTIO) in child extending abstract parent

[damian at sourceryinstitute dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77657

            Bug ID: 77657
           Summary: link error with implementation of user-defined derived
                    type input/output (UD-DTIO) in child extending
                    abstract parent
           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: ---

It appears there might be a problem with linking an abstract interface in the
module  of a parent type with a child type in another module:


$ cat ud-dtio-abstract.f90 
MODULE abstract_parent
  implicit none

  type, abstract :: parent
  contains
    procedure(write_formatted_interface), deferred :: write_formatted
    generic :: write(formatted) => write_formatted
  end type parent

  abstract interface
    subroutine write_formatted_interface(this,unit,iotype,vlist,iostat,iomsg)
      import parent
      class(parent), 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

end module

module child_module
  use abstract_parent, only : parent
  implicit none

  type, extends(parent) :: child
  contains
    procedure :: write_formatted
  end type
contains
  subroutine write_formatted(this,unit,iotype,vlist,iostat,iomsg)
    class(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
  end subroutine
end module

  use child_module, only : child
  implicit none
  write(*,*) child()

$ gfortran ud-dtio-abstract.f90 
/tmp/ccOmINhD.o: In function `MAIN__':
ud-dtio-abstract.f90:(.text+0xac): undefined reference to
`write_formatted_interface_'
collect2: error: ld returned 1 exit status

$ gfortran --version
GNU Fortran (GCC) 7.0.0 20160904 (experimental)