With incomplete or invalid dummy argument lists :


$ cat z1.f90
module m
   type t
   contains
      procedure :: s1
      procedure :: s2
      generic :: write(formatted) => s1
      generic :: write(formatted) => s2
   end type
contains
   subroutine s1 (dtv)
      class(t), intent(in) :: dtv
   end
   subroutine s2 (dtv, unit)
      class(t), intent(in) :: dtv
      integer, intent(in) :: unit
   end
end


$ cat z2.f90
module m
   type t
   contains
      procedure :: s1
      procedure :: s2
      generic :: write(formatted) => s1
      generic :: write(formatted) => s2
   end type
contains
   subroutine s1 (dtv, iomsg)
      class(t), intent(in) :: dtv
      character(*), intent(inout) :: iomsg
   end
   subroutine s2 (dtv, unit)
      class(t), intent(in) :: dtv
      integer, intent(in) :: unit
   end
end


$ gfortran-9-20180527 -c z1.f90
f951: internal compiler error: in check_dtio_interface1, at fortran/interface.c:4748
0x6b1478 check_dtio_interface1
        ../../gcc/fortran/interface.c:4748
0x6b8f05 gfc_check_dtio_interfaces(gfc_symbol*)
        ../../gcc/fortran/interface.c:4900
0x73595b do_traverse_symtree
        ../../gcc/fortran/symbol.c:4153
0x71c570 resolve_types
        ../../gcc/fortran/resolve.c:16513
0x71794c gfc_resolve(gfc_namespace*)
        ../../gcc/fortran/resolve.c:16586
0x701394 gfc_parse_file()
        ../../gcc/fortran/parse.c:6262
0x7480ef gfc_be_parse_file
        ../../gcc/fortran/f95-lang.c:204