Fortran polymorphic array assignment for `PACK`: issues - arrays

Question

I am trying to code a computationally efficient PACK operation over a polymorphic array and I am running on issues with gfortran 9.2.0:
The PACK operation has to work on a polymorphic array of a derived type quantity, and return a result on itself
For reasons I'm not explaining here, this array should not be reallocated
In general, there is overlap between the locations of the returned indices, and those of the original array: something like array(1:5) = array([2,4,6,8,10])
I'm having problems, as the only version of the assigment I've tried with gfortran is with a loop - all array-based version either produce compiler or runtime segfaults.
An example is reported in this program:
module m
implicit none
type, public :: t
integer :: i = 0
contains
procedure, private, pass(this) :: t_assign => t_to_t
generic :: assignment(=) => t_assign
end type t
type, public, extends(t) :: tt
integer :: j = 0
contains
procedure, private, pass(this) :: t_assign => t_to_tt
end type tt
contains
elemental subroutine t_to_t(this,that)
class(t), intent(inout) :: this
class(t), intent(in ) :: that
this%i = that%i
end subroutine t_to_t
elemental subroutine t_to_tt(this,that)
class(tt), intent(inout) :: this
class(t ), intent(in ) :: that
this%i = that%i
select type (thatPtr=>that)
type is (t)
this%j = 0
type is (tt)
this%j = thatPtr%j
class default
! Cannot stop here
this%i = -1
this%j = -1
end select
end subroutine t_to_tt
end module m
program test_poly_pack
use m
implicit none
integer, parameter :: n = 100
integer :: i,j
class(t), allocatable :: poly(:),otherPoly(:)
allocate(t :: poly(n))
allocate(t :: otherPoly(10))
! Assign dummy values
forall(i=1:n) poly(i)%i = i
! Array assignment with indices => ICE segfault:
! internal compiler error: Segmentation fault
otherPoly(1:10) = poly([10,20,30,40,50,60,70,80,90,100])
! Scalar assignment with loop -> OK
do i=1,10
otherPoly(i) = poly(10*i)
end do
! Array assignment with PACK => Compiles OK, Segfault on runtime. GDB returns:
! Thread 1 received signal SIGSEGV, Segmentation fault.
! 0x000000000040163d in m::t_to_t (this=..., that=...) at test_poly_pack.f90:31
! 31 this%i = that%i
otherPoly(1:10) = pack(poly,mod([(j,j=1,100)],10)==0)
do i=1,10
print *, ' polymorphic(',i,')%i = ',otherPoly(i)%i
end do
end program test_poly_pack
Am I doing anything wrong, and/or is this only a compiler bug or there is any best practices I should be following?

Answer 1

The crashes are compiler bugs. When the compiler says internal compiler error ... Please submit a full bug report, you really can trust it and you should act accordingly (and submit the bug report). The runtime crash is a compiler bug as well (wrong code).
If you know the actual types at the time of the assignment, you can use type guards
select type (p => poly)
type is (t)
select type(op => otherpoly)
type is (t)
op(1:10) = pack(p,mod([(j,j=1,100)],10)==0)
end select
end select
If you need it to be polymorphic - you probably have to reallocate
allocate(otherPoly(1:10),source = pack(poly,mod([(j,j=1,100)],10)==0))
until the bugs you hopefully reported are fixed.