Creating a subset of an array of derived type in Fortran doesn't work, what am I missing? - arrays

Question

I am having a problem trying to use subsets of an array containing instances of a type in Fortran. Creating the subset renders the contents garbage. Essentially it boils down to:
class(myType), allocatable :: instances(:)
...allocate/initialize instances here...
doSomethingWithInstances( instances ) ! Works
doSomethingWithInstances( instances((/1,2/)) ) ! Doesn't work
Here is a complete code that reproduces the problem, in this code the correct output is the values of the integers assigned to each of the instances i.e "3, 8, 16", when the subroutine is called on the subset (/1,2/) of the array it should therefore print "3, 8" however it instead prints "3, 192552":
module test
! Simple type contains integer
type :: myType
integer :: n
end type
contains
! Output the integer for myType t
subroutine saySomething(t)
class(myType) :: t
print *, t%n
end subroutine
end module
program main
use test
type(myType), allocatable :: instances(:)
! Declare an array of myType
allocate(instances(3))
instances(1) = myType(3)
instances(2) = myType(8)
instances(3) = myType(16)
! call saySomething for each element
! on the direct array and on the subset
! elements 1 and 2
print *, "Working:"
call saySomethingArray(instances)
print *, "Broken:"
call saySomethingArray(instances((/1,2/))) ! Here is the problem
contains
! Call saySomething on each element of the input array
subroutine saySomethingArray(instances)
class(myType) :: instances(:)
integer :: i
do i=1,size(instances)
call saySomething(instances(i))
enddo
end subroutine
end program
I wondered if it wasn't copying things correctly or something when it creates the subset, but I couldn't figure it out. Any help would be greatly appreciated :) cheers

Answer 1

In these cases please always tell us what is your compiler, its version and all flags used for compiling. This is a compiler bug. I can reproduce it with GCC 7. It runs correctly with Intel Fortran 16.
As a workaround, the error will go away if you declare instances in saySomethingArray as type instead of class. Also, printing the array as an array expression works correctly.
The problem also appears for other kinds of array expressions, not just vector subscripts.
Reported to GCC as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84074. Maybe it will turn up to be a duplicate of some earlier bug. This is the MCVE ([mcve]):
type :: t
integer :: n
end type
type(t) :: array(2) = [t(1),t(2)]
call sub(array((/1,2/)))
contains
subroutine sub(a)
class(t) :: a(:)
integer :: i
print *, "loop a(i) :"
do i=1,size(a)
print *,a(i)%n
enddo
print *, "a%n :",a%n
print *, "a(1:size(a))%n :",a(1:size(a))%n
end subroutine
end program
Output:
> gfortran-7 vecsubs2.f90
> ./a.out
loop a(i) :
2
0
a%n : 1 2
a(1:size(a))%n : 1 2