Error 8304 is "In an intrinsic assignment statement, variable shall not be polymorphic". This is a new feature in Fortran 2008 which we do not yet support.

The key to what you want is to use ALLOCATE with SOURCE= being a RESHAPE of the original array plus padding. Here's a contrived example:

type base
integer i
end type base

type, extends(base) :: ext
real r
end type ext

class(base), allocatable :: arr(:), tmp(:)

allocate (ext::arr(3))
select type (arr)
type is (ext)
do i=1,3
arr(i)%i = i
arr(i)%r = real(i)
end do
end select

allocate(tmp(4),source=reshape(arr,[4],pad=[arr(3)]))


call move_alloc (from=tmp, to=arr)

do i=1,4
print *, arr(i)%i
end do
end

Ah, that's a somewhat different thing. I can see the bug but am not sure yet if it is in the ALLOCATE(SOURCE=) or the MOVE_ALLOC. Let me play with this some more. The basic issue is that the t_1_v subcomponents are getting deallocated when they shouldn't be, or the ALLOCATE is failing to copy them properly.

The actual problem is that the RESHAPE is not allocating new storage for the value it gets from Y(1) and is simply reusing the existing storage for Y(1)%t_1_V. Later, when the MOVE_ALLOC is done, the old Y is deallocated and the value goes along with it. I have escalated this as issue DPD200175268, but I suspect it is actually the same issue as a previously reported problem in a different context. Even if you rewrite it as this:

allocate(x(2))
x = reshape(y,[2],pad=[t(t_1(2))])

the same problem occurs.

You would get it here. I have asked the developer, to whom this issue is assigned, for an update. I reran the test and see the issue is still not fixed.