The `ArrayCopy` was simply calling `IsOnDevice` to see if the array from
the `UnknownArrayHandle` was on a device. Seems right, but it is
actually operating on an `ArrayHandleRecombineVec`. This is a special
array that mostly behaves like other `ArrayHandle`s, but because it has
variable vec size, it breaks some `ArrayHandle` conventions.
One of the iffy things it has to do is stick the dependent `Buffer`
objects into the metadata of its own `Buffer` rather than list them in
the `Buffer` list. This means that `ArrayHandle` cannot properly check
them to see where they are located. Instead, it just sees that the one
`Buffer` it has is empty.
A recent change to `IsOnDevice` made it return true for any device if
the `Buffer` is empty. So previously this was broken in that it reported
that the array was not on any device. That changed to report that it was
on all devices, even inactive ones. So the code went from not
efficiently copying to throwing an exception.
This has been fixed by pulling one of the dependent arrays and checking
that one.
42acb9a66c