Filtering UnionArray Changes DataType

[tustvold]

Describe the bug

When filtering a UnionArray the output will only include the types that had at least one value selected by the filter. I think this is a bug, as I would not expect a filter to mutate the schema of the input

Expected behavior

This test should pass

let mut builder = UnionBuilder::new_dense(4);
builder.append::<Int32Type>("A", 1).unwrap();
builder.append::<Float64Type>("B", 3.2).unwrap();
let array = builder.build().unwrap();
let filter_array = BooleanArray::from(vec![true, false]);
let c = filter(&array, &filter_array).unwrap();

assert_eq!(array.data_type(), c.data_type())

[tfeda]

The code runs fine for me. Perhaps it got fixed in #1589 or #1632?

[tustvold]

I can't reproduce this even on commits before those changes, so possibly a PEBCAC on my part here 😅