ak.unflatten on a nested IndexedOptionArray typetracer hits a bug trying to calculate lengths

[lgray]

Version of Awkward Array

2.5.1

Description and code to reproduce

import json
import awkward as ak

fromjson = {
    "class": "ListOffsetArray",
    "offsets": "i64",
    "content": {
        "class": "ListOffsetArray",
        "offsets": "i64",
        "content": {
            "class": "IndexedOptionArray",
            "index": "i64",
            "content": {
                "class": "RecordArray",
                "fields": [
                    "pt",
                ],
                "contents": [
                    {
                        "class": "NumpyArray",
                        "primitive": "float32",
                        "inner_shape": [],
                        "parameters": {"__doc__": "pt"},
                        "form_key": None,
                    },
                ],
                "parameters": {
                    "__record__": "PFCand",
                    "__doc__": "interesting particles from various jet collections",
                    "collection_name": "PFCands",
                },
                "form_key": None,
            },
            "parameters": {},
            "form_key": None,
        },
        "parameters": {},
        "form_key": None,
    },
    "parameters": {},
    "form_key": None,
}

form = ak.forms.from_json(json.dumps(fromjson))

ttlayout, report = ak.typetracer.typetracer_with_report(form)

ttarray = ak.Array(ttlayout, behavior=ak.behavior)

flattened = ak.flatten(ttarray, axis=2)

counts = ak.flatten(ak.num(ttarray, axis=2), axis=1)

unflattened = ak.unflatten(flattened, counts, axis=1)

Yields the following error:

Traceback (most recent call last):
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/_dispatch.py", line 62, in dispatch
    next(gen_or_result)
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/operations/ak_unflatten.py", line 90, in unflatten
    return _impl(array, counts, axis, highlevel, behavior, attrs)
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/operations/ak_unflatten.py", line 98, in _impl
    ctx.unwrap(array, allow_record=False, primitive_policy="error").to_packed(),
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/contents/listoffsetarray.py", line 2219, in to_packed
    next_content = next._content[: next._offsets[-1]].to_packed()
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/contents/indexedoptionarray.py", line 1725, in to_packed
    len(original_index) - num_none
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/_nplikes/typetracer.py", line 320, in __len__
    raise AssertionError(
AssertionError: bug in Awkward Array: attempt to get length of a TypeTracerArray

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "form_madness16.py", line 54, in <module>
    unflattened = ak.unflatten(flattened, counts, axis=1)
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/_dispatch.py", line 70, in dispatch
    return gen_or_result
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/_errors.py", line 85, in __exit__
    self.handle_exception(exception_type, exception_value)
  File "/Users/lgray/miniforge3/envs/coffea-dev/lib/python3.8/site-packages/awkward/_errors.py", line 95, in handle_exception
    raise self.decorate_exception(cls, exception)
AssertionError: bug in Awkward Array: attempt to get length of a TypeTracerArray

See if this has been reported at https://github.com/scikit-hep/awkward/issues

[lgray]

@cmoore24-24