code: def test_union_int_categorical_with_nan():
      ci = CategoricalIndex([1, 2, np.nan])
      assert ci.categories.dtype.kind == "i"
  
      idx = Index([1, 2])
  
      result = idx.union(ci)
      expected = Index([1, 2, np.nan], dtype=np.float64)
      tm.assert_index_equal(result, expected)
  
      result = ci.union(idx)
      tm.assert_index_equal(result, expected)
  
imports: from datetime import datetime
  import operator
  import numpy
  import pytest
  from pandas._libs import lib
  from pandas.core.dtypes.cast import find_common_type
  from pandas import CategoricalDtype, CategoricalIndex, DatetimeTZDtype, Index, MultiIndex, PeriodDtype, RangeIndex, Series, Timestamp
  import pandas._testing
  from pandas.api.types import is_signed_integer_dtype, pandas_dtype
label_desc: Use image node_id 18 for calling a global function with example usage: test_union_int_categorical_with_nan() without return types