code: def test_to_flat_index(self, index_flat):
      # 22866
      index = index_flat
  
      result = index.to_flat_index()
      tm.assert_index_equal(result, index)
  
imports: from copy import copy, deepcopy
  import re
  import numpy
  import pytest
  from pandas.compat import IS64
  from pandas.compat.numpy import np_version_gte1p25
  from pandas.core.dtypes.common import is_integer_dtype, is_numeric_dtype
  import pandas
  from pandas import CategoricalIndex, MultiIndex, PeriodIndex, RangeIndex
  import pandas._testing
label_desc: Use image node_id 5 for calling the TestCommon obj's underlying member method code with example usage: obj.test_to_flat_index(index_flat) without return types