I'm not sure what should be confirmed. `groupby._groupby_indices` looks called from `Grouping`, but `Grouping` is not used during `TimeGrouper` process? 

If I call the function directly, I get following results.

```
import pandas as pd
idx = pd.DatetimeIndex(start='2013-01-31', freq='M', periods=12)
b = pd.core.groupby.BinGrouper([2, 2, 2, 2, 2, 2, 2, 2, 2, 6, 6, 8], idx)

pd.core.groupby._groupby_indices(b)
TypeError: len() of unsized object

pd.core.groupby._groupby_indices(b.bins)
{8: array([11]), 2: array([0, 1, 2, 3, 4, 5, 6, 7, 8]), 6: array([ 9, 10])}
```

Could you show me your expectation?