What happened?

Since #7561 with xarray-2023.5.0, the new grouper object raises an unexpected exception with an IndexVariable.

What did you expect to happen?

With xarray-2023.3.0 there was no issue, the grouper operation returned a new DataArray object.

Minimal Complete Verifiable Example

```Python import numpy as np import pandas as pd import xarray as xr

da = xr.DataArray( np.linspace(0, 1826, num=1827), coords=[pd.date_range("2000-01-01", "2004-12-31", freq="D")], dims="time", ) iv = xr.IndexVariable(dims=("time",), data=pd.Index(da.time.dt.year))

This is where the exception is raised

m = da.groupby(iv).mean() print(m) ```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

```Python

UnboundLocalError Traceback (most recent call last) Cell In[1], line 13 10 iv = xr.IndexVariable(dims=("time",), data=pd.Index(da.time.dt.year)) 12 # This is where the exception is raised ---> 13 m = da.groupby(iv).mean() 14 print(m)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/dataarray.py:6503, in DataArray.groupby(self, group, squeeze, restore_coord_dims) 6495 from xarray.core.groupby import ( 6496 DataArrayGroupBy, 6497 ResolvedUniqueGrouper, 6498 UniqueGrouper, 6499 _validate_groupby_squeeze, 6500 ) 6502 _validate_groupby_squeeze(squeeze) -> 6503 rgrouper = ResolvedUniqueGrouper(UniqueGrouper(), group, self) 6504 return DataArrayGroupBy( 6505 self, 6506 (rgrouper,), 6507 squeeze=squeeze, 6508 restore_coord_dims=restore_coord_dims, 6509 )

File <string>:6, in init(self, grouper, group, obj)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/groupby.py:335, in ResolvedGrouper.post_init(self) 334 def post_init(self) -> None: --> 335 self.group: T_Group = _resolve_group(self.obj, self.group) 337 ( 338 self.group1d, 339 self.stacked_obj, 340 self.stacked_dim, 341 self.inserted_dims, 342 ) = _ensure_1d(group=self.group, obj=self.obj)

File /tmp/py310/lib/python3.10/site-packages/xarray/core/groupby.py:640, in _resolve_group(obj, group) 637 else: 638 newgroup = group --> 640 if newgroup.size == 0: 641 raise ValueError(f"{newgroup.name} must not be empty") 643 return newgroup

UnboundLocalError: local variable 'newgroup' referenced before assignment ```

Anything else we need to know?

With xarray-2023.3.0 the output for the example is: <xarray.DataArray (time: 5)> array([ 182.5, 548. , 913. , 1278. , 1643.5]) Coordinates: * time (time) int64 2000 2001 2002 2003 2004

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.10.6 (main, Mar 10 2023, 10:55:28) [GCC 11.3.0] python-bits: 64 OS: Linux OS-release: 4.4.0-19041-Microsoft machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: C.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: None libnetcdf: None xarray: 2023.5.0 pandas: 1.5.3 numpy: 1.24.3 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None iris: None bottleneck: None dask: None distributed: None matplotlib: 3.7.1 cartopy: None seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None flox: None numpy_groupies: None setuptools: 59.6.0 pip: 23.1.2 conda: None pytest: 7.3.1 mypy: None IPython: 8.13.2 sphinx: None