issues: 1173980959

What happened?

Got a TypeError when resampling a dataset along a dimension, mapping a function to each group. The function returns a DataArray.

Failed with : TypeError: _overwrite_indexes() got an unexpected keyword argument 'variables'

What did you expect to happen?

This worked before the merging of #5692. A DataArray was returned as expected.

Minimal Complete Verifiable Example

```Python import xarray as xr

ds = xr.tutorial.open_dataset("air_temperature")

ds.resample(time="YS").map(lambda grp: grp.air.mean("time")) ```

Relevant log output

```Python

TypeError Traceback (most recent call last) Input In [37], in <module> ----> 1 ds.resample(time="YS").map(lambda grp: grp.air.mean("time"))

File /opt/miniconda3/envs/xclim-pip/lib/python3.9/site-packages/xarray/core/resample.py:300, in DatasetResample.map(self, func, args, shortcut, kwargs) 298 # ignore shortcut if set (for now) 299 applied = (func(ds, *args, kwargs) for ds in self._iter_grouped()) --> 300 combined = self._combine(applied) 302 return combined.rename({self._resample_dim: self._dim})

File /opt/miniconda3/envs/xclim-pip/lib/python3.9/site-packages/xarray/core/groupby.py:999, in DatasetGroupByBase._combine(self, applied) 997 index, index_vars = create_default_index_implicit(coord) 998 indexes = {k: index for k in index_vars} --> 999 combined = combined._overwrite_indexes(indexes, variables=index_vars) 1000 combined = self._maybe_restore_empty_groups(combined) 1001 combined = self._maybe_unstack(combined)

TypeError: _overwrite_indexes() got an unexpected keyword argument 'variables' ```

Anything else we need to know?

In the docstring of DatasetGroupBy.map it is not made clear that the passed function should return a dataset, but the opposite is also not said. This worked before and I think the issues comes from #5692, which introduced different signatures for DataArray._overwrite_indexes (which is called in my case) and Dataset._overwrite_indexes (which is expected by the new _combine).

If the function passed to Dataset.resample(...).map should only return Datasets then I believe a more explicit error is needed, as well as some notice in the docs and a breaking change entry in the changelog. If DataArrays should be accepted, then we have a regression here.

I may have time to help on this.

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.9.6 | packaged by conda-forge | (default, Jul 11 2021, 03:39:48) [GCC 9.3.0] python-bits: 64 OS: Linux OS-release: 5.16.13-arch1-1 machine: x86_64 processor: byteorder: little LC_ALL: None LANG: fr_CA.utf8 LOCALE: ('fr_CA', 'UTF-8') libhdf5: 1.12.0 libnetcdf: 4.7.4 xarray: 2022.3.1.dev16+g3ead17ea pandas: 1.4.0 numpy: 1.20.3 scipy: 1.7.1 netCDF4: 1.5.7 pydap: None h5netcdf: 0.11.0 h5py: 3.4.0 Nio: None zarr: 2.10.0 cftime: 1.5.0 nc_time_axis: 1.3.1 PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: 1.3.2 dask: 2021.08.0 distributed: 2021.08.0 matplotlib: 3.4.3 cartopy: None seaborn: None numbagg: None fsspec: 2021.07.0 cupy: None pint: 0.18 sparse: None setuptools: 57.4.0 pip: 21.2.4 conda: None pytest: 6.2.5 IPython: 8.0.1 sphinx: 4.1.2