id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
566490806,MDU6SXNzdWU1NjY0OTA4MDY=,3774,Allow xr.combine_by_coords to work with point coordinates?,555667,closed,0,,,13,2020-02-17T19:53:51Z,2021-06-23T15:58:10Z,2021-06-23T15:58:10Z,NONE,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
# Your code here

data_0 = xr.Dataset({'temperature': ('time', [10,20,30])}, coords={'time': [0,1,2]})
data_0.coords['trial'] = 0

data_1 = xr.Dataset({'temperature': ('time', [50,60,70])}, coords={'time': [0,1,2]})
data_1.coords['trial'] = 1

# the following fails with a ValueError:
# all_trials = xr.combine_by_coords([data_0, data_1])

# but this works:
all_trials = xr.combine_by_coords([data_0.expand_dims('trial'), data_1.expand_dims('trial')]) # works
```
#### Expected output
Both work and produce the same result.

#### Problem Description
<!-- this should explain why the current behavior is a problem and why the expected output is a better solution -->
I found this behavior somewhat unintuitive -- I didn't understand that I needed to call `expand_dims` to get `combine_by_coords` to work. It might make things a little easier to understand if both behaved the same way -- maybe the code that concatenates dimensions should look for full dimensions first, and point coordinates second.

This would also make code that selects dimensions with `sel` and then calls `combine_by_coords` more easy to write.

On the other hand, this could be seen as ""magic"", and it might also be a breaking change to `combine_by_coords`; I'm not sure. So I'd understand if isn't implemented.

#### Output of ``xr.show_versions()``
<details>
<pre>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 (default, Jan  8 2020, 19:13:59) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.14.0-115.10.1.el7a.ppc64le
machine: ppc64le
processor: ppc64le
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.2
libnetcdf: None

xarray: 0.15.0
pandas: 0.25.3
numpy: 1.16.5
scipy: 1.3.0
netCDF4: None
pydap: None
h5netcdf: None
h5py: 2.8.0
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.10.0
distributed: 2.10.0
matplotlib: 3.1.1
cartopy: None
seaborn: None
numbagg: None
setuptools: 44.0.0.post20200106
pip: 19.3.1
conda: None
pytest: 5.3.2
IPython: 7.11.1
sphinx: None
</pre>
</details>
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