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
1194945072,I_kwDOAMm_X85HOWow,6447,allow merging datasets where a variable might be a coordinate variable only in a subset of datasets,2448579,open,0,,,1,2022-04-06T17:53:51Z,2022-11-16T03:46:56Z,,MEMBER,,,,"### Is your feature request related to a problem?

Here are two datasets, in one `a` is a data_var, in the other `a` is a coordinate  variable. The following fails
``` python
import  xarray as xr

ds1 = xr.Dataset({""a"": ('x', [1, 2, 3])})
ds2 = ds1.set_coords(""a"")
ds2.update(ds1)
```
with
```
    649 ambiguous_coords = coord_names.intersection(noncoord_names)
    650 if ambiguous_coords:
--> 651     raise MergeError(
    652         ""unable to determine if these variables should be ""
    653         f""coordinates or not in the merged result: {ambiguous_coords}""
    654     )
    656 attrs = merge_attrs(
    657     [var.attrs for var in coerced if isinstance(var, (Dataset, DataArray))],
    658     combine_attrs,
    659 )
    661 return _MergeResult(variables, coord_names, dims, out_indexes, attrs)

MergeError: unable to determine if these variables should be coordinates or not in the merged result: {'a'}
```

### Describe the solution you'd like

I think we should replace this error with a warning and arbitrarily choose to either convert `a` to a coordinate variable or a data  variable.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/6447/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue
1194790343,I_kwDOAMm_X85HNw3H,6445,map removes non-dimensional coordinate variables,2448579,open,0,,,0,2022-04-06T15:40:40Z,2022-04-06T15:40:40Z,,MEMBER,,,,"### What happened?

``` python
ds = xr.Dataset(
    {""a"": (""x"",  [1, 2, 3])},
    coords={""c"": (""x"", [1, 2, 3]), ""d"": (""y"", [1, 2, 3, 4])}
)
print(ds.coords)
mapped = ds.map(lambda x: x)
print(mapped.coords)
```

Variables `d` gets dropped in the `map` call. It does not share any dimensions with any of the data variables.
```
Coordinates:
    c        (x) int64 1 2 3
    d        (y) int64 1 2 3 4
Coordinates:
    c        (x) int64 1 2 3
```

### What did you expect to happen?

_No response_

### Minimal Complete Verifiable Example

_No response_

### Relevant log output

_No response_

### Anything else we need to know?

_No response_

### Environment

xarray 2022.03.0","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/6445/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue