html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/1533#issuecomment-538533149,https://api.github.com/repos/pydata/xarray/issues/1533,538533149,MDEyOklzc3VlQ29tbWVudDUzODUzMzE0OQ==,2448579,2019-10-04T19:35:55Z,2019-10-04T19:35:55Z,MEMBER,"Adding 
``` python
if not isinstance(coords, Frozen):
    coords.update(indexes)
```
before the `merge_core` call fixes this. Not sure if its right.

https://github.com/pydata/xarray/blob/283b4feba601e9838ed538106b2091d0b8264c77/xarray/core/merge.py#L455-L462","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,253395960
https://github.com/pydata/xarray/issues/1533#issuecomment-537541838,https://api.github.com/repos/pydata/xarray/issues/1533,537541838,MDEyOklzc3VlQ29tbWVudDUzNzU0MTgzOA==,2448579,2019-10-02T15:16:40Z,2019-10-02T15:16:40Z,MEMBER,Haha I thought it was already in!,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,253395960
https://github.com/pydata/xarray/issues/1533#issuecomment-537518889,https://api.github.com/repos/pydata/xarray/issues/1533,537518889,MDEyOklzc3VlQ29tbWVudDUzNzUxODg4OQ==,2448579,2019-10-02T14:28:01Z,2019-10-02T14:28:01Z,MEMBER,"Nice example!

I tracked this down to
1. `as_variable` being called when extracting the index. This seems sensible.
    https://github.com/pydata/xarray/blob/21705e61503fb49f000186c0d556e5623bd5ac82/xarray/core/merge.py#L413-L420
2. `as_variable` being called again in the `expand_variable_dicts` call in `merge_core` (should be avoided):
https://github.com/pydata/xarray/blob/21705e61503fb49f000186c0d556e5623bd5ac82/xarray/core/merge.py#L493-L497

We need to replace the appropriate objects in `aligned` with IndexVariables from `indexes` if `indexes is not None`.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,253395960