GetIndex() Unable to Deserialize Response for Certain Alias Filters

NEST/Elasticsearch.Net version: 6.8.3 Elasticsearch version: 6.8.3 Description of the problem including expected versus actual behavior:

I’m having an issue with NEST loading index using await client.GetIndexAsync(indexName) method if the index has alias setup with certain filter format. Cluster accepts the filter, but NEST client fails.

Alias with filter:

{ "actions": [
	{
            "add": {
                "index": ".es-test-repro",
                "alias": ".es-test-repro-alias1",
                "filter": {
                    "bool": { "must_not": { "exists": { "field": "field1" } } }
                }
            }
	} ]
}

Error:

Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'System.Collections.Generic.IEnumerable`1[Nest.QueryContainer]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly.
To fix this error either change the JSON to a JSON array (e.g. [1,2,3]) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List<T>) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object.
Path '['.es-test-repro'].aliases['.es-test-repro-alias1'].filter.bool.must_not.exists', line 1, position 90.
Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'System.Collections.Generic.IEnumerable`1[Nest.QueryContainer]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly.
To fix this error either change the JSON to a JSON array (e.g. [1,2,3]) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List<T>) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object.
Path '['.es-test-repro'].aliases['.es-test-repro-alias1'].filter.bool.must_not.exists', line 1, position 90.

Repro project: https://github.com/opnarius/nest-alias-filter-repro

I can get around the error by reformatting the filter as follow:

 "filter": {
  "bool": 
    { 
      "must_not": [ { "exists": { "field": "field1" } }  ]
     } 
}

Should Nest support this alias filter if cluster is allowing it?

[codebrain]

Should Nest support this alias filter if cluster is allowing it?

Yes, we support all responses from the cluster, regardless of their initial origin.

From your description this sounds like a bug in the client, where we need to be more lenient in instances where the server has accepted the must_not as an object, and not an array.

Thanks for the repro, that will make it easier to fix.

[codebrain]

Closing ticket as PR https://github.com/elastic/elasticsearch-net/pull/4322 was merged.