Alias Datatype with multi-fields value

Hi All,

I am trying to creating a mapping with aliases that involve mapping to a multifield property. However, I am running into errors. I have read the docs on the alias datatype, however I don't think it is clear enough to determine whether what I am trying to do is allowed.

Below is a snippet of what I am trying to do.

    "properties": {
      "userId": {
        "type": "text",
        "fields": {
          "keyword": {
            "type": "keyword"
          }
        }
      },
      "deviceUsername": {
        "type": "alias",
        "path": "userId",
        "fields": {
          "keyword": {
            "type": "alias",
            "path": "userId.keyword"
          }
        }
      }
    }

This results in the following error:

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Mapping definition for [deviceUsername] has unsupported parameters:  [fields : {keyword={path=userId.keyword, type=alias}}]"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Failed to parse mapping [_doc]: Mapping definition for [deviceUsername] has unsupported parameters:  [fields : {keyword={path=userId.keyword, type=alias}}]",
    "caused_by": {
      "type": "mapper_parsing_exception",
      "reason": "Mapping definition for [deviceUsername] has unsupported parameters:  [fields : {keyword={path=userId.keyword, type=alias}}]"
    }
  },
  "status": 400
}

I have also tried the following:

    "properties": {
      "userId": {
        "type": "text",
        "fields": {
          "keyword": {
            "type": "keyword"
          }
        }
      },
      "deviceUsername": {
        "type": "alias",
        "path": "userId"
      },
      "deviceUsername.keyword": {
        "type": "alias",
        "path": "userId.keyword"
      }
    }

This results in the error:

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Failed to parse mapping [_doc]: Cannot merge a field alias mapping [deviceUsername] with a mapping that is not for a field alias."
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Failed to parse mapping [_doc]: Cannot merge a field alias mapping [deviceUsername] with a mapping that is not for a field alias.",
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "Cannot merge a field alias mapping [deviceUsername] with a mapping that is not for a field alias."
    }
  },
  "status": 400
}

Is there a way to achieve what I am trying to do, or is this not possible?