[BUG][Elm] specifying x-enum-varnames causes malformed output

Created by: r-k-b

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
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Description

Context: Server sends & expects int values for enum types, we want to use enum names in the Elm code.

Adding x-enum-varnames seems to be the recommended approach, but that leads to malformed Elm code being output.

E.g., in src/Api/Data.elm:

Expected:

intFromYesNoNa : YesNoNa -> Int
intFromYesNoNa model =
    case model of
        YesNoNaUnknown ->
            0

        YesNoNaYes ->
            1

        YesNoNaNo ->
            2

        YesNoNaNa ->
            999

Actual:

intFromYesNoNa : YesNoNa -> Int
intFromYesNoNa model =
    case model of
        YesNoNaUnknown ->
            Unknown

        YesNoNaYes ->
            Yes

        YesNoNaNo ->
            No

        YesNoNaNa ->
            Na

openapi-generator version

5.0.0-SNAPSHOT, from nixpkgs (commit f75c8753d80959f6bf63237748e799849c74179f). Doesn't seem like a regression.

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {"title": "Hippo API", "version": "v1"},
  "paths": {"/foo": {"get": {"tags": [], "summary": "x", "responses": {"200": {"description": "Success", "content": {"application/json": {"schema": {"$ref": "#/components/schemas/YesNoNa"}}}}}}}},
  "components": {
    "schemas": {
      "YesNoNa": {
        "enum": [0, 1, 2, 999],
        "x-enum-varnames": ["Unknown", "Yes", "No", "Na"],
        "x-enum-descriptions": ["Unknown", "Yes", "No", "N/A"],
        "type": "integer"
      }
    }
  }
}

Generation Details

$ openapi-generator-cli generate -i ./enumnames.json -g elm

Steps to reproduce

Generate as above
elm make src/Api/Data.elm

or, using the latest master:

$ docker pull openapitools/openapi-generator-cli:latest
$ docker run -v /home/rkb/projects/namedenums/:/host -it openapitools/openapi-generator-cli:latest generate -i /host/enumnames.json -g elm -o /host/dckr
$ elm make dckr/src/Api/Data.elm

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/5794

Suggest a fix

Looks like #917 added enum names support to the Java generator, with an updateEnumVarsWithExtensions() method