[BUG] Property isEnum false for referenced ENUM schemas

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

When generating the code of the Erlang server, I have noticed that the ENUM check would not work on one parameter. By reducing the file I found out that the generator wasn't putting the enum values in the generated code, not because allowableValues was null but because isEnum was set on false for referenced ENUM parameters.

openapi-generator version

4.0.0, commit b426bab

OpenAPI declaration file content or url

openapi: '3.0.0'
info:
  version: 1.0.0
  title: Enums Issue
paths: 
  /test:    
    get:
      operationId: TestEnum
      parameters:
        - name: paramA
          in: query
          required: true
          schema:
            $ref: '#/components/schemas/EnumString'
        - name: paramB
          in: query
          required: true
          schema:
            type: string
            enum:
              - first
              - second
              - third
      responses:
        default:
          description: unexpected error
          content:       
            application/json:
              schema:
                type: object
                properties:
                  message:
                    type: string
components:
  schemas:
    EnumString:
      type: string
      enum:
        - first
        - second
        - third

Command line used for generation

java -jar openapi-generator-cli.jar -i enums.yaml -g erlang-server -o server/

Steps to reproduce

• Create the file enums.yaml
• Generate the code with the command
• Open server/src/openapi_api.erl, look at function request_param_info(line 52)
• Compare the param infos of paramA and paramB

Obtained result:

request_param_info('TestEnum', 'paramA') ->
    #{
        source => qs_val  ,
        rules => [
            required
        ]
    };

request_param_info('TestEnum', 'paramB') ->
    #{
        source => qs_val  ,
        rules => [
            {type, 'binary'},
            {enum, ['first', 'second', 'third'] },
            required
        ]
    };

Expected result:

request_param_info('TestEnum', 'paramA') ->
    #{
        source => qs_val  ,
        rules => [
            {type, 'binary'},
            {enum, ['first', 'second', 'third'] },
            required
        ]
    };

request_param_info('TestEnum', 'paramB') ->
    #{
        source => qs_val  ,
        rules => [
            {type, 'binary'},
            {enum, ['first', 'second', 'third'] },
            required
        ]
    };

Suggest a fix

In DefaultCodegen, fromProperty, adding property.isEnum= true; at line 2068 after assigning allowableValues seems to fix the issue of the missing enum values, but doesn't assign any type to the property. To fix that I would move the type checking block right over the inline enum case as a function protected void setPropertyType(CodegenProperty property, Schema p, String name) and add two calls, one for p (setPropertyType(property, p, name)) and one for referencedSchema (setPropertyType(property, referencedSchema, name).

        //Referenced enum case:
        if (referencedSchema.getEnum() != null && !referencedSchema.getEnum().isEmpty()) {
            List<Object> _enum = referencedSchema.getEnum();

            Map<String, Object> allowableValues = new HashMap<String, Object>();
            allowableValues.put("values", _enum);
            if (allowableValues.size() > 0) {
                property.allowableValues = allowableValues;
                property.isEnum = true;
            }
        }

Start with DefaultCodegen.fromProperty and the referenced-enum handling described in the issue. Reproduce with enums.yaml using the openapi-generator-cli command, then compare request_param_info for paramA and paramB in server/src/openapi_api.erl. Done means both referenced and inline enum parameters include the type and enum rules.