[BUG] Example values are not generated for enum types referenced with $ref

Description

openapi-generator only produces example schemas/responses for enum types when they are inlined in the response type. When an enum is $referenced, no example value is produced. Because this happens with both openapi and spring generators, I assume this is a core bug and not specific to either generator.

With the declaration file below, I get

• "Specification" : { "example" : {"bar" : "BAR_A"} } for openapi rather than the expected "Specification" : { "example" : {"foo" : "FOO_A", "bar" : "BAR_A"} } and
• setExampleResponse(request, "application/json", "{ \"bar\" : \"BAR_A\"}") for Spring rather than the expected setExampleResponse(request, "application/json", "{ \"foo\": \"FOO_A\", \"bar\" : \"BAR_A\"}").

openapi-generator version

Problem seen at least in 4.0.3, 4.1.3, latest 4.2.0-SNAPSHOT and 5.0.0-SNAPSHOT.

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  version: 0.0.1
  title: Enum example

components:
  schemas:
    Foo:
      type: string
      enum:
        - FOO_A
        - FOO_B
    Specification:
      type: object
      properties:
        foo:
          $ref: '#/components/schemas/Foo'
        bar:
          type: string
          enum:
            - BAR_A
            - BAR_B
      required:
        - foo
        - bar

paths:
  /hello:
    get:
      responses:
        200:
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Specification'

Command line used for generation

java -jar openapi-generator-cli-4.2.0-20191028.135625-129.jar generate -i api.yaml -g openapi -o oa-420/
java -jar openapi-generator-cli-4.2.0-20191028.135625-129.jar generate -i api.yaml -g spring -o spring-420/

Steps to reproduce

Generate an Openapi JSON file or a Spring application with the above command lines and check $.components.schemas.Specification.example and HelloApi::helloGet(), respectively

Related issues/PRs

Could not find anything related to examples for enums; in:title enum example returned 0 matches.

Use the supplied OpenAPI YAML declaration to reproduce the missing example with both the openapi and spring generators, then inspect the shared example-generation path. Compare the generated Specification.example and HelloApi::helloGet() output with the expected values for both the inline and $ref enum properties; done means both generators include FOO_A and BAR_A.