[BUG][JAVA] Subclass model was classified as a free-form object even though the super class has a property in it.

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

    Response:
      type: object
      description: The response.
      oneOf:
        - $ref: '#/components/schemas/SuccessResponse'
        - $ref: '#/components/schemas/ErrorResponse'
      discriminator:
        propertyName: type
        mapping:
          SUCCESS: '#/components/schemas/SuccessResponse'
          ERROR: '#/components/schemas/ErrorResponse'

    SuccessResponse:
      type: object
      description: The success response.

    ErrorResponse:
      type: object
      description: The error response.
      properties:
        errorCode:
          type: string
      required:
        - errorCode

I'm getting Model SuccessResponse not generated since it's a free-form object even if SuccessResponse technically has a type property inherited from Response and is definitely not a free-form object.

openapi-generator version

4.0.0

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/2044 fixed the case where the object/schema has a oneOf/allOf/anyOf but it didn't fix the case where the object is part of a oneOf.

Suggest a fix

A subclass should still be generated even if it's empty as long as the superclass is not empty.

Start by reproducing Java generation with the provided oneOf schema and its inherited property, then read the approach from PR 2044. Trace the free-form-object classification for the subclass and compare it with the superclass handling. Done means the subclass is generated when its superclass has properties, with regression coverage for this schema.