[BUG] [Spring] Externally defined subtypes are not resolved when referenced through the root

Description

Given the setup below, the generator only generates Animal and AnimalFarmEntry. The should also generate Cat and Dog.

openapi-generator version

3.3.4

OpenAPI declaration file content or url

api.yaml

openapi: 3.0.0
info:
  title: Sample API
  description: API description in Markdown.
  version: 1.0.0
paths:
  /animals:
    get:
      summary: Get an animal.
      description: Optional extended description in Markdown.
      responses:
        200:
          description: OK
          content:
            application/json:
              schema:
                $ref: 'schema.yaml#/components/schemas/AnimalFarm'
               

schema.yaml

components:
  schemas:
    Dog:
      allOf:
        - $ref: '#/components/schemas/Animal'
        - type: object
          properties:
            breed:
              type: string
    Cat:
      allOf:
        - $ref: '#/components/schemas/Animal'
        - type: object
          properties:
            breed:
              type: string
    Animal:
      type: object
      discriminator:
        propertyName: className
      required:
        - className
      properties:
        className:
          type: string
        color:
          type: string
          default: red
    AnimalFarm:
      type: array
      items:
        $ref: '#/components/schemas/AnimalFarmEntry'
    AnimalFarmEntry:
      type: object
      properties:
        animal:
          $ref: "#/components/schemas/Animal"

Command line used for generation

openapi-generate generate -g spring -i api.yaml -i generated

Steps to reproduce

Invoke the generator above using the provided inputs.

Suggest a fix

This is appears to be an issue with the Swagger parser. I've filed swagger-api/swagger-parser#970.

Reproduce the issue with api.yaml and schema.yaml using the documented OpenAPI Generator command and Spring generator. Compare the generated models with the referenced Animal, Cat, and Dog schemas, then review the related swagger-api/swagger-parser#970 report. Done means generation includes Cat and Dog when AnimalFarm is referenced through the root API definition.