[BUG] [Java] Bug generating code when specs contains enum

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The OpenAPI generator generates invalid Java code with swagger 2 spec file which contains enumeration

The generated code doesn't compile

This is an example project, which demonstrates the error: https://github.com/dsurot/openapi-generator-maven-plugin-bug

openapi-generator version

It used to work with version prior to 5.3.1

OpenAPI declaration file content or url

swagger: '2.0'
info:
  description: a short description
  version: '1.0.0'
  title: A swagger 2 API with enum
  contact:
    name: API Developper
basePath: '/api/v1'
tags:
  - name: api-end-point
    description: API End Point
schemes:
  - https
paths:
  /api:
    get:
      tags:
        - api-end-point
      operationId: apiUsingGET
      produces:
        - '*/*'
      responses:
        '200':
          description: OK
          headers:
            X-TraceId:
              type: string
          schema:
            type: array
            items:
              $ref: '#/definitions/ObjectWithEnum'
        '400':
          description: Bad Request
          headers:
            X-TraceId:
              type: string
securityDefinitions:
  basicAuth:
    type: basic
  apiKey:
    type: apiKey
    name: apikey
    in: header
security:
  - basicAuth: []
  - apiKey: []
definitions:
  ObjectWithEnum:
    description: Object with enum.
    properties:
      amount:
        example: '56.99'
        format: double
        type: number
      currency:
        example: EUR
        type: string
      status:
        allOf:
          - $ref: '#/definitions/StatusEnum'
          - example: WAITING_FOR_PAYMENT
            readOnly: true
    required:
      - amount
      - currency
    type: object
  StatusEnum:
    description: "Status Enum"
    enum:
      - WAITING_FOR_PAYMENT
      - PAYMENT_OK
      - PAYMENT_KO
      - PAYMENT_NOT_ATTEMPTED
    type: string

Generation Details

Maven configuration

<plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>
        <version>5.3.1</version>
        <configuration>
          <generateModelTests>false</generateModelTests>
          <generateApiTests>false</generateApiTests>
          <configOptions>
            <dateLibrary>java8</dateLibrary>
            <serializableModel>true</serializableModel>
            <openApiNullable>false</openApiNullable>
          </configOptions>
        </configuration>
        <executions>
          <execution>
            <id>swagger2-with-enum</id>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>${project.basedir}/src/main/resources/specs/swagger2-with-enum.yml
              </inputSpec>
              <generatorName>java</generatorName>
              <library>resttemplate</library>
              <apiPackage>com.accor.issue.api</apiPackage>
              <modelPackage>com.accor.issue.client.model</modelPackage>
              <templateDirectory>${project.basedir}/src/main/resources/templates</templateDirectory>
            </configuration>
          </execution>
        </executions>
      </plugin>

Steps to reproduce

mvn clean compile on project https://github.com/dsurot/openapi-generator-maven-plugin-bug

Start by running mvn clean compile in the linked example project with the provided Swagger 2 specification, then inspect the Java generator output for the enum and its allOf usage. Trace the relevant Java generator or template entry point that produces the invalid model code. Done means the specification generates Java code that compiles with the reported Maven configuration.