openapi-generator: [java] Compilation failure when 'discriminator' is introduced

Description

I got the following error when generate code by using openapi-generator-maven-plugin.

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project swagger-vnfm: Compilation failure
[ERROR] /swagger-vnfm/target/generated-sources/src/main/java/com/openapi/test/VimConnectionInfo.java:[43,49] incompatible types: java.lang.String cannot be converted to com.openapi.test.VimType

openapi-generator version

<version>3.2.0</version>

OpenAPI declaration file content or url

swagger: '2.0'
info:
  description: This spec is mainly for testing
  version: 1.0.0
  title: Swagger Petstore
  termsOfService: 'http://swagger.io/terms/'
  contact:
    email: apiteam@swagger.io
  license:
    name: Apache-2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
host: 'petstore.swagger.io:80'
basePath: /v2
paths:
  /test:
    get:
      description: Gets all operation occurrence resources.
      responses:
        '200':
          description: Successful response
          schema:
            type: string
        default:
          description: Error payload
          schema:
            type: string
      tags:
        - test
schemes:
  - http
definitions:
  VimType:
    description: Type of the VIM info structure.
    type: string
    enum:
      - OPENSTACK_V2
      - OPENSTACK_V3
      - VMWARE_VCLOUD
      - OTHER_VIM_INFO
  VimConnectionInfo:
    type: object
    required:
      - id
      - vimType
    discriminator: vimType
    properties:
      id:
        description: Identifier of the VIM
        type: string
      vimType:
        $ref: '#/definitions/VimType'
externalDocs:
  description: Find out more about Swagger
  url: 'http://swagger.io'

Command line used for generation

mvn clean compile

Steps to reproduce

The maven pom.xml is

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.vnfm.model</groupId>
    <artifactId>swagger-vnfm</artifactId>
    <packaging>jar</packaging>
    <version>DYNAMIC-SNAPSHOT</version>
    <name>swagger-vnfm</name>
    <url>http://maven.apache.org</url>
    <dependencies>
        <dependency>
            <groupId>io.swagger</groupId>
            <artifactId>swagger-annotations</artifactId>
            <version>${swagger-core-version}</version>
        </dependency>
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>${gson-version}</version>
        </dependency>
    </dependencies>
    <properties>
        <java.version>1.8</java.version>
        <maven.compiler.source>${java.version}</maven.compiler.source>
        <maven.compiler.target>${java.version}</maven.compiler.target>
        <swagger-core-version>1.5.21</swagger-core-version>
        <gson-version>2.8.1</gson-version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>3.2.0</version>
                <executions>
                    <execution>
                        <id>lcm-generate</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/main/resources/swagger.yaml</inputSpec>
                            <language>java</language>
                            <generateApis>false</generateApis>
                            <generateModelTests>false</generateModelTests>
                            <generateModelDocumentation>false</generateModelDocumentation>
                            <generateSupportingFiles>false</generateSupportingFiles>
                            <skipOverwrite>true</skipOverwrite>
                            <configOptions>
                                <dateLibrary>java8</dateLibrary>
                            </configOptions>
                            <output>${project.build.directory}/generated-sources</output>
                            <modelPackage>com.openapi.test</modelPackage>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Related issues/PRs

No

Suggest a fix/enhancement