[BUG][Java][Spring Boot] inheritance not working as in Openapi specification described.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix

Description

I try to use the Open API specification 3.0.2 to generate an API that uses inheritance.
My reference is inheritance and I try to make use of the Polymorphism.
When I do it as described the reference to the parent type is not used and a non-existing class is referenced.
Additional there are unused classes generated DogAllOf.java and CatAllOf.java that are never used.

openapi-generator version

<plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.0</version>
</plugin>

OpenAPI declaration file content or url

https://github.com/viesure/openapi-generator-inheritance/blob/master/src/main/resources/api/openapi.yaml

Command line used for generation

./mvn clean generate-sources

Steps to reproduce

Executed with AdoptOpenJDK11.0.4

git clone git@github.com:viesure/openapi-generator-inheritance.git
cd openapi-generator-inheritance
./mvnw clean compile

To see the compile error.

I created the workaround branch that uses only the parent type as a reference and this seems to work, but I could not find a definition in the Open API specification that you should do this.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/1685
This is the one that came closest to my problem. But is using a different approach to Add the model to the API.

Suggest a fix

Depending on if it is a spec definition example on my side, giving a better example spec for an inheritance? I was not to find one within the generator.

Start with the reproduction repository and its src/main/resources/api/openapi.yaml, then run ./mvnw clean compile to inspect the generated Java compilation error. Compare the generated inheritance model classes, including DogAllOf.java and CatAllOf.java, with the OpenAPI inheritance example; done means the generated project compiles without references to nonexistent classes and does not contain unused inheritance classes.