[BUG] [Kotlin] Multiple interface composition is not implemented

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

When a type implements multiple supertypes (aka the perfect case for interfaces), the generator does not extend those interfaces properly, even though it is actually importing both

openapi-generator version

7.12.0

OpenAPI declaration file content or url

components:
  schemas:
    searchInput:
      type: object
      discriminator:
        propertyName: type
        mapping:
          DOG: '#/components/schemas/dogSearchInput'
      properties:
        query:
          type: string
      required:
        - query
    animalSearchInput:
      type: object
      discriminator:
        propertyName: type
        mapping:
          DOG: '#/components/schemas/dogSearchInput'
      properties:
        id:
          type: string
      required:
        - id
    dogSearchInput:
      allOf:
        - $ref: '#/components/schemas/animalSearchInput'
        - $ref: '#/components/schemas/searchInput'
      properties:
        name:
          type: string

This is the output of the DogSearchInput class:

package no.example

import java.util.Objects
import com.fasterxml.jackson.annotation.JsonProperty
import no.example.AnimalSearchInput
import no.example.SearchInput

/**
 * 
 * @param query 
 * @param name 
 */
data class DogSearchInput(

    @get:JsonProperty("query", required = true) val query: kotlin.String,

    @get:JsonProperty("id", required = true) override val id: kotlin.String,

    @get:JsonProperty("name") val name: kotlin.String? = null
    ) : AnimalSearchInput{

}

Generation Details

Generate the code using the cli and choose kotlin-spring as the generator

Steps to reproduce

Related issues/PRs

Suggest a fix

The fix here is to just extend both interfaces. The generator is already there in terms of importing the correct interface.

Start with the supplied OpenAPI YAML and the CLI entry point for the kotlin-spring generator, then inspect the generated DogSearchInput class. Compare its declared supertypes with the imported AnimalSearchInput and SearchInput; done means the generated Kotlin class extends both interfaces.