[BUG] Nullable object reference produce invalid code in kotlin, php, java, typescript-fetch and probably more

Description

To represent nullable object property, openapi recommend using a oneOf constuct. In version 3.0 in can be done with the nullable property, and in 3.1, with the null type.

3.0

propertyName:
          nullable: true
          oneOf:
            - $ref: '#/components/schemas/PropertyType'

3.1

        otherPropertyName:
          oneOf:
            - 'null'
            - $ref: '#/components/schemas/PropertyType'

In kotlin, the generatated type name is incorrect : OneOfLessThanPropertyTypeGreaterThan

@Json(name = "propertyName")
val propertyName: OneOfLessThanPropertyTypeGreaterThan? = null,

In php, the code is ok but the annotations produce an invalid class name : OneOfPropertyType|null

/**
     * Sets property_name
     *
     * @param OneOfPropertyType|null $property_name property_name
     *
     * @return self
     */
    public function setPropertyName($property_name)
    {
        $this->container['property_name'] = $property_name;

        return $this;
    }

In java the type is also wrong

public OneOfPropertyType getPropertyName() {
  return propertyName;
}

public void setPropertyName(OneOfPropertyType propertyName) {
  this.propertyName = propertyName;
}

In typescipt-fetch it produces a different result with 3.0 or 3.1 syntax

// 3.0

    /**
     * 
     * @type {PropertyType}
     * @memberof ModelWithNullableObjectProperty
     */
    propertyName?: PropertyType | null;

// 3.1
    /**
     * 
     * @type {Null | PropertyType}
     * @memberof ModelWithNullableObjectProperty
     */
    otherPropertyName?: Null | PropertyType | null;

openapi-generator version

master - 5.2.1

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: 'Title'
  version: latest
paths:
  '/':
    get:
      operationId: operation
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ModelWithNullableObjectProperty'
components:
  schemas:
    ModelWithNullableObjectProperty:
      properties:
        propertyName:
          nullable: true
          oneOf:
            - $ref: '#/components/schemas/PropertyType'
        otherPropertyName:
          oneOf:
            - 'null'
            - $ref: '#/components/schemas/PropertyType'
    PropertyType:
      properties:
        foo:
          type: string

Generation Details

docker run --rm -v ${PWD}:/local --user "${USER_ID}:${GROUP_ID}" openapitools/openapi-generator-cli:latest generate -i /local/model.yaml -g kotlin -o /local/kotlin
docker run --rm -v ${PWD}:/local --user "${USER_ID}:${GROUP_ID}" openapitools/openapi-generator-cli:latest generate -i /local/model.yaml -g php -o /local/php
docker run --rm -v ${PWD}:/local --user "${USER_ID}:${GROUP_ID}" openapitools/openapi-generator-cli:latest generate -i /local/model.yaml -g java -o /local/java
docker run --rm -v ${PWD}:/local --user "${USER_ID}:${GROUP_ID}" openapitools/openapi-generator-cli:latest generate -i /local/model.yaml -g typescript-fetch -o /local/typescript

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/2090

Suggest a fix

I went deep into the source code to understand how nullable types are handled.

ModelUtils.isNullable seems to have some logic to handle this but I'm unable to make an unit test pass in this code path.

I'm willing to contribute but may need some guidance.

Start with the provided OpenAPI declaration and Docker generation commands, then inspect ModelUtils.isNullable, which the report identifies as relevant to nullable handling. Regenerate the Kotlin, PHP, Java, and typescript-fetch outputs and verify that nullable object references produce valid type names and annotations in each generator.