OpenAPI: Circular references break generation

[Jolanrensen]

For example: https://youtrack.jetbrains.com/api/openapi.json
According to https://apitools.dev/swagger-parser/online/, there exists a circular reference and that would explain the cannot find ref IllegalStateException that is thrown when trying to generate @DataSchemas from this OpenAPI spec.

While circular references don't work in JSON, it is possible with optional properties to support it in an OpenAPI spec.