[BUG] OpenApiNormalizer default setting SIMPLIFY_ONEOF_ANYOF does not simplify oneOf with single non-null subSchema

[Mattias-Sehlstedt]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
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• What's the actual output vs expected output?
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Description

The normalization setting SIMPLIFY_ONEOF_ANYOF misses to simplify oneOf schemas that has one single non-null subSchema.

This seems to be due to a misconfiguration where the SubSchemas.lengh == 1 is only checked if there was at least one nullable subSchema.

Even if there are no nullable subSchemas we should still check if the schema can be simplified when there is only a single subSchema.

openapi-generator version

7,12

OpenAPI declaration file content or url

ParentWithOneOfProperty:
  type: object
  properties:
    number:
      oneOf:
        - $ref: '#/components/schemas/Number'

Generation Details

Steps to reproduce

Introduce an additional test case to this test that verifies that ParentWithOneOfProperty is properly simplified to:

ParentWithOneOfProperty:
  type: object
  properties:
    number:
      - $ref: '#/components/schemas/Number'

The test will fail since the schema isn't simplified.

Related issues/PRs

Suggest a fix

I will create a PR for a suggested fix