NullableConverter produces an invalid nested type array for multi-type nullable schemas

Start with NullableConverter.normalize! and nullable_to_type_null!, then reproduce the issue with the schema shown in the report. Verify that a multi-type schema remains a flat type array containing null, and confirm the generated document is accepted by the affected validation tools.

Summary

When an OpenAPI 3.1+ schema contains multiple non-null types and null, NullableConverter does not round-trip it correctly.

A valid schema such as:

type: [string, number, boolean, object, array, "null"]

is rewritten as:

type:
  - [string, number, boolean, object, array]
  - "null"

The nested array is invalid JSON Schema and causes consumers such as oasdiff to reject the generated document.

Reproduction

require "rspec/openapi"

schema = {
  type: ["string", "number", "boolean", "object", "array", "null"],
}

RSpec::OpenAPI::NullableConverter.normalize!(schema)
RSpec::OpenAPI::NullableConverter.to_json_schema!(schema)

pp schema

Actual result

{
  type: [
    ["string", "number", "boolean", "object", "array"],
    "null",
  ],
}

Expected result

{
  type: ["string", "number", "boolean", "object", "array", "null"],
}

The conversion should preserve a flat array of type names.

Cause

nullable_to_type_null! always wraps the existing type:

[type, "null"]

After normalize!, type can already be an array containing multiple non-null types. Wrapping that array creates the invalid nested result.

The array case could append null instead:

case type
when nil
  "null"
when Array
  type | ["null"]
else
  [type, "null"]
end

Impact

oasdiff validate and oasdiff breaking fail while loading the generated contract:

json: cannot unmarshal array into field Schema.type of type string

This also prevents tools expecting a valid JSON Schema type array from consuming the generated OpenAPI document.

Workaround

Expressing the union with anyOf avoids the faulty conversion:

anyOf:
  - type: string
  - type: number
  - type: boolean
  - type: object
  - type: array
  - type: "null"

Environment

• rspec-openapi: 0.33.0
• The same converter implementation is present in 0.33.1
• OpenAPI: 3.2.0
• Ruby: 4.0.5