Opt vs OptNil: Generator chooses Opt vs OptNil for nearly equivalent cases, both specifying nullable?

[fiendish]

What version of ogen are you using?

$ ogen --version                                                                                                                                                                                                                                            
ogen version v1.8.1 (built with go1.23.2) linux/amd64

Can this issue be reproduced with the latest version?

Yes, 1.8.1

What did you do?

If I do something like this in my openapi spec json file:

"components": {
  "schemas": {
    "ModelA": {
      "type": "object",
      "required": [],
      "properties": {
        "propA": {
            "oneOf": [
              {
                "type": "null"
              },
              {
                "type": "object"
              }
            ],
          "description": "propA is a property.",
          "$ref": "#/components/schemas/ModelB"
        }
      }
    }
  }
}

or

"components": {
  "schemas": {
    "ModelA": {
      "type": "object",
      "required": [],
      "properties": {
        "propA": {
            "oneOf": [
              {
                "type": "null"
              },
              {
                "type": "object",
                "allOf": [
                  {
                    "$ref": "#/components/schemas/Hub.CarrierIntegration.Addresses.Address"
                  }
                ]
              }
           ],
          "description": "propA is a property.",
          "$ref": "#/components/schemas/ModelB"
        }
      }
    }
  }
}

or

(I know this isn't true 3.0 or 3.1 but ogen doesn't complain about it? 😄)

"components": {
  "schemas": {
    "ModelA": {
      "type": "object",
      "required": [],
      "properties": {
        "propA": {
          "type": "object",
          "nullable": true,
          "description": "propA is a property.",
          "$ref": "#/components/schemas/ModelB"
        }
      }
    }
  }
}

The generated output is

type ModelA struct {
	// propA is a property.
	propA OptModelB `json:"propA"`
}

Where the property was nullable, but the output is Opt and not OptNil.

But if I do something like this instead:

"components": {
  "schemas": {
    "ModelA": {
      "type": "object",
      "required": [],
      "properties": {
        "propA": {
          "type": "object",
          "allOf": [
            {
              "$ref": "#/components/schemas/ModelB"
            }
          ],
          "nullable": true,
          "description": "propA is a property."
        }
      }
    }
  }
}

Then I get the expected OptNil

type ModelA struct {
	// propA is a property.
	propA OptNilModelB `json:"propA"`
}

This is weird because ogen does apply other siblings to $ref, which I believe is an oapi 3.1 behavior.

What did you expect to see?

I expect both cases to be OptNil.

[fiendish]

N.b. I ran into this by using the discovered workaround mentioned in #1338 which I guess is no longer a good workaround.

[fiendish]

Also, I'm definitely not an expert in how OpenAPI needs to be structured to be semantically correct at every version, especially around $refs and allOf/oneOf, which seem somewhat cumbersome. So if I'm just doing it wrong that will be good to know.