Enum has allOf wrapper with single $ref value

[rvinzent]

Describe the bug

I am generating what I would think is a pretty standard enum field, but the generated schema has a useless allOf wrapper around the $ref and this causes some type issues on my client generator (fabrikt; giving up on OpenAPI Tools 😆 ), and the existing code seems like it intends to remove these allOf wrappers with a single value. For some reason, when switching to openapi version 3.1.0, the safe_ref behavior is not enabled.

This is causing the generator to interpret this as a string field instead of using an enum type.

To Reproduce

Serializers look something like this:

class MyObjectSerializer(serializers.Serializer):
    status = serializers.ChoiceField(Status.choices, default=Status.AVAILABLE)


class Status(models.TextChoices):
    AVAILABLE = "Available"

I also have some enum related configuration:

SPECTACULAR_SETTINGS = {
    "OAS_VERSION": "3.1.0",
    "ENUM_NAME_OVERRIDES": {
        "Status": "api.models.utils.constants.Status",
    },
  "ENUM_ADD_EXPLICIT_BLANK_NULL_CHOICE": False,
    "COMPONENT_SPLIT_REQUEST": True,
    "POSTPROCESSING_HOOKS": [
        "drf_spectacular.hooks.postprocess_schema_enums",
    ],

Expected behavior

I would expect the OpenAPI to look like this, and I've confirmed this fixes the generated client code.

MyObject:
  properties:
    status:
      $ref: '#/components/schemas/Status'
      default: Available

This schema is valid according to the JSON Schema reference linked out by the openapi spec.

Example valid JSON schema with $ref and default on the same level (link)

   {
       "title": "Feature list",
       "type": "array",
       "prefixItems": [
           {
               "title": "Feature A",
               "properties": {
                   "enabled": {
                       "$ref": "#/$defs/enabledToggle",
                       "default": true
                   }
               }
           },
        ]
    }

Instead, the openapi schema is generated with a useless wrapper around the $ref

MyObject:
  properties:
    status:
      allOf:
        - $ref: '#/components/schemas/Status'
      default: Available

I'm not sure if this is intentional or a bug, but it seemed like existing code was intending to remove the allOf wrapper.