[BUG][Dart][built_value] additionalProperties: true on object schema causes model-to-Builder assignment error

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• [ x Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

When generating a Dart client (built_value) the deserializer assigns a concrete model instance to a Builder field when the property’s schema is an object with additionalProperties: true. This causes a build error:

type 'MyOwner' is not a subtype of type 'MyOwnerBuilder?' in type cast

Root cause: in _deserializeProperties, the generated code does

result.owner = valueDes; // valueDes is KoronaOwner

instead of assigning to the builder via .replace(...) or .toBuilder().

openapi-generator version

Using this flutter package.
https://github.com/gibahjoe/openapi-generator-dart/

But using the jar version:

"openapiGeneratorVersion": "7.15.0",

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: MRE for built_value builder assignment bug
  version: 1.0.0
paths: {}
components:
  schemas:
    MyOwner:
      type: object
      title: MyOwner
      properties:
        firstname:
          type: string
          nullable: true
        lastname:
          type: string
          nullable: true
        birthdate:
          type: string
          format: date
          nullable: true
          description: The birthdate of the owner
      additionalProperties: true   # <-- critical trigger
    PersonalizationRequest:
      type: object
      required: [ticketNumber]
      properties:
        ticketNumber:
          type: string
        owner:
          $ref: '#/components/schemas/MyOwner'

Generation Details

java -jar .dart_tool/openapi_generator_cache/openapi-generator-cli-7.15.0.jar
generate
-i ./mre.yaml
-g dart-dio
-o ./out-dart
--additional-properties=pubName=mre_client,packageName=mre_client

Steps to reproduce

Just try to compile.

  void _deserializeProperties(
    Serializers serializers,
    Object serialized, {
    FullType specifiedType = FullType.unspecified,
    required List<Object?> serializedList,
    required OmniPersonalizationRequestBuilder result,
    required List<Object?> unhandled,
  }) {
    for (var i = 0; i < serializedList.length; i += 2) {
      final key = serializedList[i] as String;
      final value = serializedList[i + 1];
      switch (key) {
        case r'ticketNumber':
          final valueDes = serializers.deserialize(
            value,
            specifiedType: const FullType(String),
          ) as String;
          result.ticketNumber = valueDes;
          break;
        case r'owner':
          final valueDes = serializers.deserialize(
            value,
            specifiedType: const FullType(MyOwner),
          ) as MyOwner;
          result.owner = valueDes; <---- Error
          break;
        default:
          unhandled.add(key);
          unhandled.add(value);
          break;
      }
    }
  }

Suggest a fix

Adjusting the template to use
.toBuilder in that case. I am not familiar with this code base so i didnt try.

Start at the Dart built_value template or generation path that produces _deserializeProperties, then reproduce the issue with the provided OpenAPI YAML and the shown 7.15.0 generation command. Done means the generated Dart client compiles for the PersonalizationRequest.owner case without the model-to-Builder assignment error.