[BUG][DART] anyOf serializer throws exception

Created by: szekelyisz

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?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

The generated Dart serializer code for anyOf schemas throws an exception when the referenced types are also schemas themselves.

openapi-generator version

6.3.0-SNAPSHOT 90e468b9

OpenAPI declaration file content or url

swagger.json

{
	"components": {
		"examples": {},
		"headers": {},
		"parameters": {},
		"requestBodies": {},
		"responses": {},
		"schemas": {
			"NetworkConfigManual": {
				"properties": {
					"method": {
						"type": "string",
						"enum": [
							"manual"
						],
						"nullable": false
					},
					"address": {
						"type": "string"
					},
					"prefix": {
						"type": "integer",
						"format": "int32"
					},
					"gateway": {
						"type": "string"
					},
					"dnss": {
						"items": {
							"type": "string"
						},
						"type": "array"
					}
				},
				"required": [
					"method",
					"address",
					"prefix",
					"dnss"
				],
				"type": "object",
				"additionalProperties": false
			},
			"NetworkConfigAuto": {
				"properties": {
					"method": {
						"type": "string",
						"enum": [
							"auto"
						],
						"nullable": false
					}
				},
				"required": [
					"method"
				],
				"type": "object",
				"additionalProperties": false
			},
			"NetworkConfig": {
				"anyOf": [
					{
						"$ref": "#/components/schemas/NetworkConfigManual"
					},
					{
						"$ref": "#/components/schemas/NetworkConfigAuto"
					}
				]
			}
		},
		"securitySchemes": {}
	},
	"info": {
		"title": "MyAPI",
		"version": "1.0.0"
	},
	"openapi": "3.0.0",
	"paths": {}
}

Generation Details

generate -i swagger.json -g dart-dio -o my_api/generated --additional-properties=legacyDiscriminatorBehavior=false

Steps to reproduce

Generate Dart code using the above schema and command.

Not sure if it's correct but this how I pass data to this serializer:

          networkConfig: (NetworkConfigBuilder()
                ..anyOf = AnyOf2(values: {
                  1: (NetworkConfigManualBuilder()
                        ..method = NetworkConfigManualMethodEnum.manual
                        ..address = address.address
                        ..prefix = prefix
                        ..gateway = gateway?.address
                        ..dnss = ListBuilder(dnss.map((e) => e.address)))
                      .build()
                }))
              .build()

Suggest a fix

Here's the generated Dart serializer code for the NetworkConfig schema from the above specification:

  @override
  Object serialize(
    Serializers serializers,
    NetworkConfig object, {
    FullType specifiedType = FullType.unspecified,
  }) {
    final anyOf = object.anyOf;
    final result = <Object?>[];
    for (var _valueEntry in anyOf.values.entries) {
      final _typeIndex = _valueEntry.key;
      final _type = anyOf.types[_typeIndex];
      final _value = _valueEntry.value;
      result.addAll(serializers.serialize(_value, specifiedType: FullType(_type)) as Iterable<Object?>);
    }
    return result;
  }

serializers become BuiltJsonSerializers in this case. The problem here is that the returned value by serializers.serialize is a _InternalLinkedHashMap<String, Object?> and it is force-cast to Iterable<Object?> which obviously throws an exception.

Maybe there needs to be a distinction whether the referenced data types are schemas or primitives and serialization needs to branch accordingly? I don't have enough insight into openapi-generator to tell this.