[BUG] $JsonSerializableConverter fails to parse List<DateTime> responses

[LeventeAsztalosAlpian]

$JsonSerializableConverter fails to parse List<DateTime> responses

Describe the bug

The generated $JsonSerializableConverter cannot handle API responses that return List<DateTime>. The converter has special handling for single DateTime responses, but when the response is List<DateTime>, it falls through to super.convertResponse() which uses cast<DateTime>() on the list of strings. This fails at runtime because strings cannot be cast to DateTime - they must be parsed using DateTime.parse().

To Reproduce

Swagger spec:

openapi: 3.0.0
paths:
  /appointments/availability:
    post:
      responses:
        '200':
          description: Available appointment slots
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string
                  format: date-time

Generated code produces a method with return type Future<Response<List<DateTime>>>.

When calling the endpoint and accessing response.body, you get:

type 'String' is not a subtype of type 'DateTime' in type cast

The issue is in the generated $JsonSerializableConverter:

class $JsonSerializableConverter extends chopper.JsonConverter {
  @override
  FutureOr<chopper.Response<ResultType>> convertResponse<ResultType, Item>(
    chopper.Response response,
  ) async {
    // ... handles empty body and String ...
    
    // Handles single DateTime ✓
    if (ResultType == DateTime) {
      return response.copyWith(
        body: DateTime.parse((response.body as String).replaceAll('"', '')) as ResultType,
      );
    }

    // List<DateTime> falls through here and fails!
    final jsonRes = await super.convertResponse(response);
    return jsonRes.copyWith<ResultType>(
      body: $jsonDecoder.decode<Item>(jsonRes.body) as ResultType,
    );
  }
}

The parent JsonConverter.decodeJson() does body.cast<InnerType>() which creates a lazy cast wrapper that fails when accessed.

Expected behavior

response.body should return a properly parsed List<DateTime>:

// API returns: ["2026-01-10T10:00:00Z", "2026-01-10T11:00:00Z"]
// response.body should be: [DateTime(2026, 1, 10, 10, 0), DateTime(2026, 1, 10, 11, 0)]

Swagger specification link

Any OpenAPI 3.0 spec with an endpoint returning array of string with format: date-time.

Library version used:

• swagger_dart_code_generator: 3.0.1 (verified still broken in 4.1.1)
• chopper: 8.x
• Dart SDK: 3.x

Additional context

Suggested fix - Add a check for Item == DateTime before calling super.convertResponse():

// Handle List<DateTime> specially since strings can't be cast to DateTime
if (Item == DateTime) {
  final List<dynamic> jsonList = jsonDecode(response.bodyString);
  final List<DateTime> dateTimeList =
      jsonList.map((e) => DateTime.parse(e as String)).toList();
  return response.copyWith<ResultType>(body: dateTimeList as ResultType);
}

File to modify: lib/src/code_generators/swagger_additions_generator.dart - the generateCustomJsonConverter() method.

Workaround: Create a custom converter that extends JsonConverter with the fix above, and use it instead of the generated $JsonSerializableConverter.