Description

When using Ferry with JsonObject type (from built_value package) and tristate_optionals: true option enabled, the generated serializer code contains calls to .replace() method on JsonObject? fields, but JsonObject doesn't have a replace method.

Important: This error only occurs when tristate_optionals: true is set in the build configuration.

Environment

• Ferry version: Latest
• built_value version: 8.11.1
• Flutter/Dart SDK: Latest stable

Reproduction Steps

1. Define a GraphQL schema with JSON scalar type:

scalar JSON

2. Configure build.yaml with type override and tristate_optionals enabled:

targets:
  $default:
    builders:
      ferry_generator|graphql_builder:
        options:
          schema: path/to/schema.graphql
          tristate_optionals: true  # ← This triggers the issue
          type_overrides:
            JSON:
              name: JsonObject
              import: "package:built_value/json_object.dart"

3. Generate code using build_runner

Generated Code Issue

The generated serializer code produces:

// In schema.schema.gql.dart
case 'params':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(_i1.JsonObject)) as _i1.JsonObject;
  builder.params.replace(_$fieldValue);  // ❌ Error here
  break;

case 'body':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(_i1.JsonObject)) as _i1.JsonObject;
  builder.body.replace(_$fieldValue);  // ❌ Error here
  break;

Error Message

Error: The method 'replace' isn't defined for type 'JsonObject?'.
 - 'JsonObject' is from 'package:built_value/json_object.dart'
Try correcting the name to the name of an existing method, or defining a method named 'replace'.
              builder.params.replace(_$fieldValue);
                             ^^^^^^^

Expected Behavior

The generated code should use proper assignment for JsonObject fields instead of calling .replace() method, similar to how other non-built types are handled:

// Should be:
builder.params = _$fieldValue;
// Instead of:
builder.params.replace(_$fieldValue);

Additional Context

• The issue does not occur when tristate_optionals: false (or omitted)
• Looking at the generated .g.dart file, the builder has simple setters for JsonObject fields:

_i1.JsonObject? _params;
_i1.JsonObject? get params => _$this._params;
set params(_i1.JsonObject? params) => _$this._params = params;

The issue seems to be that when tristate_optionals is enabled, the code generator incorrectly treats JsonObject as a built collection type that has a replace method, when it should treat it as a simple value type.

Workaround

Disabling tristate_optionals avoids the issue, but this is not ideal when you need to distinguish between null and absent values in GraphQL inputs.