[bug] json_serializable is unable to recognize generics on `@JsonKey`'s `toJson/fromJson` methods, or on `JsonConverter`

[lucavenir]

TL;DR: minimal reproducible bug:

class A<T> {}

A<T> fromJson<T>(String input) => A<T>();
String toJson<T>(A<T> input) => '$input';

@JsonSerializable()
class Serializable<T> {
  const Serializable({required this.a});
  @JsonKey(fromJson: fromJson, toJson: toJson)
  final A<T> a;
}

The above breaks the builder.

---

In an attempt to work around #1507, I end up with the following:

sealed class Slug<T> {}
class Asd extends Slug<String> {}
class Lol extends Slug<int> {}
class Rofl extends Slug<double> {}
class Lmao extends Slug<(int, int)> {}

@JsonSerializable()
class Serializable<T> {
  const Serializable(this.slug);
  final Slug<T> slug;
}

The above outputs:

[SEVERE] json_serializable on lib/src/features/models/lol_dto.dart:

Could not generate `fromJson` code for `slug`.
To support the type `Slug` you can:
* Use `JsonConverter`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
* Use `JsonKey` fields `fromJson` and `toJson`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html
package:asdlol/src/features/models/lol_dto.dart:19:17
   ╷
19 │   final Slug<T> slug;
   │                 ^^^^
   ╵

But that's expected.

So, I follow the first advice, the JsonConverter way.

@JsonSerializable()
class Serializable<T> {
  const Serializable(this.slug);
  @SlugConverter<T>() // this has been added
  final Slug<T> slug;
}

class SlugConverter<T> extends JsonConverter<Slug<T>, String> {
  const SlugConverter();

  @override
  String toJson(Slug<T> object) {
    return object.runtimeType.toString(); 
  }

  @override
  Slug<T> fromJson(String json) {
    switch (json) {
      case 'Asd':
        return Asd() as Slug<T>;
      case 'Lol':
        return Lol() as Slug<T>;
      case 'Rofl':
        return Rofl() as Slug<T>;
      case 'Lmao':
        return Lmao() as Slug<T>;
      default:
        throw Exception('Unknown slug type: $json');
    }
  }
}

But the above is straight ignored by the builder, which outputs exactly the same message.
Am I doing something wrong? If so, can I be welcomed with a more informative message? 😸

Then, I try the JsonKey way.

@JsonSerializable()
class Serializable<T> {
  const Serializable(this.slug);
  @JsonKey(fromJson: slugFromJson, toJson: slugToJson) // let's try this one now
  final Slug<T> slug;
}

String slugToJson<T>(Slug<T> object) {
  return object.runtimeType.toString();
}

Slug<T> slugFromJson<T>(String json) {
  switch (json) {
    case 'Asd':
      return Asd() as Slug<T>;
    case 'Lol':
      return Lol() as Slug<T>;
    case 'Rofl':
      return Rofl() as Slug<T>;
    case 'Lmao':
      return Lmao() as Slug<T>;
    default:
      throw Exception('Unknown slug type: $json');
  }
}

But the above leads the builder to output:

[SEVERE] json_serializable on lib/src/features/models/lol_dto.dart:

Error with `@JsonKey` on the `slug` field. The `toJson` function `slugToJson` argument type `Slug<T>` is not compatible with field type `Slug<T>`.
package:asdlol/src/features/models/lol_dto.dart:19:17
   ╷
19 │   final Slug<T> slug;
   │                 ^^^^
   ╵

Say again? Slug<T> is not compatible with Slug<T>? 😵‍💫

I'm unsure what I can actually do at this point.

[lucavenir]

I've changed the issue's name because generics seem to break even on non-sealed classes.

Take #1507 example:

enum Slug<T> { asd<String>(), lol<int>(), rofl<double>(), lmao<(int, int)>() }

@JsonSerializable()
class Serializable<T> {
  const Serializable(this.slug, this.name);
  @SlugConverter<T>()
  final Slug<T> slug;
  @WeirdConverter()
  final String name;
}

class WeirdConverter extends JsonConverter<String, int> {
  const WeirdConverter();
  @override
  String fromJson(int json) => '$json';
  @override
  int toJson(String object) => int.tryParse(object) ?? 0;
}

class SlugConverter<T> extends JsonConverter<Slug<T>, String> {
  const SlugConverter();

  @override
  String toJson(Slug<T> object) => object.runtimeType.toString();

  @override
  Slug<T> fromJson(String json) {
    return switch (json) {
      'Asd' => Slug.asd as Slug<T>,
      'Lol' => Slug.lol as Slug<T>,
      'Rofl' => Slug.rofl as Slug<T>,
      'Lmao' => Slug.lmao as Slug<T>,
      _ => throw Exception('Unknown slug type: $json'),
    };
  }
}

The above ends up generating:

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'lol_dto.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

Serializable<T> _$SerializableFromJson<T>(Map<String, dynamic> json) =>
    $checkedCreate('Serializable', json, ($checkedConvert) {
      final val = Serializable<T>(
        $checkedConvert('slug', (v) => $enumDecode(_$SlugEnumMap, v)),
        $checkedConvert(
          'name',
          (v) => const WeirdConverter().fromJson((v as num).toInt()),
        ),
      );
      return val;
    });

Map<String, dynamic> _$SerializableToJson<T>(Serializable<T> instance) =>
    <String, dynamic>{
      'slug': _$SlugEnumMap[instance.slug]!,
      'name': const WeirdConverter().toJson(instance.name),
    };

const _$SlugEnumMap = {
  Slug.asd: 'asd',
  Slug.lol: 'lol',
  Slug.rofl: 'rofl',
  Slug.lmao: 'lmao',
};

As you can see, while WeirdConverter is correctly used, SlugConverter is silently ignored

Finally, let's try injecting to the above a normal, generic class:

@JsonSerializable()
class Serializable<T> {
  // ...
  @JsonKey(fromJson: normalFromJson, toJson: normalToJson)
  final A<T> normal;
}

String normalToJson<T>(A<T> object) => object.runtimeType.toString();
A<T> normalFromJson<T>(String json) => A<T>();

The above errors the builder, which again outputs:

[SEVERE] json_serializable on lib/src/features/models/lol_dto.dart:

Error with `@JsonKey` on the `normal` field. The `toJson` function `normalToJson` argument type `A<T>` is not compatible with field type `A<T>`.
package:asdlol/src/features/models/lol_dto.dart:17:14
   ╷
17 │   final A<T> normal;
   │              ^^^^^^
   ╵

Again, AFAIK, A<T> should be compatible with itself.