Bug: Build Runner Generates Incorrect .mapper File With Nullable Generics

[gabrielmcreynolds]

When there is nullable generics the build runner is generating a .mapper file that contains an error. Below is a minimally reproducable example:

// nullable_generics.dart
import 'package:dart_mappable/dart_mappable.dart';

part 'nullable_generics.mapper.dart';

@MappableClass()
// in real-world it is T extends AnotherClass but using the default Object for simplicity here
class NullableGenerics<T extends Object> with NullableGenericsMappable<T> {
  final T? value;

  const NullableGenerics({required this.value});
}

nullable_generics.mapper.dart

// coverage:ignore-file
// GENERATED CODE - DO NOT MODIFY BY HAND
// ignore_for_file: type=lint
// ignore_for_file: unused_element, unnecessary_cast, override_on_non_overriding_member
// ignore_for_file: strict_raw_type, inference_failure_on_untyped_parameter

part of 'nullable_generics.dart';

class NullableGenericsMapper extends ClassMapperBase<NullableGenerics> {
  NullableGenericsMapper._();

  static NullableGenericsMapper? _instance;
  static NullableGenericsMapper ensureInitialized() {
    if (_instance == null) {
      MapperBase.addType<Object>();
      MapperContainer.globals.use(_instance = NullableGenericsMapper._());
    }
    return _instance!;
  }

  @override
  final String id = 'NullableGenerics';
  @override
  Function get typeFactory => <T extends Object>(f) => f<NullableGenerics<T>>();

  static Object _$value(NullableGenerics v) => v.value;
  static dynamic _arg$value<T extends Object>(f) => f<T>();
  static const Field<NullableGenerics, Object> _f$value =
      Field('value', _$value, arg: _arg$value);

  @override
  final MappableFields<NullableGenerics> fields = const {
    #value: _f$value,
  };

  static NullableGenerics<T> _instantiate<T extends Object>(DecodingData data) {
    return NullableGenerics(value: data.dec(_f$value));
  }

  @override
  final Function instantiate = _instantiate;

  static NullableGenerics<T> fromMap<T extends Object>(
      Map<String, dynamic> map) {
    return ensureInitialized().decodeMap<NullableGenerics<T>>(map);
  }

  static NullableGenerics<T> fromJson<T extends Object>(String json) {
    return ensureInitialized().decodeJson<NullableGenerics<T>>(json);
  }
}

mixin NullableGenericsMappable<T extends Object> {
  String toJson() {
    return NullableGenericsMapper.ensureInitialized()
        .encodeJson<NullableGenerics<T>>(this as NullableGenerics<T>);
  }

  Map<String, dynamic> toMap() {
    return NullableGenericsMapper.ensureInitialized()
        .encodeMap<NullableGenerics<T>>(this as NullableGenerics<T>);
  }

  NullableGenericsCopyWith<NullableGenerics<T>, NullableGenerics<T>,
          NullableGenerics<T>, T>
      get copyWith => _NullableGenericsCopyWithImpl(
          this as NullableGenerics<T>, $identity, $identity);
  @override
  String toString() {
    return NullableGenericsMapper.ensureInitialized()
        .stringifyValue(this as NullableGenerics<T>);
  }

  @override
  bool operator ==(Object other) {
    return NullableGenericsMapper.ensureInitialized()
        .equalsValue(this as NullableGenerics<T>, other);
  }

  @override
  int get hashCode {
    return NullableGenericsMapper.ensureInitialized()
        .hashValue(this as NullableGenerics<T>);
  }
}

extension NullableGenericsValueCopy<$R, $Out, T extends Object>
    on ObjectCopyWith<$R, NullableGenerics<T>, $Out> {
  NullableGenericsCopyWith<$R, NullableGenerics<T>, $Out, T>
      get $asNullableGenerics =>
          $base.as((v, t, t2) => _NullableGenericsCopyWithImpl(v, t, t2));
}

abstract class NullableGenericsCopyWith<$R, $In extends NullableGenerics<T>,
    $Out, T extends Object> implements ClassCopyWith<$R, $In, $Out> {
  $R call({T? value});
  NullableGenericsCopyWith<$R2, $In, $Out2, T> $chain<$R2, $Out2>(
      Then<$Out2, $R2> t);
}

class _NullableGenericsCopyWithImpl<$R, $Out, T extends Object>
    extends ClassCopyWithBase<$R, NullableGenerics<T>, $Out>
    implements NullableGenericsCopyWith<$R, NullableGenerics<T>, $Out, T> {
  _NullableGenericsCopyWithImpl(super.value, super.then, super.then2);

  @override
  late final ClassMapperBase<NullableGenerics> $mapper =
      NullableGenericsMapper.ensureInitialized();
  @override
  $R call({Object? value = $none}) =>
      $apply(FieldCopyWithData({if (value != $none) #value: value}));
  @override
  NullableGenerics<T> $make(CopyWithData data) =>
      NullableGenerics(value: data.get(#value, or: $value.value));

  @override
  NullableGenericsCopyWith<$R2, NullableGenerics<T>, $Out2, T>
      $chain<$R2, $Out2>(Then<$Out2, $R2> t) =>
          _NullableGenericsCopyWithImpl($value, $cast, t);
}

The error being thrown is

A value of type 'Object?' can't be returned from the method '_$value' because it has a return type of 'Object'.

I'm assuming this is coming from the T extends Object line and it is just not checking if the generic type is nullable.