Dart 3.3 extension types don't work with `dart_mappable`

[benthillerkus]

Using the definition of the SimpleMapper custom Mapper as a reference:

/// An interface to define a custom mapper.
///
/// Implementation should extend this interface and implement the
/// [decode] and [encode] methods.
/// For a generic type with one or two generic type arguments, extend the
/// [SimpleMapper1] or [SimpleMapper2] interface instead, respectively.
///
/// {@category Custom Mappers}
abstract class SimpleMapper<T extends Object> extends _SimpleMapperBase<T> {
  const SimpleMapper();

  T decode(Object value);
  Object? encode(T self);

  @override
  T _decode(Object value, DecodingContext context) {
    return decode(value);
  }

  @override
  Object? _encode(T value, EncodingContext context) {
    return encode(value);
  }
}

Extension types don't seem to extend Object, so you cannot use them as the type parameter T.
And because the return type of decode has to be the same as T, you cannot define the Mapper on the wrapped value instead.

[benthillerkus]

Ok, it turns out it does work, one just has to implement Object on the extension type 🤦