Fix incorrect behavior while deserializing maps to sealed classes

[rodrigovedovato]

This is an initial attempt at fixing an incompatibility between the deserialization of sealed classes when using json vs map

fix: #2035

By reusing some of the stuff from StreamingJsonDecoder I was able to succesfully deserialize a map that has the same structure as the test json.

Am I in the right path?

[sandwwraith]

Hi, thanks for the PR. Yeah, I think that's the right way to solve the problem; it just needs more polishing.

Please also try to fix this problem for serializing to map, so this functionality won't be half-baked.

[rodrigovedovato]

Hey, @sandwwraith! Thanks for the comments

Yeah, it definitely needs more polishing. I'll work on those things you mentioned and I'll let you know when it's ready for revision 👍

[rodrigovedovato]

Hey, @sandwwraith! I think I managed to do everything you suggested, but I have a doubt concerning the serialization process.

While it was possible to make it work by creating another method especifically for that, I think the best option would be using the built-in encodeSerializableValue. However, AbstractPolymorphicSerializer's upper bound is stricter than the one encodeSerializableValue, which allows null values, making it impossible to call findPolymorphicSerializerOrNull using the value directly. I tried changing encodeSerializableValue to work exclusively with non-nullable values, but it didn't go well.

Do you think there's a better approach to solve this?

[sandwwraith]

It seems that JSON just ignores nullability:

kotlinx.serialization/formats/json/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt

Line 26 in 93a06df

val actualSerializer = casted.findPolymorphicSerializer(this, value as Any)

It makes sense, because AbstractPolymorphicSerializer does not support nullable values (they're handled mainly via NullableSerializer). So, if deserializer is AbstractPolymorphicSerializer, you can safely use value as Any

[rodrigovedovato]

I think it's done, @sandwwraith.

[sandwwraith]

Thanks a lot for your contribution!

[rodrigovedovato]

Glad I could help, @sandwwraith!