Support generic user-defined classes

[elizarov]

It should be possible to define:

@Serializable class Box<T>(val value: T)

and the enjoy type-safe serialization of Box(42) and Box("foo") alike. This is a meta-issue to discuss design problems surrounding type-safe access to instances of serializers for generic classes.

[elizarov]

The way it can/should work is this in Kotlin < 1.3 (w/o any language changes):

@Serializable data class MyData(val x: Int)
@Serializable data class Box<T>(val v: T)

val b = Box(MyData(42))
JSON.stringify(b, Box.serializer(MyData)) // produces "{v: {x: 42}}" -- types are fully static
val c = JSON.parse("...", Box.serializer(MyData))

In short, the idea that for generic classes a serialier(...) function in their companion object will be generated by serialization plugin. This function will have as many parameters as there are generic type parameters in the class, each parameter to the serializer function being the corresponding serializer.

In Koltin 1.3 we can rollout experimental language support for serialization and redefine JSON.parse function like this:

fun <serializable T> parse(): T

The serializable modifier is going to work similarly to reified (with or without the need for inline). It will plug the corresponding type-specific serializer as an additional parameter to the parse function behind the scenes. Inside the parse it could be accessed like T::serializer.

[sandwwraith]

Generic user classes supported (modulo bugs) in 0.3 with syntax described here