Registering multiple contextual serializers for generic class fails

[mvdbos]

This PR is meant for discussion, not merging.

The use case here is serializing a third party generic class with different contents: ThirdPartyBox.
As is show in the manual, the BoxSerializer in this test is capable of serializing any ThirdPartyBox and takes as its constructor arg the serializer for T.

Different from the examples for generics in the manual, we have to use contextual here, because we don't control ThirdPartyBox.
Unfortunately, adding another contextual serializer for the generic BoxSerializer fails, because SerializersModuleBuilders.registerSerializer() only looks at the toplevel type of generic types to determine if a serializer was already registered.

This is the assertion, see the code for the full setup:

        assertFailsWith(IllegalArgumentException::class) {
            SerializersModule {
                contextual(BoxSerializer(Item.serializer()))
                contextual(BoxSerializer(AnotherItem.serializer()))
            }

        }.also { it.message!!.contains("Serializer for ${ThirdPartyBox::class} already registered in this module") }

As solution could be to add type parameter info as a differentiator to the map, but this would not be possible for JavaScript,
because KClass.typeParameters is not available in JavaScript. How to solve this?

[sandwwraith]

Hi,
We have designed a solution for this, but it is not finished yet. If you take a look at the ContextualSerializer (

kotlinx.serialization/core/commonMain/src/kotlinx/serialization/ContextualSerializer.kt

Line 45 in b275a2d

private val typeParametersSerializers: Array<KSerializer<*>>

), you find out that it does have additional typeParametersSerializers argument, which is dedicated for, surprise, type arguments of contextually serializable class. As for now plugin provides these arguments, but they're not used. Using this arguments, it is possible to add special function in module builder, that would allow to construct serializer, like that:

SerializersModule {
    contextual<Box> { typeArgs -> BoxSerializer(typeArgs[0]) }
}

[mvdbos]

Great, that would help. Any idea about ETA?

[sandwwraith]

@mvdbos No particular guarantees but I hope it would come in one next or two releases

[sandwwraith]

See here: #1416

[mvdbos]

@sandwwraith Great! Thanks