Prepare fix for contextual deserialization of polymorphic scalars

[Jojo4GH]

Second bug from #607 (as preparation for the PR)

Previously it was not possible to implement a content based polymorphic serializer in the case that one of the types involved is a scalar. Different from the JSON serialization library it was therefore impossible to implement something like:

myCats:
  myFirstCat: "Bella"
  mySecondCat:
    name: "Daisy"
    age: 2

typealias MyCats = Map<String, CatDetails>

@Serializable(with = MyContentBasedSerializer::class)
sealed interface CatDetails {

    @Serializable
    value class InlineName(val name: String) : CatDetails

    @Serializable
    data class More(
        val name: String,
        val age: String?
    ) : CatDetails
}

object MyContentBasedSerializer : YamlContentPolymorphicSerializer<CatDetails>(CatDetails::class) {
    override fun selectDeserializer(node: YamlNode) = when (node) {
        is YamlScalar -> CatDetails.InlineName.serializer()
        is YamlMap -> CatDetails.More.serializer()
        else -> error("Unsupported node type ${node::class.simpleName}")
    }
}

[charleskorn]

It's not clear to me why this change is necessary - why can't a contextual polymorphic serializer look at the value of node?

[Jojo4GH]

It's not clear to me why this change is necessary - why can't a contextual polymorphic serializer look at the value of node?

Since YamlContentPolymorphicSerializer uses PolymorphicKind.SEALED (just like the JsonContentPolymorphicSerializer) when selecting an existing serializer that expects a YamlScalar the IllegalStateException("Must call beginStructure() and use returned Decoder") gets thrown erroneously.
Remember that fun selectDeserializer(node: YamlNode): DeserializationStrategy<T> returns not a value but a deserializer.

[charleskorn]

Sorry for the delay, thanks @Jojo4GH!

[github-actions]

🎉 This PR is included in version 0.62.0 🎉

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