I haven't worked with annotations much in Java or Kotlin. I've come across a bug, that I believe is in generated code.

@Serializable
data class FooInput {
    val id: OptionalInput<@Serializable(with = UUIDSerializer::class) UUID> = OptionalInput.Undefined
}

How does the annotation in the type parameter work? It seems like it's discarded which causes the bug, but kotlinx.serialization looks a bit like magic to me.

What's the bug?

yes. https://github.com/ExpediaGroup/graphql-kotlin/blob/master/clients/graphql-kotlin-[…]xpediagroup/graphql/client/serialization/types/OptionalInput.kt

You can't use that annotation the way you have. The custom serialiser for optional input doesn't support it.

Yeah, makes sense. That code I pasted is generated by the gradle plugin

No, I don't see how that is supposed to work. It needs fixing.

it should work if you define a serializer that accepts the type, for example

class TypedOptionalInputSerializer<T : Any>(
    private val delegate: KSerializer<T>
) : KSerializer<OptionalInput<T>> {
    override val descriptor: SerialDescriptor = delegate.descriptor.nullable

    override fun serialize(encoder: Encoder, value: OptionalInput<T>) {
        encoder.encodeNullableSerializableValue(
            delegate,
            when (value) {
                is OptionalInput.Undefined -> null
                is OptionalInput.Defined -> value.value
            }
        )
    }

    override fun deserialize(decoder: Decoder): OptionalInput<T> =
        OptionalInput.Defined(decoder.decodeNullableSerializableValue(delegate.nullable))
}

@Serializable
data class FooInput {
    val id: @Serializable(with = TypedOptionalInputSerializer::class) OptionalInput<@Serializable(with = UUIDSerializer::class) UUID> = OptionalInput.Undefined
}

(untested but something along those lines)

Thanks. That looks promising. Would it be possible for TypedOptionalInputSerializer to have a default delegate of AnyKSerializer so it falls back to the current behavior? I guess I can try to work our a set of changes for this. Thanks again for your help