Explicitly add KSerializer supertype to avoid certain restrictions after update

1 files changed, 5 insertions, 2 deletions

diff --git a/docs/custom_serializers.md b/docs/custom_serializers.md
index cef1b041..2d956cc8 100644
--- a/docs/custom_serializers.md
+++ b/docs/custom_serializers.md
@@ -41,7 +41,7 @@ import kotlinx.serialization.internal.*
 @Serializable
 class MyData(val s: String) {
     @Serializer(forClass = MyData::class)
-    companion object /*: KSerializer<MyData> can be omitted or not, if you like*/{
+    companion object : KSerializer<MyData> {
         override val descriptor: SerialDescriptor = StringDescriptor
 
         override fun serialize(output: Encoder, obj: MyData) {
@@ -65,7 +65,7 @@ First, we need to correctly fill-in descriptor so all formats would know about m
 @Serializable
 class BinaryPayload(val req: ByteArray, val res: ByteArray) {
     @Serializer(forClass = BinaryPayload::class)
-    companion object {
+    companion object : KSerializer<MyData> {
         override val descriptor: SerialDescriptor = object : SerialClassDescImpl("BinaryPayload") {
             init {
                 addElement("req") // req will have index 0
@@ -167,6 +167,9 @@ If you're familiar with DI concepts, think of it as _constructor injection_ of s
 
 Note that we haven't applied `@Serializable` on the class, because we can't customize it via companion object since companion object can't have constructor arguments.
 
+*Current limitation*: Because primary constructor in such case is generated by the compiler itself, not the plugin,
+you have to override `descriptor` manually since it can't be initialized in non-synthetic constructor.
+
 See full sample [here](https://github.com/kotlin/kotlinx.serialization/blob/a4c41392bb735a36788db1d789ec60afdbad3ca8/runtime/jvm/src/test/kotlin/kotlinx/serialization/features/GenericCustomSerializerTest.kt). *Note*: is is broken on JS for now ([#244](https://github.com/Kotlin/kotlinx.serialization/issues/244)).
 
 ## Using custom serializers