Improve message about missing polymorphic serializer

Fixes #2174

Author: sandwwraith

core/commonMain/src/kotlinx/serialization/internal/AbstractPolymorphicSerializer.kt

@@ -104,7 +104,8 @@ internal fun throwSubtypeNotRegistered(subClassName: String?, baseClass: KClass<
             "Class discriminator was missing and no default polymorphic serializers were registered $scope"
         else
             "Class '$subClassName' is not registered for polymorphic serialization $scope.\n" +
-            "Mark the base class as 'sealed' or register the serializer explicitly."
+            "To be registered automatically, class '$subClassName' has to be '@Serializable', and the base class '${baseClass.simpleName}' has to be sealed and '@Serializable'.\n" +
+            "Alternatively, register the serializer for '$subClassName' explicitly in a corresponding SerializersModule."
     )
 }
 

docs/polymorphism.md

@@ -124,7 +124,8 @@ This is close to the best design for a serializable hierarchy of classes, but ru
 
 ```text 
 Exception in thread "main" kotlinx.serialization.SerializationException: Class 'OwnedProject' is not registered for polymorphic serialization in the scope of 'Project'.
-Mark the base class as 'sealed' or register the serializer explicitly.
+To be registered automatically, class 'OwnedProject' has to be '@Serializable', and the base class 'Project' has to be sealed and '@Serializable'.
+Alternatively, register the serializer for 'OwnedProject' explicitly in a corresponding SerializersModule.
 ```         
 
 <!--- TEST LINES_START -->

guide/test/PolymorphismTest.kt

@@ -24,7 +24,8 @@ class PolymorphismTest {
     fun testExamplePoly03() {
         captureOutput("ExamplePoly03") { example.examplePoly03.main() }.verifyOutputLinesStart(
             "Exception in thread \"main\" kotlinx.serialization.SerializationException: Class 'OwnedProject' is not registered for polymorphic serialization in the scope of 'Project'.",
-            "Mark the base class as 'sealed' or register the serializer explicitly."
+            "To be registered automatically, class 'OwnedProject' has to be '@Serializable', and the base class 'Project' has to be sealed and '@Serializable'.",
+            "Alternatively, register the serializer for 'OwnedProject' explicitly in a corresponding SerializersModule."
         )
     }