Tidy up the code a bit (and use the cast method on the serializer to avoid casting warnings). Use a map to allow faster lookup of serializers to class (to allow for overlapping hierarchies).

Author: pdvrieze

core/commonMain/src/kotlinx/serialization/modules/PolymorphicModuleBuilder.kt

@@ -19,15 +19,23 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
     private val baseClass: KClass<Base>,
     private val baseSerializer: KSerializer<Base>? = null
 ) {
-    private val subclasses: MutableList<Pair<KClass<out Base>, KSerializer<out Base>>> = mutableListOf()
+    private val subclasses: MutableMap<KClass<out Base>, KSerializer<out Base>> = mutableMapOf()
     private var defaultSerializerProvider: ((Base) -> SerializationStrategy<Base>?)? = null
     private var defaultDeserializerProvider: ((String?) -> DeserializationStrategy<Base>?)? = null
 
+
+    /**
+     * Registers the child serializers for the sealed [subclass] [serializer] in the resulting module under the [base class][Base].
+     */
+    public inline fun <reified T : Base> subclassesOf(): Unit =
+        subclassesOf(serializer<T>())
+
+
     /**
-     * Registers the subclasses of the given class as subclasses of the outer class. Currently this requires `baseClass`
+     * Registers the subclasses of the given class as subclasses of the outer class. This currently requires `baseClass`
      * to be sealed.
      */
-    public fun <T: Base> subclassesOf(baseClass: KClass<T>, serializer: KSerializer<T>) {
+    public fun <T: Base> subclassesOf(serializer: KSerializer<T>) {
         require(serializer is SealedClassSerializer) {
             "subClassesOf only supports automatic adding of subclasses of sealed types."
         }
@@ -42,7 +50,7 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
      * Registers a [subclass] [serializer] in the resulting module under the [base class][Base].
      */
     public fun <T : Base> subclass(subclass: KClass<T>, serializer: KSerializer<T>) {
-        subclasses.add(subclass to serializer)
+        subclasses[subclass] = serializer
     }
 
     /**
@@ -132,14 +140,8 @@ public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.
 public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.subclass(clazz: KClass<T>): Unit =
     subclass(clazz, serializer())
 
-/**
- * Registers the child serializers for the sealed [subclass] [serializer] in the resulting module under the [base class][Base].
- */
-public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.subclassesOf(serializer: KSerializer<T>): Unit =
-    subclassesOf(T::class, serializer)
-
 /**
  * Registers the child serializers for the sealed class [T] in the resulting module under the [base class][Base].
  */
 public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.subclassesOf(clazz: KClass<T>): Unit =
-    subclassesOf(clazz, serializer())
+    subclassesOf(clazz.serializer())

docs/polymorphism.md

@@ -411,7 +411,7 @@ fun main() {
 > Note: On Kotlin/Native, you should use `format.encodeToString(PolymorphicSerializer(Project::class), data))` instead due to limited reflection capabilities.
 
 ### Registering sealed children as subclasses
-A sealed parent interface or class can be used to directly register all its children using subclassesOf. This will
+A sealed parent interface or class can be used to directly register all its children using `subclassesOf`. This will
 expose all children that would be available when serializing the parent directly, but now as sealed. Please note that
 this is will remain open serialization, and the sealed parent serializer will not be used in serialization.
 

formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphicSealedChildTest.kt

@@ -34,7 +34,7 @@ class PolymorphicSealedChildTest {
 
     val sealedModule = SerializersModule {
         polymorphic(FooBase::class) {
-            subclassesOf(Foo.serializer())
+            subclassesOf<Foo>()
         }
     }