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).

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

@@ -19,7 +19,7 @@ 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
 
@@ -29,13 +29,14 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
     public fun <T : Base> subclass(subclass: KClass<T>, serializer: KSerializer<T>) {
 
         if (serializer is SealedClassSerializer) {
-            for ((subsubclass, subserializer) in serializer.class2Serializer.entries) {
-                @Suppress("UNCHECKED_CAST")
-                // We don't know the type here, but it matches if correct in the sealed serializer.
-                subclass(subsubclass as KClass<T>, subserializer as KSerializer<T>)
+            for ((subsubclass, subserializer) in serializer.class2Serializer) {
+                // Allow for overlapping hierarchies (possible with sealed interfaces)
+                if (subsubclass !in subclasses) {
+                    subclass(subsubclass, subserializer.cast())
+                }
             }
         } else {
-            subclasses.add(subclass to serializer)
+            subclasses.put(subclass, serializer)
         }
     }