Do not add any .childSerializers() stuff if user explicitly marked custom serializer as KSerializer

todo: better design for user-provided descriptors and and schema hierarchy

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/SerializerCodegen.kt

@@ -72,6 +72,9 @@ abstract class SerializerCodegen(
 
     val localSerializersFieldsDescriptors: List<PropertyDescriptor> = findLocalSerializersFieldDescriptors()
 
+    // Can be false if user specified inheritance from KSerializer explicitly
+    protected val isGeneratedSerializer = serializerDescriptor.typeConstructor.supertypes.any(::isGeneratedKSerializer)
+
     private fun findLocalSerializersFieldDescriptors(): List<PropertyDescriptor> {
         val count = serializableDescriptor.declaredTypeParameters.size
         if (count == 0) return emptyList()

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt

@@ -81,11 +81,12 @@ class SerializerIrGenerator(val irClass: IrClass, override val compilerContext:
             compilerContext.localSymbolTable.withScope(initIrBody.descriptor) {
                 initIrBody.body = compilerContext.createIrBuilder(initIrBody.symbol).irBlockBody {
                     val localDesc = irTemporary(
-                            irInvoke(null, serialClassDescImplCtor,
-                                    irString(serialName), irGet(thisAsReceiverParameter),
-                                    typeHint = serialClassDescImplCtor.owner.returnType
-                            ),
-                            nameHint = "serialDesc"
+                        irInvoke(
+                            null, serialClassDescImplCtor,
+                            irString(serialName), if (isGeneratedSerializer) irGet(thisAsReceiverParameter) else irNull(),
+                            typeHint = serialClassDescImplCtor.owner.returnType
+                        ),
+                        nameHint = "serialDesc"
                     )
 
                     fun addFieldCall(fieldName: String) =

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/js/SerializerJsTranslator.kt

@@ -58,7 +58,10 @@ class SerializerJsTranslator(descriptor: ClassDescriptor,
 
         // this.serialDesc = new SerialDescImpl(...)
         val correctThis = context.getDispatchReceiver(JsDescriptorUtils.getReceiverParameterForDeclaration(desc.containingDeclaration))
-        val value = JsNew(context.getInnerReference(serialDescImplConstructor), listOf(JsStringLiteral(serialName), correctThis))
+        val value = JsNew(
+            context.getInnerReference(serialDescImplConstructor),
+            listOf(JsStringLiteral(serialName), if (isGeneratedSerializer) correctThis else JsNullLiteral())
+        )
         val assgmnt = TranslationUtils.assignmentToBackingField(context, desc, value)
         translator.addInitializerStatement(assgmnt.makeStmt())
 

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/SerializerCodegenImpl.kt

@@ -80,7 +80,10 @@ class SerializerCodegenImpl(
         if (isStatic) {
             assert(serializerDescriptor.kind == ClassKind.OBJECT) { "Serializer for type without type parameters must be an object" }
             // static descriptor means serializer is an object. it is safer to get it from correct field
-            StackValue.singleton(serializerDescriptor, codegen.typeMapper).put(generatedSerializerType, this)
+            if (isGeneratedSerializer)
+                StackValue.singleton(serializerDescriptor, codegen.typeMapper).put(generatedSerializerType, this)
+            else
+                aconst(null)
         } else {
             load(0, serializerAsmType)
         }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializerDescriptorResolver.kt

@@ -176,6 +176,7 @@ object KSerializerDescriptorResolver {
         val isLoad = name == SerialEntityNames.LOAD_NAME &&
                 shouldAddSerializerFunction { classDescriptor.checkLoadMethodParameters(it.valueParameters) }
         val isDescriptorGetter = name == SerialEntityNames.GENERATED_DESCRIPTOR_GETTER &&
+                thisDescriptor.typeConstructor.supertypes.any(::isGeneratedKSerializer) &&
                 shouldAddSerializerFunction { true /* TODO? */ }
 
         if (isSave || isLoad || isDescriptorGetter) {