Improve error message for non-overridable methods in framework

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/objcexport/ObjCExportCodeGenerator.kt

@@ -895,9 +895,9 @@ private fun ObjCExportCodeGenerator.createTypeAdapter(
 
     exposedMethods.forEach { method ->
         val baseMethods = mapper.getBaseMethods(method)
-        val hasSelectorClash = baseMethods.map { namer.getSelector(it) }.distinct().size > 1
+        val hasSelectorAmbiguity = baseMethods.map { namer.getSelector(it) }.distinct().size > 1
 
-        if (method.isOverridable && !hasSelectorClash) {
+        if (method.isOverridable && !hasSelectorAmbiguity) {
             val baseMethod = baseMethods.first()
 
             val presentVtableBridges = mutableSetOf<Int?>(null)
@@ -929,7 +929,11 @@ private fun ObjCExportCodeGenerator.createTypeAdapter(
             // Mark it as non-overridable:
             baseMethods.distinctBy { namer.getSelector(it) }.forEach { baseMethod ->
                 reverseAdapters += KotlinToObjCMethodAdapter(
-                        namer.getSelector(baseMethod), -1, -1, NullPointer(int8Type))
+                        namer.getSelector(baseMethod),
+                        -1,
+                        vtableIndex = if (hasSelectorAmbiguity) -2 else -1, // Describes the reason.
+                        kotlinImpl = NullPointer(int8Type)
+                )
             }
 
             // TODO: some fake-overrides can be skipped.

runtime/src/main/cpp/ObjCExport.mm

@@ -794,6 +794,20 @@ static void insertOrReplace(KStdVector<MethodTableRecord>& methodTable, MethodNa
   methodTable.insert(methodTable.begin(), record);
 }
 
+static void throwIfCantBeOverridden(Class clazz, const KotlinToObjCMethodAdapter* adapter) {
+  if (adapter->kotlinImpl == nullptr) {
+    NSString* reason;
+    switch (adapter->vtableIndex) {
+      case -1: reason = @"it is final"; break;
+      case -2: reason = @"original Kotlin method has more than one selector"; break;
+      default: reason = @""; break;
+    }
+    [NSException raise:NSGenericException
+        format:@"[%s %s] can't be overridden: %@",
+        class_getName(clazz), adapter->selector, reason];
+  }
+}
+
 static const TypeInfo* createTypeInfo(Class clazz, const TypeInfo* superType) {
   Class superClass = class_getSuperclass(clazz);
 
@@ -852,12 +866,7 @@ static const TypeInfo* createTypeInfo(Class clazz, const TypeInfo* superType) {
       const KotlinToObjCMethodAdapter* adapter = &typeAdapter->reverseAdapters[i];
       if (definedSelectors.find(sel_registerName(adapter->selector)) == definedSelectors.end()) continue;
 
-      if (adapter->kotlinImpl == nullptr) {
+      throwIfCantBeOverridden(clazz, adapter);
-        [NSException raise:NSGenericException
-              format:@"[%s %s] can't be implemented",
-              class_getName(clazz), adapter->selector];
-        // TODO: describe the reasons
-      }
 
       insertOrReplace(methodTable, adapter->nameSignature, const_cast<void*>(adapter->kotlinImpl));
       if (adapter->vtableIndex != -1) vtable[adapter->vtableIndex] = adapter->kotlinImpl;
@@ -870,11 +879,7 @@ static const TypeInfo* createTypeInfo(Class clazz, const TypeInfo* superType) {
 
     for (int i = 0; i < typeAdapter->reverseAdapterNum; ++i) {
       const KotlinToObjCMethodAdapter* adapter = &typeAdapter->reverseAdapters[i];
-      if (adapter->kotlinImpl == nullptr) {
+      throwIfCantBeOverridden(clazz, adapter);
-        [NSException raise:NSGenericException
-              format:@"[%s %s] can't be implemented",
-              class_getName(clazz), adapter->selector];
-      }
 
       insertOrReplace(methodTable, adapter->nameSignature, const_cast<void*>(adapter->kotlinImpl));
       RuntimeAssert(adapter->vtableIndex == -1, "");