Enable method signature checks when overriding Objective-C method

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/ObjCInterop.kt

@@ -133,12 +133,19 @@ class ObjCOverridabilityCondition : ExternalOverridabilityCondition {
         assert(superDescriptor.name == subDescriptor.name)
 
         val superClass = superDescriptor.containingDeclaration as? ClassDescriptor
-        if (superClass == null || !superClass.isObjCClass()) {
+        val subClass = subDescriptor.containingDeclaration as? ClassDescriptor
+
+        if (superClass == null || !superClass.isObjCClass() || subClass == null) {
             return ExternalOverridabilityCondition.Result.UNKNOWN
         }
 
         return if (areSelectorsEqual(superDescriptor, subDescriptor)) {
-            ExternalOverridabilityCondition.Result.OVERRIDABLE
+            // Also check the method signatures if the subclass is user-defined:
+            if (subClass.isExternalObjCClass()) {
+                ExternalOverridabilityCondition.Result.OVERRIDABLE
+            } else {
+                ExternalOverridabilityCondition.Result.UNKNOWN
+            }
         } else {
             ExternalOverridabilityCondition.Result.INCOMPATIBLE
         }

backend.native/tests/interop/objc/smoke.kt

@@ -48,8 +48,8 @@ fun MutablePairProtocol.swap() {
 }
 
 class Bar : Foo() {
-    override fun helloWithPrinter(printer: PrinterProtocol) = memScoped {
+    override fun helloWithPrinter(printer: PrinterProtocol?) = memScoped {
-        printer.print("Hello from Kotlin".cstr.getPointer(memScope))
+        printer!!.print("Hello from Kotlin".cstr.getPointer(memScope))
     }
 }