Add unavailable stub to framework for Kotlin subclass of Obj-C class

To improve discoverability of the corresponding limitation

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

@@ -220,8 +220,8 @@ abstract class ObjCExportHeaderGenerator(
             getContributedDescriptors()
                     .asSequence()
                     .filterIsInstance<ClassDescriptor>()
-                    .filter { mapper.shouldBeExposed(it) }
                     .forEach {
+                        if (mapper.shouldBeExposed(it)) {
                             if (it.isInterface) {
                                 translateInterface(it)
                             } else {
@@ -229,6 +229,10 @@ abstract class ObjCExportHeaderGenerator(
                             }
 
                             it.unsubstitutedMemberScope.translateClasses()
+                        } else if (it.isKotlinObjCClass() && mapper.shouldBeVisible(it)) {
+                            assert(!it.isInterface)
+                            translateKotlinObjCClassAsUnavailableStub(it)
+                        }
                     }
         }
 
@@ -257,6 +261,16 @@ abstract class ObjCExportHeaderGenerator(
         return stubs
     }
 
+    private fun translateKotlinObjCClassAsUnavailableStub(descriptor: ClassDescriptor) {
+        stubs.add(objCInterface(
+                namer.getClassOrProtocolName(descriptor),
+                descriptor = descriptor,
+                superClass = "NSObject",
+                attributes = listOf("unavailable(\"Kotlin subclass of Objective-C class can't be imported\")")
+
+        ))
+    }
+
     private fun genKotlinNumbers() {
         val members = buildMembers {
             NSNumberKind.values().forEach {

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

@@ -54,7 +54,10 @@ internal fun ObjCExportMapper.shouldBeExposed(descriptor: CallableMemberDescript
         descriptor.isEffectivelyPublicApi && !descriptor.isSuspend && !descriptor.isExpect
 
 internal fun ObjCExportMapper.shouldBeExposed(descriptor: ClassDescriptor): Boolean =
-        descriptor.isEffectivelyPublicApi && !descriptor.defaultType.isObjCObjectType() && when (descriptor.kind) {
+        shouldBeVisible(descriptor) && !descriptor.defaultType.isObjCObjectType()
+
+internal fun ObjCExportMapper.shouldBeVisible(descriptor: ClassDescriptor): Boolean =
+        descriptor.isEffectivelyPublicApi && when (descriptor.kind) {
         ClassKind.CLASS, ClassKind.INTERFACE, ClassKind.ENUM_CLASS, ClassKind.OBJECT -> true
         ClassKind.ENUM_ENTRY, ClassKind.ANNOTATION_CLASS -> false
     } && !descriptor.isExpect && !isSpecialMapped(descriptor) && !descriptor.isInlined()