[FE] Prohibit inheritors of sealed classes which are declared in different package

#KT-13495

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -426,6 +426,7 @@ public interface Errors {
     DiagnosticFactory0<KtCallExpression> SEALED_CLASS_CONSTRUCTOR_CALL = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<KtTypeReference> SEALED_SUPERTYPE = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<KtTypeReference> SEALED_SUPERTYPE_IN_LOCAL_CLASS = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory2<KtTypeReference, FqName, FqName> SEALED_INHERITOR_IN_DIFFERENT_PACKAGE = DiagnosticFactory2.create(ERROR);
 
     // Companion objects
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -633,6 +633,7 @@ public class DefaultErrorMessages {
         MAP.put(DATA_CLASS_CANNOT_HAVE_CLASS_SUPERTYPES, "Data class inheritance from other classes is forbidden");
         MAP.put(SEALED_SUPERTYPE, "This type is sealed, so it can be inherited by only its own nested classes or objects");
         MAP.put(SEALED_SUPERTYPE_IN_LOCAL_CLASS, "Local class cannot extend a sealed class");
+        MAP.put(SEALED_INHERITOR_IN_DIFFERENT_PACKAGE, "Inheritor of sealed class or interface declared in package {1} but it must be in package {2} where base class is declared", TO_STRING, TO_STRING);
         MAP.put(SINGLETON_IN_SUPERTYPE, "Cannot inherit from a singleton");
         MAP.put(CLASS_CANNOT_BE_EXTENDED_DIRECTLY, "Class {0} cannot be extended directly", NAME);
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt

@@ -40,7 +40,8 @@ private val DEFAULT_DECLARATION_CHECKERS = listOf(
     FunInterfaceDeclarationChecker(),
     DeprecatedSinceKotlinAnnotationChecker,
     ContractDescriptionBlockChecker,
-    PrivateInlineFunctionsReturningAnonymousObjectsChecker
+    PrivateInlineFunctionsReturningAnonymousObjectsChecker,
+    SealedInheritorInSamePackageChecker,
 )
 
 private val DEFAULT_CALL_CHECKERS = listOf(

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/SealedInheritorInSamePackageChecker.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.checkers
+
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.descriptors.containingPackage
+import org.jetbrains.kotlin.descriptors.isSealed
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.psi.KtClassOrObject
+import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.resolve.bindingContextUtil.getAbbreviatedTypeOrType
+
+object SealedInheritorInSamePackageChecker : DeclarationChecker {
+    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
+        if (!context.languageVersionSettings.supportsFeature(LanguageFeature.FreedomForSealedClasses)) return
+        if (descriptor !is ClassDescriptor || declaration !is KtClassOrObject) return
+        val classPackage = descriptor.containingPackage() ?: return // local class, SEALED_SUPERTYPE already reported
+        for (superTypeListEntry in declaration.superTypeListEntries) {
+            val typeReference = superTypeListEntry.typeReference ?: continue
+            val superType = typeReference.getAbbreviatedTypeOrType(context.trace.bindingContext)?.unwrap() ?: continue
+            val superClass = superType.constructor.declarationDescriptor ?: continue
+            if (!superClass.isSealed()) continue
+            val superClassPackage = superClass.containingPackage() ?: continue
+            if (classPackage != superClassPackage) {
+                context.trace.report(Errors.SEALED_INHERITOR_IN_DIFFERENT_PACKAGE.on(typeReference, classPackage, superClassPackage))
+            }
+        }
+    }
+}