[FE] Prohibit implementing java sealed classes

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/checkers/ClassInheritsJavaSealedClassChecker.kt

@@ -0,0 +1,30 @@
+/*
+ * 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.jvm.checkers
+
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.load.java.lazy.descriptors.LazyJavaClassDescriptor
+import org.jetbrains.kotlin.psi.KtClassOrObject
+import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.resolve.bindingContextUtil.getAbbreviatedTypeOrType
+import org.jetbrains.kotlin.resolve.checkers.DeclarationChecker
+import org.jetbrains.kotlin.resolve.checkers.DeclarationCheckerContext
+
+object ClassInheritsJavaSealedClassChecker : DeclarationChecker {
+    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
+        if (descriptor !is ClassDescriptor || declaration !is KtClassOrObject) return
+        for (superTypeListEntry in declaration.superTypeListEntries) {
+            val typeReference = superTypeListEntry.typeReference ?: continue
+            val superType = typeReference.getAbbreviatedTypeOrType(context.trace.bindingContext)?.unwrap() ?: continue
+            val superClass = superType.constructor.declarationDescriptor as? LazyJavaClassDescriptor ?: continue
+            if (superClass.jClass.isSealed) {
+                context.trace.report(Errors.CLASS_INHERITS_JAVA_SEALED_CLASS.on(typeReference))
+            }
+        }
+    }
+}

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/platform/JvmPlatformConfigurator.kt

@@ -41,7 +41,8 @@ object JvmPlatformConfigurator : PlatformConfiguratorBase(
         JvmMultifileClassStateChecker,
         SynchronizedOnInlineMethodChecker,
         DefaultCheckerInTailrec,
-        FunctionDelegateMemberNameClashChecker
+        FunctionDelegateMemberNameClashChecker,
+        ClassInheritsJavaSealedClassChecker
     ),
 
     additionalCallCheckers = listOf(