Prohibit extending kotlin.Enum directly

#KT-7773 Fixed

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

@@ -49,9 +49,7 @@ import java.util.List;
 import java.util.Map;
 
 import static org.jetbrains.kotlin.diagnostics.PositioningStrategies.*;
-import static org.jetbrains.kotlin.diagnostics.Severity.ERROR;
-import static org.jetbrains.kotlin.diagnostics.Severity.INFO;
-import static org.jetbrains.kotlin.diagnostics.Severity.WARNING;
+import static org.jetbrains.kotlin.diagnostics.Severity.*;
 
 /**
  * For error messages, see DefaultErrorMessages and IdeErrorMessages.
@@ -272,6 +270,8 @@ public interface Errors {
     DiagnosticFactory0<KtNullableType> NULLABLE_SUPERTYPE = DiagnosticFactory0.create(ERROR, NULLABLE_TYPE);
     DiagnosticFactory0<KtTypeReference> DYNAMIC_SUPERTYPE = DiagnosticFactory0.create(ERROR);
 
+    DiagnosticFactory1<KtTypeReference, ClassDescriptor> CLASS_CANNOT_BE_EXTENDED_DIRECTLY = DiagnosticFactory1.create(ERROR);
+
     DiagnosticFactory0<KtElement> MISSING_CONSTRUCTOR_KEYWORD = DiagnosticFactory0.create(ERROR);
 
     DiagnosticFactory0<PsiElement> NON_PRIVATE_CONSTRUCTOR_IN_ENUM = DiagnosticFactory0.create(ERROR);

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

@@ -541,6 +541,7 @@ public class DefaultErrorMessages {
         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(SINGLETON_IN_SUPERTYPE, "Cannot inherit from a singleton");
+        MAP.put(CLASS_CANNOT_BE_EXTENDED_DIRECTLY, "Class {0} cannot be extended directly", NAME);
 
         MAP.put(CYCLIC_CONSTRUCTOR_DELEGATION_CALL, "There's a cycle in the delegation calls chain");
         MAP.put(CONSTRUCTOR_IN_OBJECT, "Constructors are not allowed for objects");

compiler/frontend/src/org/jetbrains/kotlin/resolve/BodyResolver.java

@@ -584,6 +584,9 @@ public class BodyResolver {
                 else if (ModalityKt.isFinalOrEnum(classDescriptor)) {
                     trace.report(FINAL_SUPERTYPE.on(typeReference));
                 }
+                else if (KotlinBuiltIns.isEnum(classDescriptor)) {
+                    trace.report(CLASS_CANNOT_BE_EXTENDED_DIRECTLY.on(typeReference, classDescriptor));
+                }
             }
         }
     }