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));
+                }
             }
         }
     }

compiler/testData/diagnostics/tests/enum/extendingEnumDirectly.kt

@@ -0,0 +1,9 @@
+class Test1 : <!CLASS_CANNOT_BE_EXTENDED_DIRECTLY!>Enum<Test1><!>("", 0)
+
+class Outer {
+    class Test2 : <!CLASS_CANNOT_BE_EXTENDED_DIRECTLY!>Enum<Test2><!>("", 0)
+}
+
+fun outer() {
+    class Test3 : <!CLASS_CANNOT_BE_EXTENDED_DIRECTLY!>Enum<Test3><!>("", 0)
+}

compiler/testData/diagnostics/tests/enum/extendingEnumDirectly.txt

@@ -0,0 +1,36 @@
+package
+
+public fun outer(): kotlin.Unit
+
+public final class Outer {
+    public constructor Outer()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public final class Test2 : kotlin.Enum<Outer.Test2> {
+        public constructor Test2()
+        public final override /*1*/ /*fake_override*/ val name: kotlin.String
+        public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+        protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+        public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: Outer.Test2): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+        public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<Outer.Test2!>!
+        public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+
+public final class Test1 : kotlin.Enum<Test1> {
+    public constructor Test1()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: Test1): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<Test1!>!
+    public final override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -7363,6 +7363,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("extendingEnumDirectly.kt")
+            public void testExtendingEnumDirectly() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/enum/extendingEnumDirectly.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("extensionNamedAsEnumEntry.kt")
             public void testExtensionNamedAsEnumEntry() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/enum/extensionNamedAsEnumEntry.kt");

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -1040,6 +1040,10 @@ public abstract class KotlinBuiltIns {
         return descriptor.getContainingDeclaration() == getAny();
     }
 
+    public static boolean isEnum(@NotNull ClassDescriptor descriptor) {
+        return classFqNameEquals(descriptor, FQ_NAMES._enum);
+    }
+
     public static boolean isCharSequence(@Nullable KotlinType type) {
         return type != null && isNotNullConstructedFromGivenClass(type, FQ_NAMES.charSequence);
     }