Report a separate diagnostic on inheritance from singletons

compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java

@@ -139,6 +139,7 @@ public interface Errors {
 
 
     DiagnosticFactory0<JetTypeReference> FINAL_SUPERTYPE = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<JetTypeReference> SINGLETON_IN_SUPERTYPE = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<JetNullableType> NULLABLE_SUPERTYPE = DiagnosticFactory0.create(ERROR, NULLABLE_TYPE);
 
     // Trait-specific

compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/rendering/DefaultErrorMessages.java

@@ -17,8 +17,8 @@
 package org.jetbrains.jet.lang.diagnostics.rendering;
 
 import org.jetbrains.annotations.NotNull;
-import org.jetbrains.jet.lang.diagnostics.DiagnosticFactory;
 import org.jetbrains.jet.lang.diagnostics.Diagnostic;
+import org.jetbrains.jet.lang.diagnostics.DiagnosticFactory;
 import org.jetbrains.jet.lang.diagnostics.Errors;
 import org.jetbrains.jet.lang.psi.JetExpression;
 import org.jetbrains.jet.lang.psi.JetSimpleNameExpression;
@@ -318,6 +318,7 @@ public class DefaultErrorMessages {
         MAP.put(CONSTRUCTOR_IN_TRAIT, "A trait may not have a constructor");
         MAP.put(SUPERTYPE_APPEARS_TWICE, "A supertype appears twice");
         MAP.put(FINAL_SUPERTYPE, "This type is final, so it cannot be inherited from");
+        MAP.put(SINGLETON_IN_SUPERTYPE, "Cannot inherit from a singleton");
 
         MAP.put(ILLEGAL_SELECTOR, "Expression ''{0}'' cannot be a selector (occur after a dot)", TO_STRING);
 

compiler/frontend/src/org/jetbrains/jet/lang/resolve/BodyResolver.java

@@ -256,10 +256,13 @@ public class BodyResolver {
                 JetType supertype = trace.getBindingContext().get(BindingContext.TYPE, typeReference);
                 recordSupertype(typeReference, supertype);
                 if (supertype == null) return;
-                ClassDescriptor classDescriptor = TypeUtils.getClassDescriptor(supertype);
+                ClassDescriptor superClass = TypeUtils.getClassDescriptor(supertype);
-                if (classDescriptor == null) return;
+                if (superClass == null) return;
-                if (descriptor.getKind() != ClassKind.TRAIT && !classDescriptor.getConstructors().isEmpty() &&
+                if (superClass.getKind().isSingleton()) {
-                    !ErrorUtils.isError(classDescriptor) && classDescriptor.getKind() != ClassKind.TRAIT) {
+                    // A "singleton in supertype" diagnostic will be reported later
+                    return;
+                }
+                if (descriptor.getKind() != ClassKind.TRAIT && !superClass.getConstructors().isEmpty() && !ErrorUtils.isError(superClass)) {
                     trace.report(SUPERTYPE_NOT_INITIALIZED.on(specifier));
                 }
             }
@@ -322,7 +325,10 @@ public class BodyResolver {
                 trace.report(SUPERTYPE_APPEARS_TWICE.on(typeReference));
             }
 
-            if (constructor.isFinal() && !allowedFinalSupertypes.contains(constructor)) {
+            if (DescriptorUtils.isSingleton(classDescriptor)) {
+                trace.report(SINGLETON_IN_SUPERTYPE.on(typeReference));
+            }
+            else if (constructor.isFinal() && !allowedFinalSupertypes.contains(constructor)) {
                 trace.report(FINAL_SUPERTYPE.on(typeReference));
             }
         }

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

@@ -0,0 +1,5 @@
+enum class E {
+    ENTRY
+}
+
+class A : <!SINGLETON_IN_SUPERTYPE!>E.ENTRY<!>

compiler/testData/diagnostics/tests/objects/Objects.kt

@@ -14,7 +14,7 @@ package toplevelObjectDeclarations
     }
   }
 
-  object B : <!SUPERTYPE_NOT_INITIALIZED, FINAL_SUPERTYPE!>A<!> {}
+  object B : <!SINGLETON_IN_SUPERTYPE!>A<!> {}
 
   val x = A.foo()
 

compiler/testData/diagnostics/tests/objects/ObjectsInheritance.kt

@@ -1,6 +1,5 @@
-
 package toplevelObjectDeclarations
 
 object CObj {}
 
-object DOjb : <!SUPERTYPE_NOT_INITIALIZED, FINAL_SUPERTYPE!>CObj<!> {}
+object DOjb : <!SINGLETON_IN_SUPERTYPE!>CObj<!> {}

compiler/testData/diagnostics/tests/scopes/ImportObjectAndUseAsSupertype.kt

@@ -12,7 +12,7 @@ package baz
 
 import foo.Bar
 
-class C: <!SUPERTYPE_NOT_INITIALIZED, FINAL_SUPERTYPE!>Bar<!>
+class C: <!SINGLETON_IN_SUPERTYPE!>Bar<!>
 
 fun test() {
     Bar.bar()

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -2577,6 +2577,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/enum/importEnumFromJava.kt");
             }
             
+            @TestMetadata("inheritFromEnumEntry.kt")
+            public void testInheritFromEnumEntry() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/enum/inheritFromEnumEntry.kt");
+            }
+            
             @TestMetadata("inheritanceFromEnum.kt")
             public void testInheritanceFromEnum() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/enum/inheritanceFromEnum.kt");

core/descriptors/src/org/jetbrains/jet/lang/resolve/DescriptorUtils.java

@@ -243,7 +243,7 @@ public class DescriptorUtils {
         return isKindOf(descriptor, ClassKind.ENUM_ENTRY);
     }
 
-    public static boolean isSingleton(@NotNull DeclarationDescriptor classifier) {
+    public static boolean isSingleton(@Nullable DeclarationDescriptor classifier) {
         if (classifier instanceof ClassDescriptor) {
             ClassDescriptor clazz = (ClassDescriptor) classifier;
             return clazz.getKind().isSingleton();