Report warning on 'class object'

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

@@ -191,6 +191,8 @@ public interface Errors {
     DiagnosticFactory0<JetObjectDeclaration> MANY_DEFAULT_OBJECTS = DiagnosticFactory0.create(ERROR, DEFAULT_OBJECT);
     DiagnosticFactory0<JetObjectDeclaration> DEFAULT_OBJECT_NOT_ALLOWED = DiagnosticFactory0.create(ERROR, DEFAULT_OBJECT);
 
+    DiagnosticFactory0<JetObjectDeclaration> DEPRECATED_CLASS_OBJECT_SYNTAX = DiagnosticFactory0.create(WARNING);
+
     // Objects
 
     DiagnosticFactory1<JetObjectDeclaration, ClassDescriptor> LOCAL_OBJECT_NOT_ALLOWED = DiagnosticFactory1.create(ERROR, DECLARATION_NAME);

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

@@ -272,6 +272,9 @@ public class DefaultErrorMessages {
 
         MAP.put(MANY_DEFAULT_OBJECTS, "Only one default object is allowed per class");
         MAP.put(DEFAULT_OBJECT_NOT_ALLOWED, "A default object is not allowed here");
+
+        MAP.put(DEPRECATED_CLASS_OBJECT_SYNTAX, "'class object' syntax for default objects was deprecated. Use 'default' modifier instead");
+
         MAP.put(LOCAL_OBJECT_NOT_ALLOWED, "Named object ''{0}'' is a singleton and cannot be local. Try to use anonymous object instead", NAME);
         MAP.put(LOCAL_ENUM_NOT_ALLOWED, "Enum class ''{0}'' cannot be local", NAME);
         MAP.put(DELEGATION_IN_TRAIT, "Traits cannot use delegation");

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

@@ -256,12 +256,19 @@ public class DeclarationsChecker {
     }
 
     private void checkObject(JetObjectDeclaration declaration, ClassDescriptor classDescriptor) {
+        checkDeprecatedClassObjectSyntax(declaration);
         reportErrorIfHasIllegalModifier(declaration);
         if  (declaration.isLocal() && !declaration.isDefault() && !declaration.isObjectLiteral()) {
             trace.report(LOCAL_OBJECT_NOT_ALLOWED.on(declaration, classDescriptor));
         }
     }
 
+    private void checkDeprecatedClassObjectSyntax(@NotNull JetObjectDeclaration declaration) {
+        if (declaration.getClassKeyword() != null) {
+            trace.report(DEPRECATED_CLASS_OBJECT_SYNTAX.on(declaration));
+        }
+    }
+
     private void checkClass(BodiesResolveContext c, JetClass aClass, ClassDescriptorWithResolutionScopes classDescriptor) {
         checkOpenMembers(classDescriptor);
         checkConstructorParameters(aClass);

compiler/testData/diagnostics/tests/classObjects/deprecatedClassObjectSyntax.kt

@@ -1,17 +1,21 @@
 trait G
 
 class A {
-    class object A {
+    <!DEPRECATED_CLASS_OBJECT_SYNTAX!>class object A<!> {
 
     }
 }
 
 trait B {
-    class object : G {
+    <!DEPRECATED_CLASS_OBJECT_SYNTAX!>class object<!> : G {
 
     }
 }
 
+class C {
+    default <!DEPRECATED_CLASS_OBJECT_SYNTAX!>class object<!>
+}
+
 fun main() {
     A
     A.A

compiler/testData/diagnostics/tests/classObjects/deprecatedClassObjectSyntax.txt

@@ -31,6 +31,20 @@ internal trait B {
     }
 }
 
+internal final class C {
+    public constructor C()
+    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
+
+    internal default object Default {
+        private constructor Default()
+        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
+    }
+}
+
 internal trait G {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int