KT-1934 Check inherited signatures for compatibility

Overridden signatures should have compatible return types
(equal types for 'var').
Only relevant overrides should be taken into account.
Refactor inherited signatures check,
introduce a strategy interface for problem reporting.

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

@@ -324,6 +324,11 @@ public interface Errors {
     DiagnosticFactory2<KtNamedDeclaration, CallableMemberDescriptor, CallableMemberDescriptor> PROPERTY_TYPE_MISMATCH_ON_OVERRIDE =
             DiagnosticFactory2.create(ERROR, DECLARATION_RETURN_TYPE);
 
+    DiagnosticFactory2<KtClassOrObject, CallableMemberDescriptor, CallableMemberDescriptor> RETURN_TYPE_MISMATCH_ON_INHERITANCE =
+            DiagnosticFactory2.create(ERROR, DECLARATION_NAME);
+    DiagnosticFactory2<KtClassOrObject, CallableMemberDescriptor, CallableMemberDescriptor> PROPERTY_TYPE_MISMATCH_ON_INHERITANCE =
+            DiagnosticFactory2.create(ERROR, DECLARATION_NAME);
+
     DiagnosticFactory2<KtClassOrObject, KtClassOrObject, CallableMemberDescriptor> ABSTRACT_MEMBER_NOT_IMPLEMENTED =
             DiagnosticFactory2.create(ERROR, DECLARATION_NAME);
     DiagnosticFactory2<KtClassOrObject, KtClassOrObject, CallableMemberDescriptor> ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED =

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

@@ -555,9 +555,13 @@ public class DefaultErrorMessages {
 
         MAP.put(RETURN_TYPE_MISMATCH_ON_OVERRIDE, "Return type of ''{0}'' is not a subtype of the return type of the overridden member ''{1}''",
                 NAME, FQ_NAMES_IN_TYPES);
+        MAP.put(RETURN_TYPE_MISMATCH_ON_INHERITANCE, "''{0}'' clashes with ''{1}'': return types are incompatible",
+                SHORT_NAMES_IN_TYPES, SHORT_NAMES_IN_TYPES);
 
         MAP.put(PROPERTY_TYPE_MISMATCH_ON_OVERRIDE, "Type of ''{0}'' doesn''t match the type of the overridden var-property ''{1}''",
                 NAME, FQ_NAMES_IN_TYPES);
+        MAP.put(PROPERTY_TYPE_MISMATCH_ON_INHERITANCE, "''{0}'' clashes with ''{1}'': property types are incompatible",
+                SHORT_NAMES_IN_TYPES, SHORT_NAMES_IN_TYPES);
 
         MAP.put(VAR_OVERRIDDEN_BY_VAL, "Var-property {0} cannot be overridden by val-property {1}", FQ_NAMES_IN_TYPES, FQ_NAMES_IN_TYPES);
 

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

@@ -257,7 +257,7 @@ public class OverrideResolver {
         }
     }
 
-    private void checkOverridesInAClass(@NotNull ClassDescriptorWithResolutionScopes classDescriptor, @NotNull KtClassOrObject klass) {
+    private void checkOverridesInAClass(@NotNull ClassDescriptorWithResolutionScopes classDescriptor, @NotNull final KtClassOrObject klass) {
         // Check overrides for internal consistency
         for (CallableMemberDescriptor member : classDescriptor.getDeclaredCallableMembers()) {
             checkOverrideForMember(member);
@@ -265,13 +265,53 @@ public class OverrideResolver {
 
         // Check if everything that must be overridden, actually is
         // More than one implementation or no implementations at all
-        Set<CallableMemberDescriptor> abstractNoImpl = Sets.newLinkedHashSet();
-        Set<CallableMemberDescriptor> manyImpl = Sets.newLinkedHashSet();
-        Set<CallableMemberDescriptor> abstractInBaseClassNoImpl = Sets.newLinkedHashSet();
-        Set<CallableMemberDescriptor> conflictingInterfaceOverrides = Sets.newLinkedHashSet();
-        collectMissingImplementations(classDescriptor,
-                                      abstractNoImpl, manyImpl,
-                                      abstractInBaseClassNoImpl, conflictingInterfaceOverrides);
+        final Set<CallableMemberDescriptor> abstractNoImpl = Sets.newLinkedHashSet();
+        final Set<CallableMemberDescriptor> manyImpl = Sets.newLinkedHashSet();
+        final Set<CallableMemberDescriptor> abstractInBaseClassNoImpl = Sets.newLinkedHashSet();
+        final Set<CallableMemberDescriptor> conflictingInterfaceOverrides = Sets.newLinkedHashSet();
+
+        checkInheritedSignatures(
+                classDescriptor,
+                new CheckInheritedSignaturesReportingStrategy() {
+                    private boolean returnTypeMismatch = false;
+                    private boolean propertyTypeMismatch = false;
+
+                    @Override
+                    public void abstractMemberNoImpl(CallableMemberDescriptor descriptor) {
+                        abstractNoImpl.add(descriptor);
+                    }
+
+                    @Override
+                    public void abstractBaseClassMemberNoImpl(CallableMemberDescriptor descriptor) {
+                        abstractInBaseClassNoImpl.add(descriptor);
+                    }
+
+                    @Override
+                    public void manyImplMemberNoImpl(CallableMemberDescriptor descriptor) {
+                        manyImpl.add(descriptor);
+                    }
+
+                    @Override
+                    public void conflictingMemberFromInterface(CallableMemberDescriptor descriptor) {
+                        conflictingInterfaceOverrides.add(descriptor);
+                    }
+
+                    @Override
+                    public void clashingWithReturnType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2) {
+                        if (!returnTypeMismatch) {
+                            returnTypeMismatch = true;
+                            trace.report(RETURN_TYPE_MISMATCH_ON_INHERITANCE.on(klass, descriptor1, descriptor2));
+                        }
+                    }
+
+                    @Override
+                    public void clashingWithPropertyType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2) {
+                        if (!propertyTypeMismatch) {
+                            propertyTypeMismatch = true;
+                            trace.report(PROPERTY_TYPE_MISMATCH_ON_INHERITANCE.on(klass, descriptor1, descriptor2));
+                        }
+                    }
+                });
 
         if (!classCanHaveAbstractMembers(classDescriptor)) {
             if (!abstractInBaseClassNoImpl.isEmpty()) {
@@ -292,34 +332,68 @@ public class OverrideResolver {
 
     @NotNull
     public static Set<CallableMemberDescriptor> getMissingImplementations(@NotNull ClassDescriptor classDescriptor) {
-        Set<CallableMemberDescriptor> shouldImplement = new LinkedHashSet<CallableMemberDescriptor>();
-        Set<CallableMemberDescriptor> dontCare = new HashSet<CallableMemberDescriptor>();
-        collectMissingImplementations(classDescriptor, shouldImplement, shouldImplement, dontCare, dontCare);
-        return shouldImplement;
+        CollectMissingImplementationsStrategy collector = new CollectMissingImplementationsStrategy();
+        checkInheritedSignatures(classDescriptor, collector);
+        return collector.shouldImplement;
     }
 
-    private static void collectMissingImplementations(
+    private interface CheckInheritedSignaturesReportingStrategy {
+        void abstractMemberNoImpl(CallableMemberDescriptor descriptor);
+        void abstractBaseClassMemberNoImpl(CallableMemberDescriptor descriptor);
+        void manyImplMemberNoImpl(CallableMemberDescriptor descriptor);
+        void conflictingMemberFromInterface(CallableMemberDescriptor descriptor);
+        void clashingWithReturnType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2);
+        void clashingWithPropertyType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2);
+    }
+
+    private static class CollectMissingImplementationsStrategy implements CheckInheritedSignaturesReportingStrategy {
+        private Set<CallableMemberDescriptor> shouldImplement = new LinkedHashSet<CallableMemberDescriptor>();
+
+        @Override
+        public void abstractMemberNoImpl(CallableMemberDescriptor descriptor) {
+            shouldImplement.add(descriptor);
+        }
+
+        @Override
+        public void abstractBaseClassMemberNoImpl(CallableMemberDescriptor descriptor) {
+            // don't care
+        }
+
+        @Override
+        public void manyImplMemberNoImpl(CallableMemberDescriptor descriptor) {
+            shouldImplement.add(descriptor);
+        }
+
+        @Override
+        public void conflictingMemberFromInterface(CallableMemberDescriptor descriptor) {
+            // don't care
+        }
+
+        @Override
+        public void clashingWithReturnType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2) {
+            // don't care
+        }
+
+        @Override
+        public void clashingWithPropertyType(CallableMemberDescriptor descriptor1, CallableMemberDescriptor descriptor2) {
+            // don't care
+        }
+    }
+
+    private static void checkInheritedSignatures(
             @NotNull ClassDescriptor classDescriptor,
-            @NotNull Set<CallableMemberDescriptor> abstractNoImpl,
-            @NotNull Set<CallableMemberDescriptor> manyImpl,
-            @NotNull Set<CallableMemberDescriptor> abstractInBaseClassNoImpl,
-            @NotNull Set<CallableMemberDescriptor> conflictingInterfaceOverrides
+            @NotNull CheckInheritedSignaturesReportingStrategy reportingStrategy
     ) {
         for (DeclarationDescriptor member : DescriptorUtils.getAllDescriptors(classDescriptor.getDefaultType().getMemberScope())) {
             if (member instanceof CallableMemberDescriptor) {
-                collectMissingImplementations((CallableMemberDescriptor) member,
-                                              abstractNoImpl, manyImpl,
-                                              abstractInBaseClassNoImpl, conflictingInterfaceOverrides);
+                checkInheritedSignatures((CallableMemberDescriptor) member, reportingStrategy);
             }
         }
     }
 
-    private static void collectMissingImplementations(
+    private static void checkInheritedSignatures(
             @NotNull CallableMemberDescriptor descriptor,
-            @NotNull Set<CallableMemberDescriptor> abstractNoImpl,
-            @NotNull Set<CallableMemberDescriptor> manyImpl,
-            @NotNull Set<CallableMemberDescriptor> abstractInBaseClassNoImpl,
-            @NotNull Set<CallableMemberDescriptor> conflictingInterfaceOverrides
+            @NotNull CheckInheritedSignaturesReportingStrategy reportingStrategy
     ) {
         if (descriptor.getKind().isReal()) return;
         if (descriptor.getVisibility() == Visibilities.INVISIBLE_FAKE) return;
@@ -341,7 +415,9 @@ public class OverrideResolver {
         Set<CallableMemberDescriptor> relevantDirectlyOverridden =
                 getRelevantDirectlyOverridden(overriddenDeclarationsByDirectParent, allFilteredOverriddenDeclarations);
 
-        collectJava8MissingOverrides(relevantDirectlyOverridden, abstractInBaseClassNoImpl, conflictingInterfaceOverrides);
+        checkInheritedSignaturesForFakeOverride(descriptor, relevantDirectlyOverridden, reportingStrategy);
+
+        collectJava8MissingOverrides(relevantDirectlyOverridden, reportingStrategy);
 
         List<CallableMemberDescriptor> implementations = collectImplementations(relevantDirectlyOverridden);
         if (implementations.size() == 1 && isReturnTypeOkForOverride(descriptor, implementations.get(0))) return;
@@ -351,20 +427,27 @@ public class OverrideResolver {
         filterNotSynthesizedDescriptorsByModality(allFilteredOverriddenDeclarations, abstractOverridden, concreteOverridden);
 
         if (implementations.isEmpty()) {
-            abstractNoImpl.addAll(abstractOverridden);
+            for (CallableMemberDescriptor member : abstractOverridden) {
+                reportingStrategy.abstractMemberNoImpl(member);
+            }
         }
         else if (implementations.size() > 1) {
-            manyImpl.addAll(concreteOverridden);
+            for (CallableMemberDescriptor member : concreteOverridden) {
+                reportingStrategy.manyImplMemberNoImpl(member);
+            }
         }
         else {
-            abstractNoImpl.addAll(collectAbstractMethodsWithMoreSpecificReturnType(abstractOverridden, implementations.get(0)));
+            List<CallableMemberDescriptor> membersWithMoreSpecificReturnType =
+                    collectAbstractMethodsWithMoreSpecificReturnType(abstractOverridden, implementations.get(0));
+            for (CallableMemberDescriptor member : membersWithMoreSpecificReturnType) {
+                reportingStrategy.abstractMemberNoImpl(member);
+            }
         }
     }
 
     private static void collectJava8MissingOverrides(
-            Set<CallableMemberDescriptor> relevantDirectlyOverridden,
-            @NotNull Set<CallableMemberDescriptor> abstractInBaseClassNoImpl,
-            @NotNull Set<CallableMemberDescriptor> conflictingInterfaceOverrides
+            @NotNull Set<CallableMemberDescriptor> relevantDirectlyOverridden,
+            @NotNull CheckInheritedSignaturesReportingStrategy reportingStrategy
     ) {
         // Java 8:
         // -- class should implement an abstract member of a super-class,
@@ -396,11 +479,13 @@ public class OverrideResolver {
         }
 
         if (overridesAbstractInBaseClass != null) {
-            abstractInBaseClassNoImpl.add(overridesAbstractInBaseClass);
+            reportingStrategy.abstractBaseClassMemberNoImpl(overridesAbstractInBaseClass);
         }
 
         if (!overridesClassMember && overridesNonAbstractInterfaceMember && overriddenInterfaceMembers.size() > 1) {
-            conflictingInterfaceOverrides.addAll(overriddenInterfaceMembers);
+            for (CallableMemberDescriptor member : overriddenInterfaceMembers) {
+                reportingStrategy.conflictingMemberFromInterface(member);
+            }
         }
     }
 
@@ -638,6 +723,39 @@ public class OverrideResolver {
         }
     }
 
+    private static void checkInheritedSignaturesForFakeOverride(
+            @NotNull CallableMemberDescriptor fakeOverride,
+            @NotNull Collection<CallableMemberDescriptor> relevantDirectlyOverridden,
+            @NotNull CheckInheritedSignaturesReportingStrategy reportingStrategy
+    ) {
+        assert fakeOverride.getKind() == FAKE_OVERRIDE
+                : "Fake override expected; actual: " + fakeOverride + " of kind " + fakeOverride.getKind();
+
+        // FIXME This algorithm depends on transitiveness of sub-typing relation, which is broken in presence of flexible types.
+        // See override/clashesOnInheritance/flexibleReturnType.kt.
+        if (relevantDirectlyOverridden.size() > 1) {
+            Iterator<CallableMemberDescriptor> overriddenIterator = relevantDirectlyOverridden.iterator();
+            CallableMemberDescriptor mostSpecificOverridden = overriddenIterator.next();
+            while (overriddenIterator.hasNext()) {
+                CallableMemberDescriptor overriddenDescriptor = overriddenIterator.next();
+                if (OverridingUtil.isMoreSpecific(overriddenDescriptor, mostSpecificOverridden)) {
+                    mostSpecificOverridden = overriddenDescriptor;
+                }
+            }
+
+            for (CallableMemberDescriptor overriddenDescriptor : relevantDirectlyOverridden) {
+                if (!OverridingUtil.isMoreSpecific(mostSpecificOverridden, overriddenDescriptor)) {
+                    if (overriddenDescriptor instanceof PropertyDescriptor) {
+                        reportingStrategy.clashingWithPropertyType(mostSpecificOverridden, overriddenDescriptor);
+                    }
+                    else {
+                        reportingStrategy.clashingWithReturnType(mostSpecificOverridden, overriddenDescriptor);
+                    }
+                }
+            }
+        }
+    }
+
     private static void checkOverridesForMemberMarkedOverride(
             @NotNull CallableMemberDescriptor declared,
             boolean checkIfOverridesNothing,

compiler/testData/codegen/box/bridges/kt1939.kt

@@ -3,7 +3,7 @@ abstract class Foo<T> {
 }
 
 interface Tr {
-    fun hello(s : String)
+    fun hello(s : String): String
 }
 
 class Bar: Foo<String>(), Tr {

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndNamesInSupertypes.kt

@@ -7,7 +7,8 @@ open class B {
 }
 
 interface C {
-    fun foo(y: Int)
+    fun foo(y: Int): Int
 }
 
-<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE, DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES!>class Z<!> : A, B(), C
+// TODO DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES reported twice
+<!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE, DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES, DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES!>class Z<!> : A, B(), C

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndNamesInSupertypes.txt

@@ -17,7 +17,7 @@ public open class B {
 
 public interface C {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract fun foo(/*0*/ y: kotlin.Int): kotlin.Unit
+    public abstract fun foo(/*0*/ y: kotlin.Int): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
@@ -25,7 +25,7 @@ public interface C {
 public final class Z : A, B, C {
     public constructor Z()
     public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public final override /*3*/ /*fake_override*/ fun foo(/*0*/ x: kotlin.Int = ...): kotlin.Int
+    public final override /*3*/ /*fake_override*/ fun foo(/*0*/ y: kotlin.Int = ...): kotlin.Int
     public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
 }

compiler/testData/diagnostics/tests/override/clashesOnInheritance/covariantOverrides.kt

@@ -0,0 +1,37 @@
+interface IBase {
+    fun copy(): IBase
+}
+
+interface ILeft : IBase {
+    override fun copy(): ILeft
+}
+
+open class CLeft : ILeft {
+    override fun copy(): ILeft = CLeft()
+}
+
+interface IRight : IBase {
+    override fun copy(): IRight
+}
+
+interface IDerived : ILeft, IRight {
+    override fun copy(): IDerived
+}
+
+// Error: ILeft::copy and IRight::copy have unrelated return types
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE, ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class CDerivedInvalid1<!> : ILeft, IRight
+
+// Error: CLeft::copy and IRight::copy have unrelated return types
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class CDerivedInvalid2<!> : CLeft(), IRight
+
+// OK: CDerived1::copy overrides both ILeft::copy and IRight::copy
+class CDerived1 : ILeft, IRight {
+    override fun copy(): CDerived1 = CDerived1()
+}
+
+// Although ILeft::copy and IRight::copy return types are unrelated, IDerived::copy return type is the most specific of three.
+abstract class CDerived2 : ILeft, IRight, IDerived
+
+class CDerived2a : ILeft, IRight, IDerived {
+    override fun copy(): IDerived = CDerived2a()
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/covariantOverrides.txt

@@ -0,0 +1,77 @@
+package
+
+public final class CDerived1 : ILeft, IRight {
+    public constructor CDerived1()
+    public open override /*2*/ fun copy(): CDerived1
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class CDerived2 : ILeft, IRight, IDerived {
+    public constructor CDerived2()
+    public abstract override /*3*/ /*fake_override*/ fun copy(): IDerived
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class CDerived2a : ILeft, IRight, IDerived {
+    public constructor CDerived2a()
+    public open override /*3*/ fun copy(): IDerived
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class CDerivedInvalid1 : ILeft, IRight {
+    public constructor CDerivedInvalid1()
+    public abstract override /*2*/ /*fake_override*/ fun copy(): ILeft
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class CDerivedInvalid2 : CLeft, IRight {
+    public constructor CDerivedInvalid2()
+    public open override /*2*/ /*fake_override*/ fun copy(): ILeft
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public open class CLeft : ILeft {
+    public constructor CLeft()
+    public open override /*1*/ fun copy(): ILeft
+    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 interface IBase {
+    public abstract fun copy(): IBase
+    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 interface IDerived : ILeft, IRight {
+    public abstract override /*2*/ fun copy(): IDerived
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface ILeft : IBase {
+    public abstract override /*1*/ fun copy(): ILeft
+    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 interface IRight : IBase {
+    public abstract override /*1*/ fun copy(): IRight
+    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
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/flexibleReturnType.kt

@@ -0,0 +1,30 @@
+// FILE: J.java
+public interface J {
+    String foo(); // String!
+}
+
+// FILE: K.kt
+interface K1 {
+    fun foo(): String
+}
+
+interface K2 {
+    fun foo(): String?
+}
+
+interface KDerived1a : K1, J
+
+interface KDerived1b : J, K1
+
+interface KDerived2a : K2, J
+
+interface KDerived2b : J, K2
+
+interface KDerived12a : K1, K2, J
+// public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String!
+
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>interface KDerived12b<!> : K1, J, K2 // TODO
+// public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String?
+
+interface KDerived12c : J, K1, K2
+// public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String

compiler/testData/diagnostics/tests/override/clashesOnInheritance/flexibleReturnType.txt

@@ -0,0 +1,73 @@
+package
+
+public /*synthesized*/ fun J(/*0*/ function: () -> kotlin.String!): J
+
+public interface J {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String!
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface K1 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface K2 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived12a : K1, K2, J {
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String!
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived12b : K1, J, K2 {
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String?
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived12c : J, K1, K2 {
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.String
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived1a : K1, J {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived1b : J, K1 {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived2a : K2, J {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface KDerived2b : J, K2 {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String?
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/propertyTypeMismatch.kt

@@ -0,0 +1,29 @@
+open class A {
+    open val foo: Boolean = true
+}
+
+interface IA {
+    val foo: String
+}
+
+interface IAA {
+    val foo: Int
+}
+
+interface IGA<T> {
+    val foo: T
+}
+
+<!PROPERTY_TYPE_MISMATCH_ON_INHERITANCE!>class B1<!>: A(), IA
+
+<!PROPERTY_TYPE_MISMATCH_ON_INHERITANCE!>class B2<!>: A(), IA, IAA
+
+abstract <!PROPERTY_TYPE_MISMATCH_ON_INHERITANCE!>class B3<!>: IA, IAA
+
+class BS1: A(), IGA<Boolean>
+
+class BS2: A(), IGA<Any>
+
+<!PROPERTY_TYPE_MISMATCH_ON_INHERITANCE!>class BS3<!>: A(), IGA<String>
+
+<!PROPERTY_TYPE_MISMATCH_ON_INHERITANCE!>class BG1<!><T>: A(), IGA<T>

compiler/testData/diagnostics/tests/override/clashesOnInheritance/propertyTypeMismatch.txt

@@ -0,0 +1,86 @@
+package
+
+public open class A {
+    public constructor A()
+    public open val foo: kotlin.Boolean = true
+    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 B1 : A, IA {
+    public constructor B1()
+    public open override /*2*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class B2 : A, IA, IAA {
+    public constructor B2()
+    public open override /*3*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class B3 : IA, IAA {
+    public constructor B3()
+    public abstract override /*2*/ /*fake_override*/ val foo: kotlin.String
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BG1</*0*/ T> : A, IGA<T> {
+    public constructor BG1</*0*/ T>()
+    public open override /*2*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS1 : A, IGA<kotlin.Boolean> {
+    public constructor BS1()
+    public open override /*2*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS2 : A, IGA<kotlin.Any> {
+    public constructor BS2()
+    public open override /*2*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS3 : A, IGA<kotlin.String> {
+    public constructor BS3()
+    public open override /*2*/ /*fake_override*/ val foo: kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IA {
+    public abstract val foo: kotlin.String
+    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 interface IAA {
+    public abstract val foo: kotlin.Int
+    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 interface IGA</*0*/ T> {
+    public abstract val foo: T
+    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
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/returnTypeMismatch.kt

@@ -0,0 +1,29 @@
+open class A {
+    open fun foo(): Boolean = true
+}
+
+interface IA {
+    fun foo(): String
+}
+
+interface IAA {
+    fun foo(): Int
+}
+
+interface IGA<T> {
+    fun foo(): T
+}
+
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class B1<!>: A(), IA
+
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class B2<!>: A(), IA, IAA
+
+abstract <!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class B3<!>: IA, IAA
+
+class BS1: A(), IGA<Boolean>
+
+class BS2: A(), IGA<Any>
+
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class BS3<!>: A(), IGA<String>
+
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class BG1<!><T>: A(), IGA<T>

compiler/testData/diagnostics/tests/override/clashesOnInheritance/returnTypeMismatch.txt

@@ -0,0 +1,86 @@
+package
+
+public open class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open fun foo(): 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 B1 : A, IA {
+    public constructor B1()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class B2 : A, IA, IAA {
+    public constructor B2()
+    public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class B3 : IA, IAA {
+    public constructor B3()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BG1</*0*/ T> : A, IGA<T> {
+    public constructor BG1</*0*/ T>()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS1 : A, IGA<kotlin.Boolean> {
+    public constructor BS1()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS2 : A, IGA<kotlin.Any> {
+    public constructor BS2()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class BS3 : A, IGA<kotlin.String> {
+    public constructor BS3()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IA {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IAA {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGA</*0*/ T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): T
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/unrelatedInherited.kt

@@ -0,0 +1,33 @@
+interface IA {
+    fun method(): String
+    val propVal: String
+    var propVar: String
+}
+
+interface IB1 : IA
+interface IB2 : IA
+
+interface IGA<T> {
+    fun method(): T
+    val propVal: T
+    var propVar: T
+}
+
+interface IGB1Str : IGA<String>
+interface IGB2Str : IGA<String>
+interface IGB3Int : IGA<Int>
+
+interface IGB4T<T> : IGA<T>
+interface IGB5T<T> : IGA<T>
+
+interface IC : IB1, IB2
+
+interface IGC1 : IGB1Str, IGB2Str
+
+interface IGC2 : <!INCONSISTENT_TYPE_PARAMETER_VALUES!>IGB1Str, IGB3Int<!>
+
+interface IGC3<T> : IGB4T<T>, IGB5T<T>
+
+interface IGC4<T> : <!INCONSISTENT_TYPE_PARAMETER_VALUES!>IGB4T<T>, IGB5T<String><!>
+
+interface IGC5 : IGB4T<String>, IGB5T<String>

compiler/testData/diagnostics/tests/override/clashesOnInheritance/unrelatedInherited.txt

@@ -0,0 +1,136 @@
+package
+
+public interface IA {
+    public abstract val propVal: kotlin.String
+    public abstract var propVar: kotlin.String
+    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 abstract fun method(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IB1 : IA {
+    public abstract override /*1*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*1*/ /*fake_override*/ var propVar: kotlin.String
+    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 abstract override /*1*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IB2 : IA {
+    public abstract override /*1*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*1*/ /*fake_override*/ var propVar: kotlin.String
+    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 abstract override /*1*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IC : IB1, IB2 {
+    public abstract override /*2*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*2*/ /*fake_override*/ var propVar: kotlin.String
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGA</*0*/ T> {
+    public abstract val propVal: T
+    public abstract var propVar: T
+    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 abstract fun method(): T
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGB1Str : IGA<kotlin.String> {
+    public abstract override /*1*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*1*/ /*fake_override*/ var propVar: kotlin.String
+    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 abstract override /*1*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGB2Str : IGA<kotlin.String> {
+    public abstract override /*1*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*1*/ /*fake_override*/ var propVar: kotlin.String
+    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 abstract override /*1*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGB3Int : IGA<kotlin.Int> {
+    public abstract override /*1*/ /*fake_override*/ val propVal: kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ var propVar: kotlin.Int
+    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 abstract override /*1*/ /*fake_override*/ fun method(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGB4T</*0*/ T> : IGA<T> {
+    public abstract override /*1*/ /*fake_override*/ val propVal: T
+    public abstract override /*1*/ /*fake_override*/ var propVar: T
+    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 abstract override /*1*/ /*fake_override*/ fun method(): T
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGB5T</*0*/ T> : IGA<T> {
+    public abstract override /*1*/ /*fake_override*/ val propVal: T
+    public abstract override /*1*/ /*fake_override*/ var propVar: T
+    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 abstract override /*1*/ /*fake_override*/ fun method(): T
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGC1 : IGB1Str, IGB2Str {
+    public abstract override /*2*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*2*/ /*fake_override*/ var propVar: kotlin.String
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGC2 : IGB1Str, IGB3Int {
+    public abstract override /*2*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*2*/ /*fake_override*/ var propVar: kotlin.String
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGC3</*0*/ T> : IGB4T<T>, IGB5T<T> {
+    public abstract override /*2*/ /*fake_override*/ val propVal: T
+    public abstract override /*2*/ /*fake_override*/ var propVar: T
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): T
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGC4</*0*/ T> : IGB4T<T>, IGB5T<kotlin.String> {
+    public abstract override /*2*/ /*fake_override*/ val propVal: T
+    public abstract override /*2*/ /*fake_override*/ var propVar: T
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): T
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IGC5 : IGB4T<kotlin.String>, IGB5T<kotlin.String> {
+    public abstract override /*2*/ /*fake_override*/ val propVal: kotlin.String
+    public abstract override /*2*/ /*fake_override*/ var propVar: kotlin.String
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*2*/ /*fake_override*/ fun method(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}

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

@@ -11295,6 +11295,45 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance")
+            @TestDataPath("$PROJECT_ROOT")
+            @RunWith(JUnit3RunnerWithInners.class)
+            public static class ClashesOnInheritance extends AbstractDiagnosticsTest {
+                public void testAllFilesPresentInClashesOnInheritance() throws Exception {
+                    KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/diagnostics/tests/override/clashesOnInheritance"), Pattern.compile("^(.+)\\.kt$"), true);
+                }
+
+                @TestMetadata("covariantOverrides.kt")
+                public void testCovariantOverrides() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/covariantOverrides.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("flexibleReturnType.kt")
+                public void testFlexibleReturnType() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/flexibleReturnType.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("propertyTypeMismatch.kt")
+                public void testPropertyTypeMismatch() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/propertyTypeMismatch.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("returnTypeMismatch.kt")
+                public void testReturnTypeMismatch() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/returnTypeMismatch.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("unrelatedInherited.kt")
+                public void testUnrelatedInherited() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/unrelatedInherited.kt");
+                    doTest(fileName);
+                }
+            }
+
             @TestMetadata("compiler/testData/diagnostics/tests/override/parameterNames")
             @TestDataPath("$PROJECT_ROOT")
             @RunWith(JUnit3RunnerWithInners.class)

core/descriptors/src/org/jetbrains/kotlin/resolve/OverridingUtil.java

@@ -319,7 +319,7 @@ public class OverridingUtil {
         }
     }
 
-    private static boolean isMoreSpecific(@NotNull CallableMemberDescriptor a, @NotNull CallableMemberDescriptor b) {
+    public static boolean isMoreSpecific(@NotNull CallableMemberDescriptor a, @NotNull CallableMemberDescriptor b) {
         if (a instanceof SimpleFunctionDescriptor) {
             assert b instanceof SimpleFunctionDescriptor : "b is " + b.getClass();
 
@@ -333,12 +333,14 @@ public class OverridingUtil {
         if (a instanceof PropertyDescriptor) {
             assert b instanceof PropertyDescriptor : "b is " + b.getClass();
 
-            if (((PropertyDescriptor) a).isVar() || ((PropertyDescriptor) b).isVar()) {
-                return ((PropertyDescriptor) a).isVar();
+            PropertyDescriptor pa = (PropertyDescriptor) a;
+            PropertyDescriptor pb = (PropertyDescriptor) b;
+            if (pa.isVar() && pb.isVar()) {
+                return KotlinTypeChecker.DEFAULT.equalTypes(pa.getType(), pb.getType());
             }
 
-            // both vals
-            return KotlinTypeChecker.DEFAULT.isSubtypeOf(((PropertyDescriptor) a).getType(), ((PropertyDescriptor) b).getType());
+            // both vals or var <? val
+            return KotlinTypeChecker.DEFAULT.isSubtypeOf(pa.getType(), pb.getType());
         }
         throw new IllegalArgumentException("Unexpected callable: " + a.getClass());
     }

idea/idea-analysis/src/org/jetbrains/kotlin/idea/highlighter/IdeErrorMessages.java

@@ -96,9 +96,13 @@ public class IdeErrorMessages {
 
         MAP.put(RETURN_TYPE_MISMATCH_ON_OVERRIDE, "<html>Return type is ''{0}'', which is not a subtype of overridden<br/>" +
                                                   "{1}</html>", HTML_RENDER_RETURN_TYPE, DescriptorRenderer.HTML);
+        MAP.put(RETURN_TYPE_MISMATCH_ON_INHERITANCE, "<html>Return types of inherited members are incompatible:<br/>{0},<br/>{1}</html>",
+                DescriptorRenderer.HTML, DescriptorRenderer.HTML);
 
         MAP.put(PROPERTY_TYPE_MISMATCH_ON_OVERRIDE, "<html>Var-property type is ''{0}'', which is not a type of overridden<br/>" +
                                                   "{1}</html>", HTML_RENDER_RETURN_TYPE, DescriptorRenderer.HTML);
+        MAP.put(PROPERTY_TYPE_MISMATCH_ON_INHERITANCE, "<html>Property types of inherited members are incompatible:<br/>{0},<br/>{1}</html>",
+                DescriptorRenderer.HTML, DescriptorRenderer.HTML);
 
         MAP.put(VAR_OVERRIDDEN_BY_VAL, "<html>Val-property cannot override var-property<br />" +
                                        "{1}</html>", DescriptorRenderer.HTML, DescriptorRenderer.HTML);