Change "most specific return type" definition for fake overrides.

Given overridden descriptors D = d[i].
1. Find D*, subset of D:
  returnType(d* from D*) <: returnType(d) for each d from D.
  Always prefer var to val.
2. Prefer non-flexible return type to flexible.

Check for var/val overrides properly
(NB: this will report PROPERTY_TYPE_MISMATCH_ON_OVERRIDE
for all properties, not just overrides involving vars as it was before).

compiler/testData/diagnostics/tests/CovariantOverrideType.kt

@@ -16,11 +16,11 @@ abstract class B<H>() : A<H> {
     override fun foo2() : <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Unit<!> {
     }
 
-    override val a : <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Double<!> = 1.toDouble()
-    override val <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>a1<!> = 1.toDouble()
+    override val a : <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>Double<!> = 1.toDouble()
+    override val <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>a1<!> = 1.toDouble()
 
     abstract override fun <X> g() : <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Int<!>
     abstract override fun <X> g1() : <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>List<X><!>
 
-    abstract override val g : <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Iterator<Int><!>
+    abstract override val g : <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>Iterator<Int><!>
 }

compiler/testData/diagnostics/tests/delegatedProperty/delegatedPropertyOverridedInTraitTypeMismatch.kt

@@ -7,7 +7,7 @@ interface A {
 }
 
 class AImpl: A  {
-    override val <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>prop<!> by Delegate()
+    override val <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>prop<!> by Delegate()
 }
 
 fun foo() {

compiler/testData/diagnostics/tests/delegation/Delegation_ClashingFunctions.kt

@@ -10,9 +10,9 @@ interface Three {
     public fun foo(): String
 }
 
-<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test123<!>(val v1: One, val v2: Two, val v3: Three) : One by v1, Two by v2, Three by v3 { }
-<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test132<!>(val v1: One, val v2: Two, val v3: Three) : One by v1, Three by v3, Two by v2 { }
+<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED, RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class Test123<!>(val v1: One, val v2: Two, val v3: Three) : One by v1, Two by v2, Three by v3 { }
+<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED, RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class Test132<!>(val v1: One, val v2: Two, val v3: Three) : One by v1, Three by v3, Two by v2 { }
 <!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test312<!>(val v1: One, val v2: Two, val v3: Three) : Three by v3, One by v1, Two by v2 { }
 <!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test321<!>(val v1: One, val v2: Two, val v3: Three) : Three by v3, Two by v2, One by v1 { }
-<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test231<!>(val v1: One, val v2: Two, val v3: Three) : Two by v2, Three by v3, One by v1 { }
-<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test213<!>(val v1: One, val v2: Two, val v3: Three) : Two by v2, One by v1, Three by v3 { }
+<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED, RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class Test231<!>(val v1: One, val v2: Two, val v3: Three) : Two by v2, Three by v3, One by v1 { }
+<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED, MANY_IMPL_MEMBER_NOT_IMPLEMENTED, RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class Test213<!>(val v1: One, val v2: Two, val v3: Three) : Two by v2, One by v1, Three by v3 { }

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

@@ -60,7 +60,7 @@ public interface KDerived1b : J, K1 {
 
 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 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/flexibleReturnTypeList.kt

@@ -30,9 +30,14 @@ interface Test5 : ILNS, IMLS, J
 interface Test6 : ILNS, J, IMLS
 interface Test7 : J, ILNS, IMLS
 
-// ILS and IMLNS are incompatible
+// Return types of ILS::foo and IMLNS::foo are incompatible themselves.
+// However, return type of J::foo is (Mutable)List<String!>!,
+// which is subtype of both List<String> and MutalbeList<String?>.
+// Thus, inheriting from J, IMLNS, and ILS is Ok,
+// but inheriting from IMLNS and ILS is not.
+
+interface Test8 : J, IMLNS, ILS
+interface Test9 : IMLNS, J, ILS
+interface Test10 : IMLNS, ILS, J
 
-<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>interface Test8<!> : J, IMLNS, ILS
-<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>interface Test9<!> : IMLNS, J, ILS
-<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>interface Test10<!> : IMLNS, ILS, J
 <!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>interface Test11<!> : IMLNS, ILS

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

@@ -46,7 +46,7 @@ public interface Test1 : ILNS, J {
 
 public interface Test10 : IMLNS, ILS, J {
     public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.MutableList<kotlin.String?>
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.(Mutable)List<kotlin.String!>!
     public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
 }
@@ -102,14 +102,14 @@ public interface Test7 : J, ILNS, IMLS {
 
 public interface Test8 : J, IMLNS, ILS {
     public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.MutableList<kotlin.String?>
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.(Mutable)List<kotlin.String!>!
     public open override /*3*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*3*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
 public interface Test9 : IMLNS, J, ILS {
     public open override /*3*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.MutableList<kotlin.String?>
+    public abstract override /*3*/ /*fake_override*/ fun foo(): kotlin.(Mutable)List<kotlin.String!>!
     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/genericWithUpperBound.kt

@@ -0,0 +1,56 @@
+// FILE: JFooWithUpperBound.java
+public interface JFooWithUpperBound<T extends IBase> {
+    T foo();
+}
+
+// FILE: JFooWithUpperBoundDerived.java
+public interface JFooWithUpperBoundDerived<T extends IBase> extends JFooWithUpperBound<T> {
+}
+
+// FILE: JCFooWithUpperBound.java
+public class JCFooWithUpperBound<T extends IBase> {
+    public T foo() {
+        return null;
+    }
+}
+
+// FILE: JCFooWithUpperBoundDerived.java
+public class JCFooWithUpperBoundDerived<T extends IBase> extends JCFooWithUpperBound<T> {
+}
+
+// FILE: K.kt
+interface IBase
+
+interface IDerived : IBase
+
+interface IFooWithUpperBound<T : IBase> {
+    fun foo(): T
+}
+
+interface IFooT<T> {
+    fun foo(): T
+}
+
+interface IFoo {
+    fun foo(): IBase
+}
+
+interface IFooDerived : IFoo {
+    override fun foo(): IDerived
+}
+
+interface IFooWithUpperBoundDerived<T : IBase> : IFooWithUpperBound<T>
+
+interface Test1<T : IBase> : IFooWithUpperBound<T>, IFoo
+
+interface Test2<T : IBase> : IFooT<T>, IFoo
+
+interface Test3<T : IDerived> : IFooWithUpperBoundDerived<T>, IFooDerived
+
+interface Test4<T : IBase> : JFooWithUpperBound<T>, IFoo
+
+interface Test5<T : IDerived> : JFooWithUpperBoundDerived<T>, IFooDerived
+
+class Test6<T : IBase> : JCFooWithUpperBound<T>(), IFoo
+
+class Test7<T : IDerived> : JCFooWithUpperBoundDerived<T>(), IFooDerived

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

@@ -0,0 +1,132 @@
+package
+
+public /*synthesized*/ fun </*0*/ T : IBase!> JFooWithUpperBound(/*0*/ function: () -> T!): JFooWithUpperBound<T>
+public /*synthesized*/ fun </*0*/ T : IBase!> JFooWithUpperBoundDerived(/*0*/ function: () -> T!): JFooWithUpperBoundDerived<T>
+
+public interface 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 : 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 IFoo {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): IBase
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IFooDerived : IFoo {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ fun foo(): IDerived
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface IFooT</*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
+}
+
+public interface IFooWithUpperBound</*0*/ T : IBase> {
+    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
+}
+
+public interface IFooWithUpperBoundDerived</*0*/ T : IBase> : IFooWithUpperBound<T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(): T
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public open class JCFooWithUpperBound</*0*/ T : IBase!> {
+    public constructor JCFooWithUpperBound</*0*/ T : IBase!>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open fun foo(): T!
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public open class JCFooWithUpperBoundDerived</*0*/ T : IBase!> : JCFooWithUpperBound<T!> {
+    public constructor JCFooWithUpperBoundDerived</*0*/ T : IBase!>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun foo(): T!
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface JFooWithUpperBound</*0*/ T : IBase!> {
+    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
+}
+
+public interface JFooWithUpperBoundDerived</*0*/ T : IBase!> : JFooWithUpperBound<T!> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(): T!
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Test1</*0*/ T : IBase> : IFooWithUpperBound<T>, IFoo {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): T
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Test2</*0*/ T : IBase> : IFooT<T>, IFoo {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): T
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Test3</*0*/ T : IDerived> : IFooWithUpperBoundDerived<T>, IFooDerived {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): T
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Test4</*0*/ T : IBase> : JFooWithUpperBound<T>, IFoo {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): T!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Test5</*0*/ T : IDerived> : JFooWithUpperBoundDerived<T>, IFooDerived {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*2*/ /*fake_override*/ fun foo(): T!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Test6</*0*/ T : IBase> : JCFooWithUpperBound<T>, IFoo {
+    public constructor Test6</*0*/ T : IBase>()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): T!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Test7</*0*/ T : IDerived> : JCFooWithUpperBoundDerived<T>, IFooDerived {
+    public constructor Test7</*0*/ T : IDerived>()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ /*fake_override*/ fun foo(): T!
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}