Refine modality of fake overrides inherited from abstract expected members

In an open expected class inheriting an expected interface, abstract
members are now inherited as _open_ fake overrides, not final. Final was
technically safer but also stricter and thus could be unexpected by the
user. In a final class, abstract members are still inherited as _final_
fake overrides. So, the general rule is now the following: the modality
of an expected fake override, which overrides only abstract members, in
a non-abstract class is equal to the modality of that class

 #KT-22031 Fixed

compiler/testData/diagnostics/tests/multiplatform/headerClass/dontOverrideMethodsFromInterfaceInCommonCode.txt

@@ -17,14 +17,14 @@ public interface Foo {
 
 public final expect class ImplicitFoo : Foo {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
 public final expect class ImplicitFooCheck : Foo {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }

compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithAbstractMember.kt

@@ -9,7 +9,7 @@ expect open class BaseAImpl() : BaseA
 
 class DerivedA1 : BaseAImpl()
 class DerivedA2 : BaseAImpl() {
-    <!OVERRIDING_FINAL_MEMBER!>override<!> fun foo() = super.foo()
+    override fun foo() = super.foo()
 }
 
 
@@ -21,7 +21,7 @@ expect open class BaseBImpl() : BaseB
 
 class DerivedB1 : BaseBImpl()
 class DerivedB2 : BaseBImpl() {
-    <!OVERRIDING_FINAL_MEMBER!>override<!> fun foo() = super.foo()
+    override fun foo() = super.foo()
 }
 
 
@@ -53,3 +53,10 @@ expect interface BaseE {
 sealed class BaseEImpl() : BaseE {
     fun bar() = super.<!ABSTRACT_SUPER_CALL!>foo<!>()
 }
+
+
+
+expect interface BaseF {
+    fun foo()
+}
+expect class BaseFImpl() : BaseF

compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithAbstractMember.txt

@@ -11,7 +11,7 @@ public abstract expect class BaseA {
 public open expect class BaseAImpl : BaseA {
     public constructor BaseAImpl()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public final expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
@@ -26,7 +26,7 @@ public expect interface BaseB {
 public open expect class BaseBImpl : BaseB {
     public constructor BaseBImpl()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public final expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
@@ -78,10 +78,25 @@ public sealed class BaseEImpl : BaseE {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public expect interface BaseF {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract expect fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final expect class BaseFImpl : BaseF {
+    public constructor BaseFImpl()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public final class DerivedA1 : BaseAImpl {
     public constructor DerivedA1()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public final expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
@@ -97,7 +112,7 @@ public final class DerivedA2 : BaseAImpl {
 public final class DerivedB1 : BaseBImpl {
     public constructor DerivedB1()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public final expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    public open expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }

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

@@ -659,9 +659,7 @@ public class OverridingUtil {
         // Optimization: avoid creating hash sets in frequent cases when modality can be computed trivially
         boolean hasOpen = false;
         boolean hasAbstract = false;
-        boolean hasExpect = false;
         for (CallableMemberDescriptor descriptor : descriptors) {
-            hasExpect |= descriptor.isExpect();
             switch (descriptor.getModality()) {
                 case FINAL:
                     return Modality.FINAL;
@@ -676,33 +674,39 @@ public class OverridingUtil {
             }
         }
 
-        if (!hasExpect) {
-            if (hasOpen && !hasAbstract) return Modality.OPEN;
-            if (!hasOpen && hasAbstract) return Modality.ABSTRACT;
+        // Fake overrides of abstract members in non-abstract expected classes should not be abstract, because otherwise it would be
+        // impossible to inherit a non-expected class from that expected class in common code.
+        // We're making their modality that of the containing class, because this is the least confusing behavior for the users.
+        // However, it may cause problems if we reuse resolution results of common code when compiling platform code (see KT-15220)
+        boolean transformAbstractToClassModality =
+                current.isExpect() && (current.getModality() != Modality.ABSTRACT && current.getModality() != Modality.SEALED);
+
+        if (hasOpen && !hasAbstract) {
+            return Modality.OPEN;
+        }
+        if (!hasOpen && hasAbstract) {
+            return transformAbstractToClassModality ? current.getModality() : Modality.ABSTRACT;
         }
 
         Set<CallableMemberDescriptor> allOverriddenDeclarations = new HashSet<CallableMemberDescriptor>();
         for (CallableMemberDescriptor descriptor : descriptors) {
             allOverriddenDeclarations.addAll(getOverriddenDeclarations(descriptor));
         }
-        // Fake overrides of abstract members in non-abstract expected classes should not be abstract, because otherwise it would be
-        // impossible to inherit a non-expected class from that expected class in common code.
-        // We cannot assume that they're open though, because the actual abstract function from an expected super class
-        // can be implemented with a final function in the actual class for this class.
-        boolean transformAbstractToFinal =
-                current.isExpect() && (current.getModality() != Modality.ABSTRACT && current.getModality() != Modality.SEALED);
-        return getMinimalModality(filterOutOverridden(allOverriddenDeclarations), transformAbstractToFinal);
+        return getMinimalModality(filterOutOverridden(allOverriddenDeclarations), transformAbstractToClassModality, current.getModality());
     }
 
     @NotNull
     private static Modality getMinimalModality(
             @NotNull Collection<CallableMemberDescriptor> descriptors,
-            boolean transformAbstractToFinal
+            boolean transformAbstractToClassModality,
+            @NotNull Modality classModality
     ) {
         Modality result = Modality.ABSTRACT;
         for (CallableMemberDescriptor descriptor : descriptors) {
             Modality effectiveModality =
-                    transformAbstractToFinal && descriptor.getModality() == Modality.ABSTRACT ? Modality.FINAL : descriptor.getModality();
+                    transformAbstractToClassModality && descriptor.getModality() == Modality.ABSTRACT
+                    ? classModality
+                    : descriptor.getModality();
             if (effectiveModality.compareTo(result) < 0) {
                 result = effectiveModality;
             }