Improve fake override construction for expected classes

Fake overrides for abstract members from expected classes should become
non-abstract (final, in fact) in non-abstract expected subclasses

 #KT-22031 Fixed

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

@@ -0,0 +1,55 @@
+// !LANGUAGE: +MultiPlatformProjects
+// MODULE: m1-common
+// FILE: common.kt
+
+expect abstract class BaseA() {
+    abstract fun foo()
+}
+expect open class BaseAImpl() : BaseA
+
+class DerivedA1 : BaseAImpl()
+class DerivedA2 : BaseAImpl() {
+    <!OVERRIDING_FINAL_MEMBER!>override<!> fun foo() = super.foo()
+}
+
+
+
+expect interface BaseB {
+    fun foo()
+}
+expect open class BaseBImpl() : BaseB
+
+class DerivedB1 : BaseBImpl()
+class DerivedB2 : BaseBImpl() {
+    <!OVERRIDING_FINAL_MEMBER!>override<!> fun foo() = super.foo()
+}
+
+
+
+expect interface BaseC {
+    fun foo()
+}
+expect abstract class BaseCImpl() : BaseC
+
+<!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class DerivedC1<!> : BaseCImpl()
+class DerivedC2 : BaseCImpl() {
+    override fun foo() = super.<!ABSTRACT_SUPER_CALL!>foo<!>()
+}
+
+
+
+expect interface BaseD {
+    fun foo()
+}
+abstract class BaseDImpl() : BaseD {
+    fun bar() = super.<!ABSTRACT_SUPER_CALL!>foo<!>()
+}
+
+
+
+expect interface BaseE {
+    fun foo()
+}
+sealed class BaseEImpl() : BaseE {
+    fun bar() = super.<!ABSTRACT_SUPER_CALL!>foo<!>()
+}

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

@@ -0,0 +1,127 @@
+package
+
+public abstract expect class BaseA {
+    public constructor BaseA()
+    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 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 override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public expect interface BaseB {
+    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 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 override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public expect interface BaseC {
+    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 abstract expect class BaseCImpl : BaseC {
+    public constructor BaseCImpl()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract 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 expect interface BaseD {
+    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 abstract class BaseDImpl : BaseD {
+    public constructor BaseDImpl()
+    public final fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract 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 expect interface BaseE {
+    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 sealed class BaseEImpl : BaseE {
+    private constructor BaseEImpl()
+    public final fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract 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 override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class DerivedA2 : BaseAImpl {
+    public constructor DerivedA2()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ 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 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 override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class DerivedB2 : BaseBImpl {
+    public constructor DerivedB2()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ 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 DerivedC1 : BaseCImpl {
+    public constructor DerivedC1()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract 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 DerivedC2 : BaseCImpl {
+    public constructor DerivedC2()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ 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/extendExpectedClassWithoutExplicitOverrideOfMethod.txt

@@ -24,7 +24,7 @@ public final expect class DerivedExplicitCheck : Base {
 
 public final expect class DerivedImplicit : Base {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
-    public abstract expect override /*1*/ /*fake_override*/ fun foo(): kotlin.Unit
+    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
 }

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

@@ -14452,6 +14452,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("extendExpectedClassWithAbstractMember.kt")
+                public void testExtendExpectedClassWithAbstractMember() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithAbstractMember.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("extendExpectedClassWithoutExplicitOverrideOfMethod.kt")
                 public void testExtendExpectedClassWithoutExplicitOverrideOfMethod() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithoutExplicitOverrideOfMethod.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -14452,6 +14452,12 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                     doTest(fileName);
                 }
 
+                @TestMetadata("extendExpectedClassWithAbstractMember.kt")
+                public void testExtendExpectedClassWithAbstractMember() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithAbstractMember.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("extendExpectedClassWithoutExplicitOverrideOfMethod.kt")
                 public void testExtendExpectedClassWithoutExplicitOverrideOfMethod() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/extendExpectedClassWithoutExplicitOverrideOfMethod.kt");

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

@@ -625,7 +625,7 @@ public class OverridingUtil {
         boolean allInvisible = visibleOverridables.isEmpty();
         Collection<CallableMemberDescriptor> effectiveOverridden = allInvisible ? overridables : visibleOverridables;
 
-        Modality modality = determineModality(effectiveOverridden);
+        Modality modality = determineModalityForFakeOverride(effectiveOverridden, current);
         Visibility visibility = allInvisible ? Visibilities.INVISIBLE_FAKE : Visibilities.INHERITED;
 
         // FIXME doesn't work as expected for flexible types: should create a refined signature.
@@ -652,11 +652,16 @@ public class OverridingUtil {
     }
 
     @NotNull
-    private static Modality determineModality(@NotNull Collection<CallableMemberDescriptor> descriptors) {
+    private static Modality determineModalityForFakeOverride(
+            @NotNull Collection<CallableMemberDescriptor> descriptors,
+            @NotNull ClassDescriptor current
+    ) {
         // 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;
@@ -671,25 +676,38 @@ public class OverridingUtil {
             }
         }
 
-        if (hasOpen && !hasAbstract) return Modality.OPEN;
+        if (!hasExpect) {
-        if (!hasOpen && hasAbstract) return Modality.ABSTRACT;
+            if (hasOpen && !hasAbstract) return Modality.OPEN;
+            if (!hasOpen && hasAbstract) return Modality.ABSTRACT;
+        }
 
         Set<CallableMemberDescriptor> allOverriddenDeclarations = new HashSet<CallableMemberDescriptor>();
         for (CallableMemberDescriptor descriptor : descriptors) {
             allOverriddenDeclarations.addAll(getOverriddenDeclarations(descriptor));
         }
-        return getMinimalModality(filterOutOverridden(allOverriddenDeclarations));
+        // 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);
     }
 
     @NotNull
-    private static Modality getMinimalModality(@NotNull Collection<CallableMemberDescriptor> descriptors) {
+    private static Modality getMinimalModality(
-        Modality modality = Modality.ABSTRACT;
+            @NotNull Collection<CallableMemberDescriptor> descriptors,
+            boolean transformAbstractToFinal
+    ) {
+        Modality result = Modality.ABSTRACT;
         for (CallableMemberDescriptor descriptor : descriptors) {
-            if (descriptor.getModality().compareTo(modality) < 0) {
+            Modality effectiveModality =
-                modality = descriptor.getModality();
+                    transformAbstractToFinal && descriptor.getModality() == Modality.ABSTRACT ? Modality.FINAL : descriptor.getModality();
+            if (effectiveModality.compareTo(result) < 0) {
+                result = effectiveModality;
             }
         }
-        return modality;
+        return result;
     }
 
     @NotNull