Allow open impl class for final header class

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/HeaderImplDeclarationChecker.kt

@@ -319,7 +319,8 @@ class HeaderImplDeclarationChecker(val moduleToCheck: ModuleDescriptor? = null)
 
         val substitutor = Substitutor(aTypeParams, bTypeParams, parentSubstitutor)
 
-        if (a.modality != b.modality) return Incompatible.Modality
+        if (a.modality != b.modality && !(a.modality == Modality.FINAL && b.modality == Modality.OPEN)) return Incompatible.Modality
+
         if (a.visibility != b.visibility) return Incompatible.Visibility
 
         areCompatibleTypeParameters(aTypeParams, bTypeParams, substitutor).let { if (it != Compatible) return it }

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

@@ -0,0 +1,16 @@
+// !LANGUAGE: +MultiPlatformProjects
+// MODULE: m1-common
+// FILE: common.kt
+
+header class Foo
+
+header fun getFoo(): Foo
+
+// MODULE: m2-jvm(m1-common)
+// FILE: jvm.kt
+
+impl open class Foo
+
+class Bar : Foo()
+
+impl fun getFoo(): Foo = Bar()

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

@@ -0,0 +1,31 @@
+// -- Module: <m1-common> --
+package
+
+public header fun getFoo(): Foo
+
+public final header class Foo {
+    public constructor Foo()
+    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
+}
+
+
+// -- Module: <m2-jvm> --
+package
+
+public impl fun getFoo(): Foo
+
+public final class Bar : Foo {
+    public constructor Bar()
+    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 open impl class Foo {
+    public constructor Foo()
+    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/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -13098,6 +13098,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("implOpenClass.kt")
+                public void testImplOpenClass() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/implOpenClass.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("simpleHeaderClass.kt")
                 public void testSimpleHeaderClass() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/multiplatform/headerClass/simpleHeaderClass.kt");