Add function supertype: make abstract function in sealed class

#KT-30769 Fixed

idea/src/org/jetbrains/kotlin/idea/quickfix/AddFunctionToSupertypeFix.kt

@@ -196,7 +196,11 @@ class AddFunctionToSupertypeFix private constructor(
         ): FunctionDescriptor {
             // TODO: support for generics.
 
-            val modality = if (typeDescriptor.kind == ClassKind.INTERFACE) Modality.ABSTRACT else typeDescriptor.modality
+            val modality = if (typeDescriptor.kind == ClassKind.INTERFACE || typeDescriptor.modality == Modality.SEALED) {
+                Modality.ABSTRACT
+            } else {
+                typeDescriptor.modality
+            }
 
             return functionDescriptor.copy(
                 typeDescriptor,

idea/testData/quickfix/override/nothingToOverride/addFunctionSealedClass.kt

@@ -0,0 +1,6 @@
+// "Add 'abstract fun f()' to 'A'" "true"
+sealed class A {
+}
+class B : A() {
+    <caret>override fun f() {}
+}

idea/testData/quickfix/override/nothingToOverride/addFunctionSealedClass.kt.after

@@ -0,0 +1,7 @@
+// "Add 'abstract fun f()' to 'A'" "true"
+sealed class A {
+    abstract fun f()
+}
+class B : A() {
+    <caret>override fun f() {}
+}

idea/tests/org/jetbrains/kotlin/idea/quickfix/QuickFixTestGenerated.java

@@ -9286,6 +9286,11 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
                 runTest("idea/testData/quickfix/override/nothingToOverride/addFunctionNonUnitReturnType.kt");
             }
 
+            @TestMetadata("addFunctionSealedClass.kt")
+            public void testAddFunctionSealedClass() throws Exception {
+                runTest("idea/testData/quickfix/override/nothingToOverride/addFunctionSealedClass.kt");
+            }
+
             @TestMetadata("addFunctionTrait.kt")
             public void testAddFunctionTrait() throws Exception {
                 runTest("idea/testData/quickfix/override/nothingToOverride/addFunctionTrait.kt");