kotlinx.serialization: Support @Serializable on sealed interfaces.

Interfaces (regular and sealed) are by default polymorphic. To benefit
from sealing (i.e. knowledge of all inheritors in compile-time), @Serializable
annotation may be added on sealed interface, generating the same serializer
that can be used for sealed classes.

Synthetic nested classes are not generated in DEFAULT_IMPLS mode because
it causes problems when adding a synthetic companion to an interface.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1576

compiler/backend/src/org/jetbrains/kotlin/codegen/ClassBodyCodegen.java

@@ -105,12 +105,14 @@ public abstract class ClassBodyCodegen extends MemberCodegen<KtPureClassOrObject
         }
 
         // Generate synthetic nested classes
-        Collection<DeclarationDescriptor> classifiers = descriptor
+        if (kind != OwnerKind.DEFAULT_IMPLS) {
-                .getUnsubstitutedMemberScope()
+            Collection<DeclarationDescriptor> classifiers = descriptor
-                .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS, MemberScope.Companion.getALL_NAME_FILTER());
+                    .getUnsubstitutedMemberScope()
-        for (DeclarationDescriptor memberDescriptor : classifiers) {
+                    .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS, MemberScope.Companion.getALL_NAME_FILTER());
-            if (memberDescriptor instanceof SyntheticClassOrObjectDescriptor) {
+            for (DeclarationDescriptor memberDescriptor : classifiers) {
-                genSyntheticClassOrObject((SyntheticClassOrObjectDescriptor) memberDescriptor);
+                if (memberDescriptor instanceof SyntheticClassOrObjectDescriptor) {
+                    genSyntheticClassOrObject((SyntheticClassOrObjectDescriptor) memberDescriptor);
+                }
             }
         }
 

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt

@@ -81,7 +81,7 @@ fun AbstractSerialGenerator.allSealedSerializableSubclassesFor(
     klass: ClassDescriptor,
     module: ModuleDescriptor
 ): Pair<List<KotlinType>, List<ClassDescriptor>> {
-    assert(klass.kind == ClassKind.CLASS && klass.modality == Modality.SEALED)
+    assert(klass.modality == Modality.SEALED)
     fun recursiveSealed(klass: ClassDescriptor): Collection<ClassDescriptor> {
         return klass.sealedSubclasses.flatMap { if (it.modality == Modality.SEALED) recursiveSealed(it) else setOf(it) }
     }
@@ -155,7 +155,7 @@ fun findTypeSerializer(module: ModuleDescriptor, kType: KotlinType): ClassDescri
     val stdSer = findStandardKotlinTypeSerializer(module, kType) // see if there is a standard serializer
         ?: findEnumTypeSerializer(module, kType)
     if (stdSer != null) return stdSer
-    if (kType.isInterface()) return module.getClassFromSerializationPackage(SpecialBuiltins.polymorphicSerializer)
+    if (kType.isInterface() && kType.toClassDescriptor?.isSealedSerializableInterface == false) return module.getClassFromSerializationPackage(SpecialBuiltins.polymorphicSerializer)
     return kType.toClassDescriptor?.classSerializer // check for serializer defined on the type
 }
 

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginDeclarationChecker.kt

@@ -420,4 +420,4 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
 }
 
 internal val ClassDescriptor.serializableAnnotationIsUseless: Boolean
-    get() = hasSerializableAnnotationWithoutArgs && !isInternalSerializable && !hasCompanionObjectAsSerializer && !isSerializableEnum()
+    get() = hasSerializableAnnotationWithoutArgs && !isInternalSerializable && !hasCompanionObjectAsSerializer && !isSerializableEnum() && !isSealedSerializableInterface

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginErrorsRendering.kt

@@ -40,8 +40,8 @@ object SerializationPluginErrorsRendering : DefaultErrorMessages.Extension {
         )
         MAP.put(
             SerializationErrors.SERIALIZABLE_ANNOTATION_IGNORED,
-            "@Serializable annotation is ignored because it is impossible to serialize automatically interfaces or enums. " +
+            "@Serializable annotation without arguments can be used only on sealed interfaces." +
-                    "Provide serializer manually via e.g. companion object"
+                    "Non-sealed interfaces are polymorphically serializable by default."
         )
         MAP.put(
             SerializationErrors.NON_SERIALIZABLE_PARENT_MUST_HAVE_NOARG_CTOR,

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializationUtil.kt

@@ -101,7 +101,7 @@ val KotlinType?.toClassDescriptor: ClassDescriptor?
     }
 
 internal val ClassDescriptor.shouldHaveGeneratedMethodsInCompanion: Boolean
-    get() = this.isSerializableObject || this.isSerializableEnum() || this.kind == ClassKind.CLASS && hasSerializableAnnotation
+    get() = this.isSerializableObject || this.isSerializableEnum() || this.kind == ClassKind.CLASS && hasSerializableAnnotation || this.isSealedSerializableInterface
 
 internal val ClassDescriptor.isSerializableObject: Boolean
     get() = kind == ClassKind.OBJECT && hasSerializableAnnotation
@@ -109,6 +109,9 @@ internal val ClassDescriptor.isSerializableObject: Boolean
 internal val ClassDescriptor.isInternallySerializableObject: Boolean
     get() = kind == ClassKind.OBJECT && hasSerializableAnnotationWithoutArgs
 
+internal val ClassDescriptor.isSealedSerializableInterface: Boolean
+    get() = kind == ClassKind.INTERFACE && modality == Modality.SEALED && hasSerializableAnnotation
+
 internal val ClassDescriptor.isInternalSerializable: Boolean //todo normal checking
     get() {
         if (kind != ClassKind.CLASS) return false
@@ -158,6 +161,7 @@ internal fun ClassDescriptor.isAbstractOrSealedSerializableClass(): Boolean =
 
 internal fun ClassDescriptor.polymorphicSerializerIfApplicableAutomatically(): ClassDescriptor? {
     val serializer = when {
+        kind == ClassKind.INTERFACE && modality == Modality.SEALED -> SpecialBuiltins.sealedSerializer
         kind == ClassKind.INTERFACE -> SpecialBuiltins.polymorphicSerializer
         isInternalSerializable && modality == Modality.ABSTRACT -> SpecialBuiltins.polymorphicSerializer
         isInternalSerializable && modality == Modality.SEALED -> SpecialBuiltins.sealedSerializer
@@ -212,6 +216,7 @@ internal fun ClassDescriptor.needSerializerFactory(): Boolean {
     val serializableClass = getSerializableClassDescriptorByCompanion(this) ?: return false
     if (serializableClass.isSerializableObject) return true
     if (serializableClass.isAbstractOrSealedSerializableClass()) return true
+    if (serializableClass.isSealedSerializableInterface) return true
     if (serializableClass.declaredTypeParameters.isEmpty()) return false
     return true
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/test/org/jetbrains/kotlinx/serialization/SerializationIrBoxTestGenerated.java

@@ -30,4 +30,10 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
     public void testMultimoduleInheritance() throws Exception {
         runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/boxIr/multimoduleInheritance.kt");
     }
+
+    @Test
+    @TestMetadata("sealedInterfaces.kt")
+    public void testSealedInterfaces() throws Exception {
+        runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/boxIr/sealedInterfaces.kt");
+    }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/boxIr/sealedInterfaces.kt

@@ -0,0 +1,37 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// TARGET_BACKEND: JVM_IR
+
+// WITH_RUNTIME
+
+package a
+
+import kotlinx.serialization.*
+
+import kotlinx.serialization.descriptors.*
+import kotlin.test.assertEquals
+
+interface I
+
+sealed interface SI
+
+@Serializable
+sealed interface SSI
+
+@Serializable
+class Holder(
+    val i: I,
+    val si: SI,
+    val ssi: SSI
+)
+
+fun SerialDescriptor.checkKind(index: Int, kind: String) {
+    assertEquals(kind, getElementDescriptor(index).kind.toString())
+}
+
+fun box(): String {
+    val desc = Holder.serializer().descriptor
+    desc.checkKind(0, "OPEN")
+    desc.checkKind(1, "OPEN")
+    desc.checkKind(2, "SEALED")
+    return "OK"
+}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Sealed.ir.txt

@@ -9,7 +9,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
           INVOKESPECIAL (Container$$serializer, <init>, ()V)
           PUTSTATIC (Container$$serializer, INSTANCE, LContainer$$serializer;)
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           NEW (kotlinx/serialization/internal/PluginGeneratedSerialDescriptor)
           DUP
           LDC (Container)
@@ -26,13 +26,13 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
           CHECKCAST (kotlinx/serialization/descriptors/SerialDescriptor)
           PUTSTATIC (Container$$serializer, descriptor, Lkotlinx/serialization/descriptors/SerialDescriptor;)
         LABEL (L1)
-        LINENUMBER (14)
+        LINENUMBER (19)
           RETURN
     }
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -41,7 +41,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 
     public kotlinx.serialization.KSerializer[] childSerializers() {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ICONST_1
           ANEWARRAY (kotlinx/serialization/KSerializer)
           ASTORE (1)
@@ -61,7 +61,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
           LDC (decoder)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (0)
           INVOKEVIRTUAL (Container$$serializer, getDescriptor, ()Lkotlinx/serialization/descriptors/SerialDescriptor;)
           ASTORE (2)
@@ -146,7 +146,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 
     public java.lang.Object deserialize(kotlinx.serialization.encoding.Decoder decoder) {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (0)
           ALOAD (1)
           INVOKEVIRTUAL (Container$$serializer, deserialize, (Lkotlinx/serialization/encoding/Decoder;)LContainer;)
@@ -166,7 +166,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 public final class Container$Companion : java/lang/Object {
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -183,7 +183,7 @@ public final class Container$Companion : java/lang/Object {
 
     public final kotlinx.serialization.KSerializer serializer() {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           GETSTATIC (Container$$serializer, INSTANCE, LContainer$$serializer;)
           CHECKCAST (kotlinx/serialization/KSerializer)
           ARETURN
@@ -211,11 +211,11 @@ public final class Container : java/lang/Object {
           LDC (r)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
         LABEL (L2)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           ALOAD (1)
           PUTFIELD (Container, r, LResult;)
@@ -225,7 +225,7 @@ public final class Container : java/lang/Object {
 
     public void <init>(int seen1, Result r, kotlinx.serialization.internal.SerializationConstructorMarker serializationConstructorMarker) {
         LABEL (L0)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ICONST_1
           ICONST_1
           ILOAD (1)
@@ -260,7 +260,7 @@ public final class Container : java/lang/Object {
           LDC (serialDesc)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (13)
+        LINENUMBER (18)
           ALOAD (1)
           ALOAD (2)
           ICONST_0
@@ -297,7 +297,7 @@ final class Result$Companion$$cachedSerializer$delegate$1 : kotlin/jvm/internal/
 
     public final kotlinx.serialization.KSerializer invoke() {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           NEW (kotlinx/serialization/SealedClassSerializer)
           DUP
           LDC (Result)
@@ -343,7 +343,7 @@ final class Result$Companion$$cachedSerializer$delegate$1 : kotlin/jvm/internal/
 
     public java.lang.Object invoke() {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           ALOAD (0)
           INVOKEVIRTUAL (Result$Companion$$cachedSerializer$delegate$1, invoke, ()Lkotlinx/serialization/KSerializer;)
           ARETURN
@@ -354,7 +354,7 @@ final class Result$Companion$$cachedSerializer$delegate$1 : kotlin/jvm/internal/
 public final class Result$Companion : java/lang/Object {
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -373,7 +373,7 @@ public final class Result$Companion : java/lang/Object {
 
     public final kotlinx.serialization.KSerializer serializer() {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           ALOAD (0)
           INVOKESPECIAL (Result$Companion, get$cachedSerializer$delegate, ()Lkotlin/Lazy;)
           INVOKEINTERFACE (kotlin/Lazy, getValue, ()Ljava/lang/Object;)
@@ -405,7 +405,7 @@ final class Result$Err$$cachedSerializer$delegate$1 : kotlin/jvm/internal/Lambda
 
     public final kotlinx.serialization.KSerializer invoke() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           NEW (kotlinx/serialization/internal/ObjectSerializer)
           DUP
           LDC (Result.Err)
@@ -418,7 +418,7 @@ final class Result$Err$$cachedSerializer$delegate$1 : kotlin/jvm/internal/Lambda
 
     public java.lang.Object invoke() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           ALOAD (0)
           INVOKEVIRTUAL (Result$Err$$cachedSerializer$delegate$1, invoke, ()Lkotlinx/serialization/KSerializer;)
           ARETURN
@@ -437,7 +437,7 @@ public final class Result$Err : Result {
           INVOKESPECIAL (Result$Err, <init>, ()V)
           PUTSTATIC (Result$Err, INSTANCE, LResult$Err;)
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           GETSTATIC (kotlin/LazyThreadSafetyMode, PUBLICATION, Lkotlin/LazyThreadSafetyMode;)
           GETSTATIC (Result$Err$$cachedSerializer$delegate$1, INSTANCE, LResult$Err$$cachedSerializer$delegate$1;)
           CHECKCAST (kotlin/jvm/functions/Function0)
@@ -448,7 +448,7 @@ public final class Result$Err : Result {
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           ALOAD (0)
           ACONST_NULL
           INVOKESPECIAL (Result, <init>, (Lkotlin/jvm/internal/DefaultConstructorMarker;)V)
@@ -460,7 +460,7 @@ public final class Result$Err : Result {
 
     public final kotlinx.serialization.KSerializer serializer() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           ALOAD (0)
           INVOKESPECIAL (Result$Err, get$cachedSerializer$delegate, ()Lkotlin/Lazy;)
           INVOKEINTERFACE (kotlin/Lazy, getValue, ()Ljava/lang/Object;)
@@ -481,7 +481,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
           INVOKESPECIAL (Result$OK$$serializer, <init>, ()V)
           PUTSTATIC (Result$OK$$serializer, INSTANCE, LResult$OK$$serializer;)
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           NEW (kotlinx/serialization/internal/PluginGeneratedSerialDescriptor)
           DUP
           LDC (Result.OK)
@@ -502,7 +502,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -511,7 +511,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     public kotlinx.serialization.KSerializer[] childSerializers() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ICONST_1
           ANEWARRAY (kotlinx/serialization/KSerializer)
           ASTORE (1)
@@ -531,7 +531,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
           LDC (decoder)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKEVIRTUAL (Result$OK$$serializer, getDescriptor, ()Lkotlinx/serialization/descriptors/SerialDescriptor;)
           ASTORE (2)
@@ -607,7 +607,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     public java.lang.Object deserialize(kotlinx.serialization.encoding.Decoder decoder) {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           ALOAD (1)
           INVOKEVIRTUAL (Result$OK$$serializer, deserialize, (Lkotlinx/serialization/encoding/Decoder;)LResult$OK;)
@@ -627,7 +627,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 public final class Result$OK$Companion : java/lang/Object {
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -644,7 +644,7 @@ public final class Result$OK$Companion : java/lang/Object {
 
     public final kotlinx.serialization.KSerializer serializer() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           GETSTATIC (Result$OK$$serializer, INSTANCE, LResult$OK$$serializer;)
           CHECKCAST (kotlinx/serialization/KSerializer)
           ARETURN
@@ -672,7 +672,7 @@ public final class Result$OK : Result {
           LDC (s)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           ACONST_NULL
           INVOKESPECIAL (Result, <init>, (Lkotlin/jvm/internal/DefaultConstructorMarker;)V)
@@ -685,7 +685,7 @@ public final class Result$OK : Result {
 
     public void <init>(int seen1, java.lang.String s, kotlinx.serialization.internal.SerializationConstructorMarker serializationConstructorMarker) {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ICONST_1
           ICONST_1
           ILOAD (1)
@@ -722,7 +722,7 @@ public final class Result$OK : Result {
           LDC (serialDesc)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           CHECKCAST (Result)
           ALOAD (1)
@@ -739,7 +739,7 @@ public final class Result$OK : Result {
     }
 }
 
-public abstract class Result : java/lang/Object {
+public abstract class Result : java/lang/Object, X {
     private final static kotlin.Lazy $cachedSerializer$delegate
 
     public final static Result$Companion Companion
@@ -751,7 +751,7 @@ public abstract class Result : java/lang/Object {
           INVOKESPECIAL (Result$Companion, <init>, (Lkotlin/jvm/internal/DefaultConstructorMarker;)V)
           PUTSTATIC (Result, Companion, LResult$Companion;)
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           GETSTATIC (kotlin/LazyThreadSafetyMode, PUBLICATION, Lkotlin/LazyThreadSafetyMode;)
           GETSTATIC (Result$Companion$$cachedSerializer$delegate$1, INSTANCE, LResult$Companion$$cachedSerializer$delegate$1;)
           CHECKCAST (kotlin/jvm/functions/Function0)
@@ -762,18 +762,18 @@ public abstract class Result : java/lang/Object {
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
         LABEL (L1)
-        LINENUMBER (8)
+        LINENUMBER (13)
           RETURN
         LABEL (L2)
     }
 
     public void <init>(int seen1, kotlinx.serialization.internal.SerializationConstructorMarker serializationConstructorMarker) {
         LABEL (L0)
-        LINENUMBER (7)
+        LINENUMBER (12)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -790,6 +790,8 @@ public abstract class Result : java/lang/Object {
 
     public final static kotlin.Lazy access$get$cachedSerializer$delegate$cp()
 
+    public void def()
+
     public final static void write$Self(Result self, kotlinx.serialization.encoding.CompositeEncoder output, kotlinx.serialization.descriptors.SerialDescriptor serialDesc) {
         LABEL (L0)
           ALOAD (0)
@@ -805,3 +807,86 @@ public abstract class Result : java/lang/Object {
         LABEL (L1)
     }
 }
+
+public final class X$Companion : java/lang/Object {
+    final static X$Companion $$INSTANCE
+
+    static void <clinit>() {
+          NEW (X$Companion)
+          DUP
+          INVOKESPECIAL (X$Companion, <init>, ()V)
+          PUTSTATIC (X$Companion, $$INSTANCE, LX$Companion;)
+          RETURN
+    }
+
+    private void <init>() {
+        LABEL (L0)
+        LINENUMBER (6)
+          ALOAD (0)
+          INVOKESPECIAL (java/lang/Object, <init>, ()V)
+          RETURN
+        LABEL (L1)
+    }
+
+    public final kotlinx.serialization.KSerializer serializer() {
+        LABEL (L0)
+        LINENUMBER (6)
+          NEW (kotlinx/serialization/SealedClassSerializer)
+          DUP
+          LDC (X)
+          LDC (LX;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          ICONST_2
+          ANEWARRAY (kotlin/reflect/KClass)
+          ASTORE (1)
+          ALOAD (1)
+          ICONST_0
+          LDC (LResult$OK;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          AASTORE
+          ALOAD (1)
+          ICONST_1
+          LDC (LResult$Err;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          AASTORE
+          ALOAD (1)
+          ICONST_2
+          ANEWARRAY (kotlinx/serialization/KSerializer)
+          ASTORE (1)
+          ALOAD (1)
+          ICONST_0
+          GETSTATIC (Result$OK$$serializer, INSTANCE, LResult$OK$$serializer;)
+          CHECKCAST (kotlinx/serialization/KSerializer)
+          AASTORE
+          ALOAD (1)
+          ICONST_1
+          NEW (kotlinx/serialization/internal/ObjectSerializer)
+          DUP
+          LDC (Result.Err)
+          GETSTATIC (Result$Err, INSTANCE, LResult$Err;)
+          INVOKESPECIAL (kotlinx/serialization/internal/ObjectSerializer, <init>, (Ljava/lang/String;Ljava/lang/Object;)V)
+          CHECKCAST (kotlinx/serialization/KSerializer)
+          AASTORE
+          ALOAD (1)
+          INVOKESPECIAL (kotlinx/serialization/SealedClassSerializer, <init>, (Ljava/lang/String;Lkotlin/reflect/KClass;[Lkotlin/reflect/KClass;[Lkotlinx/serialization/KSerializer;)V)
+          CHECKCAST (kotlinx/serialization/KSerializer)
+          ARETURN
+        LABEL (L1)
+    }
+}
+
+public final class X$DefaultImpls : java/lang/Object {
+    public static void def(X this)
+}
+
+public abstract interface X : java/lang/Object {
+    public final static X$Companion Companion
+
+    static void <clinit>() {
+          GETSTATIC (X$Companion, $$INSTANCE, LX$Companion;)
+          PUTSTATIC (X, Companion, LX$Companion;)
+          RETURN
+    }
+
+    public abstract void def()
+}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Sealed.kt

@@ -3,9 +3,14 @@
 
 import kotlinx.serialization.*
 
+@Serializable
+sealed interface X {
+    fun def() {}
+}
+
 // do not forget to update this test with custom serialinfo annotation when serialization 1.3.0 is released
 @Serializable
-sealed class Result {
+sealed class Result: X {
     @Serializable class OK(val s: String): Result()
     @Serializable object Err: Result()
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Sealed.txt

@@ -5,7 +5,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 
     static void <clinit>() {
         LABEL (L0)
-        LINENUMBER (14)
+        LINENUMBER (19)
           NEW (Container$$serializer)
           DUP
           INVOKESPECIAL (Container$$serializer, <init>, ()V)
@@ -31,7 +31,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -222,7 +222,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 
     public java.lang.Object deserialize(kotlinx.serialization.encoding.Decoder p0) {
         LABEL (L0)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           ALOAD (1)
           INVOKEVIRTUAL (Container$$serializer, deserialize, (Lkotlinx/serialization/encoding/Decoder;)LContainer;)
@@ -241,7 +241,7 @@ public final class Container$$serializer : java/lang/Object, kotlinx/serializati
 public final class Container$Companion : java/lang/Object {
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -250,7 +250,7 @@ public final class Container$Companion : java/lang/Object {
 
     public void <init>(kotlin.jvm.internal.DefaultConstructorMarker $constructor_marker) {
         LABEL (L0)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           INVOKESPECIAL (Container$Companion, <init>, ()V)
           RETURN
@@ -286,7 +286,7 @@ public final class Container : java/lang/Object {
           LDC (r)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (14)
+        LINENUMBER (19)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           ALOAD (0)
@@ -458,7 +458,7 @@ public final class Result$Companion : java/lang/Object {
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (8)
+        LINENUMBER (13)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -467,7 +467,7 @@ public final class Result$Companion : java/lang/Object {
 
     public void <init>(kotlin.jvm.internal.DefaultConstructorMarker $constructor_marker) {
         LABEL (L0)
-        LINENUMBER (8)
+        LINENUMBER (13)
           ALOAD (0)
           INVOKESPECIAL (Result$Companion, <init>, ()V)
           RETURN
@@ -531,7 +531,7 @@ public final class Result$Err : Result {
 
     static void <clinit>() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           NEW (Result$Err)
           DUP
           INVOKESPECIAL (Result$Err, <init>, ()V)
@@ -548,11 +548,11 @@ public final class Result$Err : Result {
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (10)
+        LINENUMBER (15)
           ALOAD (0)
           ACONST_NULL
         LABEL (L1)
-        LINENUMBER (10)
+        LINENUMBER (15)
           INVOKESPECIAL (Result, <init>, (Lkotlin/jvm/internal/DefaultConstructorMarker;)V)
           RETURN
         LABEL (L2)
@@ -575,7 +575,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     static void <clinit>() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           NEW (Result$OK$$serializer)
           DUP
           INVOKESPECIAL (Result$OK$$serializer, <init>, ()V)
@@ -601,7 +601,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -691,7 +691,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 
     public java.lang.Object deserialize(kotlinx.serialization.encoding.Decoder p0) {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           ALOAD (1)
           INVOKEVIRTUAL (Result$OK$$serializer, deserialize, (Lkotlinx/serialization/encoding/Decoder;)LResult$OK;)
@@ -710,7 +710,7 @@ public final class Result$OK$$serializer : java/lang/Object, kotlinx/serializati
 public final class Result$OK$Companion : java/lang/Object {
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -719,7 +719,7 @@ public final class Result$OK$Companion : java/lang/Object {
 
     public void <init>(kotlin.jvm.internal.DefaultConstructorMarker $constructor_marker) {
         LABEL (L0)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           INVOKESPECIAL (Result$OK$Companion, <init>, ()V)
           RETURN
@@ -755,7 +755,7 @@ public final class Result$OK : Result {
           LDC (s)
           INVOKESTATIC (kotlin/jvm/internal/Intrinsics, checkNotNullParameter, (Ljava/lang/Object;Ljava/lang/String;)V)
         LABEL (L1)
-        LINENUMBER (9)
+        LINENUMBER (14)
           ALOAD (0)
           ACONST_NULL
           INVOKESPECIAL (Result, <init>, (Lkotlin/jvm/internal/DefaultConstructorMarker;)V)
@@ -819,7 +819,7 @@ public final class Result$OK : Result {
     }
 }
 
-public abstract class Result : java/lang/Object {
+public abstract class Result : java/lang/Object, X {
     public final static Result$Companion Companion
 
     static void <clinit>() {
@@ -833,7 +833,7 @@ public abstract class Result : java/lang/Object {
 
     private void <init>() {
         LABEL (L0)
-        LINENUMBER (8)
+        LINENUMBER (13)
           ALOAD (0)
           INVOKESPECIAL (java/lang/Object, <init>, ()V)
           RETURN
@@ -842,7 +842,7 @@ public abstract class Result : java/lang/Object {
 
     public void <init>(kotlin.jvm.internal.DefaultConstructorMarker $constructor_marker) {
         LABEL (L0)
-        LINENUMBER (8)
+        LINENUMBER (13)
           ALOAD (0)
           INVOKESPECIAL (Result, <init>, ()V)
           RETURN
@@ -857,6 +857,8 @@ public abstract class Result : java/lang/Object {
         LABEL (L1)
     }
 
+    public void def()
+
     public final static void write$Self(Result self, kotlinx.serialization.encoding.CompositeEncoder output, kotlinx.serialization.descriptors.SerialDescriptor serialDesc) {
         LABEL (L0)
           ALOAD (0)
@@ -872,3 +874,83 @@ public abstract class Result : java/lang/Object {
         LABEL (L1)
     }
 }
+
+public final class X$Companion : java/lang/Object {
+    final static X$Companion $$INSTANCE
+
+    static void <clinit>() {
+        LABEL (L0)
+        LINENUMBER (7)
+          NEW (X$Companion)
+          DUP
+          INVOKESPECIAL (X$Companion, <init>, ()V)
+          ASTORE (0)
+          ALOAD (0)
+          PUTSTATIC (X$Companion, $$INSTANCE, LX$Companion;)
+          RETURN
+    }
+
+    private void <init>() {
+        LABEL (L0)
+        LINENUMBER (7)
+          ALOAD (0)
+          INVOKESPECIAL (java/lang/Object, <init>, ()V)
+          RETURN
+        LABEL (L1)
+    }
+
+    public final kotlinx.serialization.KSerializer serializer() {
+        LABEL (L0)
+          NEW (kotlinx/serialization/SealedClassSerializer)
+          DUP
+          LDC (X)
+          LDC (LX;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          ICONST_2
+          ANEWARRAY (kotlin/reflect/KClass)
+          DUP
+          ICONST_0
+          LDC (LResult$OK;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          AASTORE
+          DUP
+          ICONST_1
+          LDC (LResult$Err;)
+          INVOKESTATIC (kotlin/jvm/internal/Reflection, getOrCreateKotlinClass, (Ljava/lang/Class;)Lkotlin/reflect/KClass;)
+          AASTORE
+          ICONST_2
+          ANEWARRAY (kotlinx/serialization/KSerializer)
+          DUP
+          ICONST_0
+          GETSTATIC (Result$OK$$serializer, INSTANCE, LResult$OK$$serializer;)
+          CHECKCAST (kotlinx/serialization/KSerializer)
+          AASTORE
+          DUP
+          ICONST_1
+          NEW (kotlinx/serialization/internal/ObjectSerializer)
+          DUP
+          LDC (Result.Err)
+          GETSTATIC (Result$Err, INSTANCE, LResult$Err;)
+          INVOKESPECIAL (kotlinx/serialization/internal/ObjectSerializer, <init>, (Ljava/lang/String;Ljava/lang/Object;)V)
+          AASTORE
+          INVOKESPECIAL (kotlinx/serialization/SealedClassSerializer, <init>, (Ljava/lang/String;Lkotlin/reflect/KClass;[Lkotlin/reflect/KClass;[Lkotlinx/serialization/KSerializer;)V)
+          ARETURN
+        LABEL (L1)
+    }
+}
+
+public final class X$DefaultImpls : java/lang/Object {
+    public static void def(X $this)
+}
+
+public abstract interface X : java/lang/Object {
+    public final static X$Companion Companion
+
+    static void <clinit>() {
+          GETSTATIC (X$Companion, $$INSTANCE, LX$Companion;)
+          PUTSTATIC (X, Companion, LX$Companion;)
+          RETURN
+    }
+
+    public abstract void def()
+}

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableIgnored.kt

@@ -1,7 +1,11 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
+// SKIP_TXT
 
 // FILE: test.kt
 import kotlinx.serialization.*
 
 <!SERIALIZABLE_ANNOTATION_IGNORED!>@Serializable<!>
 interface INonSerializable
+
+@Serializable
+sealed interface SealedSerializable

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/diagnostics/SerializableIgnored.txt

@@ -1,7 +0,0 @@
-package
-
-@kotlinx.serialization.Serializable public interface INonSerializable {
-    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
-}