K2 plugins: create companions with same expect/actual as their owners

#KT-59299 Fixed

compiler/fir/plugin-utils/src/org/jetbrains/kotlin/fir/plugin/ClassBuildingContext.kt

@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.fir.declarations.FirTypeParameterRef
 import org.jetbrains.kotlin.fir.declarations.builder.buildOuterClassTypeParameterRef
 import org.jetbrains.kotlin.fir.declarations.builder.buildRegularClass
 import org.jetbrains.kotlin.fir.declarations.origin
+import org.jetbrains.kotlin.fir.declarations.utils.isExpect
 import org.jetbrains.kotlin.fir.declarations.utils.isLocal
 import org.jetbrains.kotlin.fir.extensions.FirDeclarationGenerationExtension
 import org.jetbrains.kotlin.fir.extensions.FirExtension
@@ -159,6 +160,7 @@ public fun FirExtension.createCompanionObject(
         modality = Modality.FINAL
         status {
             isCompanion = true
+            isExpect = owner.isExpect
         }
     }.build()
 }

plugins/kotlinx-serialization/testData/boxIr/expectActualSealedClass.kt

@@ -0,0 +1,25 @@
+// LANGUAGE: +MultiPlatformProjects
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+// IGNORE REASON: multimodule MPP tests are not supported for K1
+// WITH_STDLIB
+
+// MODULE: m1-common
+// FILE: common.kt
+
+import kotlinx.serialization.*
+
+@Serializable
+expect sealed class Some
+
+fun s(): KSerializer<Some> = Some.Companion.serializer()
+
+// MODULE: m2-jvm()()(m1-common)
+// FILE: jvm.kt
+
+import kotlinx.serialization.*
+
+@Serializable
+actual sealed class Some
+
+fun box(): String = if (s().descriptor.toString() == Some.serializer().descriptor.toString()) "OK" else "fail"

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirLightTreeBlackBoxTestGenerated.java

@@ -93,6 +93,12 @@ public class SerializationFirLightTreeBlackBoxTestGenerated extends AbstractSeri
             runTest("plugins/kotlinx-serialization/testData/boxIr/expectActual.kt");
         }
 
+        @Test
+        @TestMetadata("expectActualSealedClass.kt")
+        public void testExpectActualSealedClass() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/expectActualSealedClass.kt");
+        }
+
         @Test
         @TestMetadata("externalSerialierJava.kt")
         public void testExternalSerialierJava() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java

@@ -91,6 +91,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/expectActual.kt");
     }
 
+    @Test
+    @TestMetadata("expectActualSealedClass.kt")
+    public void testExpectActualSealedClass() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/expectActualSealedClass.kt");
+    }
+
     @Test
     @TestMetadata("externalSerialierJava.kt")
     public void testExternalSerialierJava() throws Exception {