Support @Serializable(ContextualSerializer) on class

It was fixed some time ago (https://github.com/Kotlin/kotlinx.serialization/issues/957)
but for some reason only for old backend.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1776

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/BaseIrGenerator.kt

@@ -403,6 +403,7 @@ abstract class BaseIrGenerator(private val currentClass: IrClass, final override
                     // new signature of context serializer
                     args = args + mutableListOf<IrExpression>().apply {
                         val fallbackDefaultSerializer = findTypeSerializer(pluginContext, kType)
+                            .takeIf { it?.owner?.classId != contextSerializerId }
                         add(instantiate(fallbackDefaultSerializer, kType) ?: irNull())
                         add(
                             createArrayOfExpression(

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

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+
+@Serializable(with = ContextualSerializer::class)
+class Ref(
+    val id: String,
+)
+
+fun box(): String {
+    val kind = Ref.serializer().descriptor.kind.toString()
+    if (kind != "CONTEXTUAL") return kind
+    return "OK"
+}

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

@@ -39,6 +39,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/boxIr/classSerializerAsObject.kt");
         }
 
+        @Test
+        @TestMetadata("contextualByDefault.kt")
+        public void testContextualByDefault() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/contextualByDefault.kt");
+        }
+
         @Test
         @TestMetadata("enumsAreCached.kt")
         public void testEnumsAreCached() throws Exception {

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

@@ -37,6 +37,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/classSerializerAsObject.kt");
     }
 
+    @Test
+    @TestMetadata("contextualByDefault.kt")
+    public void testContextualByDefault() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/contextualByDefault.kt");
+    }
+
     @Test
     @TestMetadata("enumsAreCached.kt")
     public void testEnumsAreCached() throws Exception {