[KxSerialization] Fixed access private custom serializer on property

When a custom serializer is specified on a type and this type is used in a property of another serializable class, then on the JVM this leads to an error accessing the custom serializer class - because it is private and located in another package.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2495


Merge-request: KT-MR-12877
Merged-by: Sergei Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -225,6 +225,12 @@ public class SerializationFirLightTreeBlackBoxTestGenerated extends AbstractSeri
             runTest("plugins/kotlinx-serialization/testData/boxIr/namedCompanions.kt");
         }
 
+        @Test
+        @TestMetadata("privateCustomSerializer.kt")
+        public void testPrivateCustomSerializer() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/privateCustomSerializer.kt");
+        }
+
         @Test
         @TestMetadata("repeatableSerialInfo.kt")
         public void testRepeatableSerialInfo() throws Exception {

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

@@ -223,6 +223,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/namedCompanions.kt");
     }
 
+    @Test
+    @TestMetadata("privateCustomSerializer.kt")
+    public void testPrivateCustomSerializer() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/privateCustomSerializer.kt");
+    }
+
     @Test
     @TestMetadata("repeatableSerialInfo.kt")
     public void testRepeatableSerialInfo() throws Exception {