Check for name when searching Companion.serializer() function to generate

Name check was forgotten during refactoring, so any user function with
matching signature could be overwritten by the plugin, while correct
function would not have body.

#KT-55682 Fixed

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

@@ -35,10 +35,13 @@ class SerializableCompanionIrGenerator(
 
     companion object {
         fun getSerializerGetterFunction(serializableIrClass: IrClass): IrSimpleFunction? {
-            val irClass = if (serializableIrClass.isSerializableObject) serializableIrClass else serializableIrClass.companionObject() ?: return null
+            val irClass =
+                if (serializableIrClass.isSerializableObject) serializableIrClass else serializableIrClass.companionObject() ?: return null
             return irClass.findDeclaration<IrSimpleFunction> {
-                (it.valueParameters.size == serializableIrClass.typeParameters.size
-                        && it.valueParameters.all { p -> p.type.isKSerializer() }) && it.returnType.isKSerializer()
+                it.name == SerialEntityNames.SERIALIZER_PROVIDER_NAME
+                        && it.valueParameters.size == serializableIrClass.typeParameters.size
+                        && it.valueParameters.all { p -> p.type.isKSerializer() }
+                        && it.returnType.isKSerializer()
             }
         }
 
@@ -57,10 +60,12 @@ class SerializableCompanionIrGenerator(
     }
 
     fun generate() {
-        val serializerGetterFunction = getSerializerGetterFunction(serializableIrClass) ?: throw IllegalStateException(
-            "Can't find synthesized 'Companion.serializer()' function to generate, " +
-                    "probably clash with user-defined function has occurred"
-        )
+        val serializerGetterFunction =
+            getSerializerGetterFunction(serializableIrClass)?.takeIf { it.isFromPlugin(compilerContext.afterK2) }
+                ?: throw IllegalStateException(
+                    "Can't find synthesized 'Companion.serializer()' function to generate, " +
+                            "probably clash with user-defined function has occurred"
+                )
 
         if (serializableIrClass.isSerializableObject
             || serializableIrClass.isAbstractOrSealedSerializableClass
@@ -115,7 +120,8 @@ class SerializableCompanionIrGenerator(
             )
         )
 
-        val kSerializerIrClass = compilerContext.referenceClass(ClassId(SerializationPackages.packageFqName, SerialEntityNames.KSERIALIZER_NAME))!!.owner
+        val kSerializerIrClass =
+            compilerContext.referenceClass(ClassId(SerializationPackages.packageFqName, SerialEntityNames.KSERIALIZER_NAME))!!.owner
         val targetIrType =
             kSerializerIrClass.defaultType.substitute(mapOf(kSerializerIrClass.typeParameters[0].symbol to compilerContext.irBuiltIns.anyType))
 
@@ -154,7 +160,8 @@ class SerializableCompanionIrGenerator(
 
     private fun getOrCreateSerializerVarargFactory(): IrSimpleFunction {
         irClass.findDeclaration<IrSimpleFunction> {
-            it.valueParameters.size == 1
+            it.name == SerialEntityNames.SERIALIZER_PROVIDER_NAME
+                    && it.valueParameters.size == 1
                     && it.valueParameters.first().isVararg
                     && it.returnType.isKSerializer()
                     && it.isFromPlugin(compilerContext.afterK2)

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

@@ -0,0 +1,37 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.builtins.*
+
+
+@Serializable
+data class Main(val fields: MainFields) {
+    companion object  {
+        fun fieldsSerializer(): KSerializer<MainFields> = MainFields.serializer()
+    }
+}
+
+@Serializable
+data class MainFields(val firstName: String?)
+
+@Serializable
+data class Box<T>(val boxed: T) {
+    companion object {
+        fun <T> serializerLike(tSer: KSerializer<T>): KSerializer<List<Box<T>>> = ListSerializer(serializer(tSer))
+    }
+}
+
+
+fun box(): String {
+    if (Main.fieldsSerializer().descriptor.toString() != "MainFields(firstName: kotlin.String?)") return "Error1"
+    if (MainFields.serializer().descriptor.toString() != "MainFields(firstName: kotlin.String?)") return "Error2"
+    if (Main.serializer().descriptor.toString() != "Main(fields: MainFields)") return "Error3"
+    val boxListDesc = Box.serializerLike(String.serializer()).descriptor
+    if (boxListDesc.toString() != "kotlin.collections.ArrayList(Box(boxed: kotlin.String))") return boxListDesc.toString()
+    val boxDesc = Box.serializer(String.serializer()).descriptor
+    if (boxDesc.toString() != "Box(boxed: kotlin.String)") return boxDesc.toString()
+    return "OK"
+}

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

@@ -158,6 +158,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
         public void testUseSerializersChain() throws Exception {
             runTest("plugins/kotlinx-serialization/testData/boxIr/useSerializersChain.kt");
         }
+
+        @Test
+        @TestMetadata("userDefinedSerializerInCompanion.kt")
+        public void testUserDefinedSerializerInCompanion() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/userDefinedSerializerInCompanion.kt");
+        }
     }
 
     @Nested

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

@@ -156,4 +156,10 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
     public void testUseSerializersChain() throws Exception {
         runTest("plugins/kotlinx-serialization/testData/boxIr/useSerializersChain.kt");
     }
+
+    @Test
+    @TestMetadata("userDefinedSerializerInCompanion.kt")
+    public void testUserDefinedSerializerInCompanion() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/userDefinedSerializerInCompanion.kt");
+    }
 }