Use correct check in FirSerializationPluginClassChecker to determine if the given type is a enum class.

Otherwise, findTypeSerializerOrContextUnchecked works incorrectly when analyzing code
compiled with Kotlin 1.7.20 and serialization 1.4.1.

#KT-57704 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/K2KotlinxSerializationIT.kt

@@ -8,6 +8,10 @@ package org.jetbrains.kotlin.gradle
 import org.gradle.util.GradleVersion
 import org.jetbrains.kotlin.gradle.testbase.*
 import org.junit.jupiter.api.DisplayName
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
+import kotlin.io.path.listDirectoryEntries
+import kotlin.test.assertTrue
 
 @OtherGradlePluginTests
 class K2KotlinxSerializationIT : KGPBaseTest() {
@@ -32,4 +36,23 @@ class K2KotlinxSerializationIT : KGPBaseTest() {
             }
         }
     }
+
+    @DisplayName("Compile code with kotlinx.serialization K2 against old K1 library without enum factory support (KT-57704).")
+    @GradleTest
+    fun `test kotlinx serialization K2 against K1 library`(gradleVersion: GradleVersion, @TempDir tempDir: Path) {
+        project("kotlinxSerializationK2AgainstK1Lib/lib", gradleVersion, localRepoDir = tempDir) {
+            build(":publish") {
+                assertTasksExecuted(":publish")
+                val publishedLib = tempDir.resolve("com/example/serialization_lib/serialization_lib/1.0")
+                assertDirectoryExists(publishedLib)
+                assertTrue(publishedLib.listDirectoryEntries().isNotEmpty())
+            }
+        }
+
+        project("kotlinxSerializationK2AgainstK1Lib/app", gradleVersion, localRepoDir = tempDir) {
+            build(":run") {
+                assertTasksExecuted(":run")
+            }
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/app/build.gradle.kts

@@ -0,0 +1,33 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+
+plugins {
+    kotlin("jvm")
+    kotlin("plugin.serialization")
+    `maven-publish`
+    application
+}
+
+repositories {
+    mavenLocal()
+    maven("<localRepo>")
+    mavenCentral()
+}
+
+group = "com.example.serialization_app"
+version = "1.0"
+
+dependencies {
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.4.1")
+    implementation("com.example.serialization_lib:serialization_lib:1.0")
+}
+
+val compileKotlin: KotlinCompile by tasks
+
+compileKotlin.compilerOptions {
+    languageVersion.set(KotlinVersion.KOTLIN_2_0)
+}
+
+application {
+    mainClass.set("com.example.serialization_app.MainKt")
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/app/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "serialization_app"

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/app/src/main/kotlin/Main.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+package com.example.serialization_app
+
+import com.example.serialization_lib.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.*
+
+@Serializable
+class EnumUsage(val access: Access)
+
+fun main() {
+    val s = Json.encodeToString(EnumUsage.serializer(), EnumUsage(Access.ReadWrite))
+    if (s != """{"access":"rw"}""") throw AssertionError("Unexpected serialized form: $s")
+    val usage = Json.decodeFromString<EnumUsage>(s)
+    if (usage.access != Access.ReadWrite) throw AssertionError("Unexpected deserialized result: ${usage.access}")
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/lib/build.gradle.kts

@@ -0,0 +1,29 @@
+plugins {
+    kotlin("jvm") version "1.7.20"
+    kotlin("plugin.serialization") version "1.7.20"
+    `maven-publish`
+}
+
+repositories {
+    mavenLocal()
+    mavenCentral()
+}
+
+group = "com.example.serialization_lib"
+version = "1.0"
+
+dependencies {
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.4.1")
+}
+
+publishing {
+    repositories {
+        maven("<localRepo>")
+    }
+
+    publications {
+        create<MavenPublication>("Maven") {
+            from(components["java"])
+        }
+    }
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/lib/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "serialization_lib"

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2AgainstK1Lib/lib/src/main/kotlin/Base.kt

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+package com.example.serialization_lib
+
+import kotlinx.serialization.*
+
+@Serializable
+enum class Access {
+    Read, Write, @SerialName("rw") ReadWrite, Unknown
+}
+

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -536,7 +536,7 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
             }
             checkTypeArguments(type, source, reporter)
         } else {
-            if (!type.isEnum) {
+            if (type.toRegularClassSymbol(session)?.isEnumClass != true) {
                 // enums are always serializable
                 reporter.reportOn(source, FirSerializationErrors.SERIALIZER_NOT_FOUND, type)
             }