Pin the plugin's Kotlin language level on 2.0

build.gradle.kts

@@ -36,8 +36,8 @@ kotlin {
   explicitApi()
   compilerOptions {
     // https://docs.gradle.org/current/userguide/compatibility.html#kotlin
-    @Suppress("DEPRECATION") // TODO: bump apiVersion to 2.0 to match Gradle 9.0
-    apiVersion = KotlinVersion.KOTLIN_1_8
+    apiVersion = KotlinVersion.KOTLIN_2_0
+    languageVersion = apiVersion
     jvmTarget = JvmTarget.JVM_11
     freeCompilerArgs.addAll(
       "-Xjvm-default=all",

docs/changes/README.md

@@ -2,6 +2,11 @@
 
 ## [Unreleased](https://github.com/GradleUp/shadow/compare/9.0.0-beta14...HEAD) - 2025-xx-xx
 
+**Fixed**
+
+- Pin the plugin's Kotlin language level on 2.0. ([#1448](https://github.com/GradleUp/shadow/pull/1448))  
+  The language level used in `9.0.0-beta14` is 2.2, which may cause compatibility issues for the plugins depending on
+  Shadow.
 
 ## [9.0.0-beta14](https://github.com/GradleUp/shadow/releases/tag/9.0.0-beta14) - 2025-05-28
 

src/documentTest/kotlin/com/github/jengelman/gradle/plugins/shadow/DocCodeSnippetTest.kt

@@ -10,7 +10,6 @@ import org.junit.jupiter.api.io.TempDir
 
 class DocCodeSnippetTest {
 
-  @OptIn(ExperimentalStdlibApi::class)
   @TestFactory
   fun provideDynamicTests(@TempDir root: Path): List<DynamicTest> {
     val langExecutables = DslLang.entries.map { executor ->

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformer.kt

@@ -238,7 +238,6 @@ public open class PropertiesFileTransformer @Inject constructor(
     public companion object {
       @JvmStatic
       public fun from(value: String): MergeStrategy {
-        @OptIn(ExperimentalStdlibApi::class)
         return entries.find { it.name.equals(value, ignoreCase = true) }
           ?: throw IllegalArgumentException("Unknown merge strategy: $value")
       }