Add Kotlin 1.9 compatibility tests

src/sbt-test/kotlin/kotlin-1.9-compat/build.sbt

@@ -0,0 +1,14 @@
+import kotlin.Keys._
+
+enablePlugins(KotlinPlugin)
+kotlinLib("stdlib")
+
+kotlinVersion := "1.9.20"
+kotlincJvmTarget := "1.8"
+
+val listClasses = taskKey[Unit]("listClasses")
+
+listClasses := {
+  val classes = (Compile / classDirectory).value.listFiles()
+  streams.value.log.info("classes: " + classes.mkString("Array(", ", ", ")"))
+}

src/sbt-test/kotlin/kotlin-1.9-compat/project/plugins.sbt

@@ -0,0 +1,6 @@
+sys.props.get("plugin.version") match {
+  case Some(version) => addSbtPlugin("org.jetbrains.scala" % "sbt-kotlin-plugin" % version)
+  case _ => sys.error(
+    """The system property 'plugin.version' is not defined.
+      |Specify this property using the scriptedLaunchOpts -Dplugin.version.""".stripMargin)
+}

src/sbt-test/kotlin/kotlin-1.9-compat/src/main/kotlin/SimpleScript.kts

@@ -0,0 +1,3 @@
+// This script is not supposed to be inside source root. Since Kotlin 1.9 it is ignored during the module compilation.
+// See https://youtrack.jetbrains.com/issue/KT-52735
+println("Hello world!")

src/sbt-test/kotlin/kotlin-1.9-compat/src/main/kotlin/demo/simple.kt

@@ -0,0 +1,34 @@
+package demo
+
+fun main(args: Array<String>) {
+    // Test some Kotlin 1.9 features
+    println(findByRgb("#FF0000"))
+    println(Person("John", "Doe"))
+}
+
+enum class Color(val colorName: String, val rgb: String) {
+    RED("Red", "#FF0000"),
+    ORANGE("Orange", "#FF7F00"),
+    YELLOW("Yellow", "#FFFF00")
+}
+
+// In 1.8.20, the entries property for enum classes was introduced as an Experimental feature.
+// The entries property is a modern and performant replacement for the synthetic values() function.
+// In 1.9.0, the entries property is Stable.
+fun findByRgb(rgb: String): Color? = Color.entries.find { it.rgb == rgb }
+
+@JvmInline
+value class Person(private val fullName: String) {
+    // Allowed since Kotlin 1.4.30:
+    init {
+        check(fullName.isNotBlank()) {
+            "Full name shouldn't be empty"
+        }
+    }
+    // Allowed by default since Kotlin 1.9.0:
+    constructor(name: String, lastName: String) : this("$name $lastName") {
+        check(lastName.isNotBlank()) {
+            "Last name shouldn't be empty"
+        }
+    }
+}

src/sbt-test/kotlin/kotlin-1.9-compat/test

@@ -0,0 +1,4 @@
+> compile
+> listClasses
+$ exists target/scala-2.12/classes/demo/SimpleKt.class
+$ absent target/scala-2.12/classes/SimpleScript.class