Upgrade Kotlin to version 2.0.0

Kotlin 2.0.0 is the latest version that still supports Kotlin Language
version 1.4 which is required for Gradle 7.6 support, see

https://docs.gradle.org/current/userguide/compatibility.html#kotlin

Author: sschuberth

build.gradle.kts

@@ -1,3 +1,7 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
     // Apply the Java Gradle plugin development plugin to add support for developing Gradle plugins
     `java-gradle-plugin`
@@ -62,6 +66,23 @@ val functionalTest by tasks.registering(Test::class) {
 
 gradlePlugin.testSourceSets.add(functionalTestSourceSet)
 
+tasks.named<JavaCompile>("compileJava") {
+    targetCompatibility = JvmTarget.JVM_17.target
+}
+
+tasks.named<KotlinCompile>("compileKotlin") {
+    compilerOptions {
+        optIn = listOf("kotlin.io.path.ExperimentalPathApi")
+
+        // See https://docs.gradle.org/current/userguide/compatibility.html#java_runtime.
+        jvmTarget = JvmTarget.JVM_17
+
+        // See https://docs.gradle.org/current/userguide/compatibility.html#kotlin.
+        languageVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_4
+        apiVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_4
+    }
+}
+
 tasks.named<Task>("check") {
     // Run the functional tests as part of `check`
     dependsOn(functionalTest)

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 assertj = "3.27.3"
 git-semver-plugin = "0.16.1"
 jgit = "7.3.0.202506031305-r"
-kotlin = "1.9.10"
+kotlin = "2.0.0"
 plugin-publish = "1.3.1"
 slf4j = "2.0.17"
 

src/main/kotlin/git/semver/plugin/semver/VersionFinder.kt

@@ -143,7 +143,7 @@ class VersionFinder(private val settings: SemverSettings, private val tags: Map<
         return when {
             parentSemVersions.isEmpty() -> versionZero()
             parentSemVersions.size == 1 -> parentSemVersions[0]
-            else -> parentSemVersions.max().mergeChanges(parentSemVersions)
+            else -> checkNotNull(parentSemVersions.maxOrNull()).mergeChanges(parentSemVersions)
         }
     }