Add Kotlin version override (Kotlin#245)

- Add a dependency constraint in the root project to allow overriding the Kotlin version.
- The Kotlin version is not overridden in the Benchmark Gradle plugin - this should use the version of Kotlin embedded into Gradle.
- Add a check to ensure that the version is overridden.
- add some comments to explain Kotlin version override

build.gradle

@@ -1,3 +1,4 @@
+import org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapperKt
 import tasks.CheckReadmeTask
 
 buildscript {
@@ -10,6 +11,16 @@ buildscript {
 
     dependencies {
         classpath(libs.kotlinx.teamInfraGradlePlugin)
+
+        String kotlinVersion = providers.gradleProperty("kotlin_version").orNull
+        if (kotlinVersion != null && !kotlinVersion.isBlank()) {
+            // In addition to overriding the Kotlin version in the Version Catalog,
+            // also enforce the KGP version using a dependency constraint.
+            // Constraints are stricter than Version Catalog.
+            constraints {
+                classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion")
+            }
+        }
     }
 }
 
@@ -54,8 +65,22 @@ afterEvaluate {
 }
 //endregion
 
+String currentKgpVersion = KotlinPluginWrapperKt.getKotlinPluginVersion(project)
+logger.info("Using Kotlin Gradle Plugin ${currentKgpVersion}")
+
+String kotlinVersionOverride = providers.gradleProperty("kotlin_version").getOrNull()
+
+if (kotlinVersionOverride != null) {
+    String versionCatalogKotlinVersion = libs.versions.kotlin.get()
+    if (kotlinVersionOverride != versionCatalogKotlinVersion) {
+        throw new IllegalStateException("Kotlin version in Version Catalog was not overridden. Expected:$kotlinVersionOverride, actual:$versionCatalogKotlinVersion.")
+    }
+    if (kotlinVersionOverride != currentKgpVersion) {
+        throw new IllegalStateException("Kotlin Gradle Plugin version was not overridden. Expected:$kotlinVersionOverride, actual:$currentKgpVersion.")
+    }
+}
+
 allprojects {
-    logger.info("Using Kotlin ${libs.versions.kotlin.get()} for project $it")
     repositories {
         KotlinCommunity.addDevRepositoryIfEnabled(delegate, project)
     }

gradle/libs.versions.toml

@@ -1,5 +1,6 @@
 [versions]
 
+# Note: Kotlin version can be overridden by passing `-Pkotlin_version=<version>`
 kotlin = "1.9.21"
 kotlinx-binaryCompatibilityValidator = "0.15.0-Beta.1"
 kotlinx-teamInfra = "0.4.0-dev-81"

plugin/build.gradle

@@ -13,6 +13,10 @@ buildscript {
 
     dependencies {
         classpath(libs.kotlinx.teamInfraGradlePlugin)
+        // Note: unlike the root project, don't override KGP version in this project.
+        // Gradle plugins should only use the embedded-kotlin version.
+        // kotlinx-benchmark uses an external KGP the moment... but that should be fixed
+        // https://github.com/Kotlin/kotlinx-benchmark/issues/244
     }
 }
 

settings.gradle

@@ -7,6 +7,20 @@ pluginManagement {
     }
 }
 
+dependencyResolutionManagement {
+    versionCatalogs {
+        create("libs") {
+            String kotlinVersion = providers.gradleProperty("kotlin_version").orNull
+            if (kotlinVersion != null && !kotlinVersion.isBlank()) {
+                // Override the default Kotlin version.
+                // The only intended use-case is for testing dev Kotlin builds using kotlinx-benchmark.
+                // The Kotlin version should not be overridden during regular development.
+                version("kotlin", kotlinVersion)
+            }
+        }
+    }
+}
+
 rootProject.name = 'kotlinx-benchmark'
 
 includeBuild("plugin")