Upgrade to Kotlin 1.6.20-M1

This commit upgrades the Kotlin dependencies to 1.6.20-M1 and configures
the build to generate Java 17 bytecode for Kotlin classes.

Closes gh-27814

build.gradle

@@ -3,8 +3,8 @@ plugins {
 	id 'io.spring.nohttp' version '0.0.10'
 	id "io.freefair.aspectj" version '6.4.0' apply false
 	id 'org.jetbrains.dokka' version '1.6.0' apply false
-	id 'org.jetbrains.kotlin.jvm' version '1.6.10' apply false
-	id "org.jetbrains.kotlin.plugin.serialization" version '1.6.10' apply false
+	id 'org.jetbrains.kotlin.jvm' version '1.6.20-M1' apply false
+	id "org.jetbrains.kotlin.plugin.serialization" version '1.6.20-M1' apply false
 	id 'org.asciidoctor.jvm.convert' version '3.3.2'
 	id 'org.asciidoctor.jvm.pdf' version '3.3.2'
 	id "org.unbroken-dome.xjc" version '2.0.0' apply false
@@ -33,7 +33,7 @@ configure(allprojects) { project ->
 			mavenBom "io.r2dbc:r2dbc-bom:Borca-RELEASE"
 			mavenBom "io.rsocket:rsocket-bom:1.1.1"
 			mavenBom "org.eclipse.jetty:jetty-bom:11.0.7"
-			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.6.10"
+			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.6.20-M1"
 			mavenBom "org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.6.0"
 			mavenBom "org.jetbrains.kotlinx:kotlinx-serialization-bom:1.3.2"
 			mavenBom "org.junit:junit-bom:5.8.2"
@@ -277,13 +277,13 @@ configure([rootProject] + javaProjects) { project ->
 			kotlinOptions {
 				languageVersion = "1.6"
 				apiVersion = "1.6"
-				freeCompilerArgs = ["-Xjsr305=strict", "-Xsuppress-version-warnings", "-Xopt-in=kotlin.RequiresOptIn"]
+				freeCompilerArgs = ["-Xjsr305=strict", "-Xsuppress-version-warnings", "-opt-in=kotlin.RequiresOptIn"]
 				allWarningsAsErrors = true
 			}
 		}
 		compileTestKotlin {
 			kotlinOptions {
-				freeCompilerArgs = ["-Xjsr305=strict"]
+				freeCompilerArgs = ["-Xjsr305=strict", "-opt-in=kotlin.RequiresOptIn"]
 			}
 		}
 	}

gradle/toolchains.gradle

@@ -5,10 +5,10 @@
  * One can choose the toolchain to use for compiling the MAIN sources and/or compiling
  * and running the TEST sources. These options apply to Java, Kotlin and Groovy sources
  * when available.
- * {@code "./gradlew check -PmainToolchain=8 -PtestToolchain=11"} will use:
+ * {@code "./gradlew check -PmainToolchain=17 -PtestToolchain=18"} will use:
  * <ul>
- *     <li>a JDK8 toolchain for compiling the main SourceSet
- *     <li>a JDK11 toolchain for compiling and running the test SourceSet
+ *     <li>a JDK17 toolchain for compiling the main SourceSet
+ *     <li>a JDK18 toolchain for compiling and running the test SourceSet
  * </ul>
  *
  * By default, the build will fall back to using the current JDK and 17 language level for all sourceSets.
@@ -21,11 +21,11 @@
  *
  * We can also configure ENV variables and let Gradle know about them:
  * {@code
- * $ echo JDK11
- * /opt/openjdk/java11
- * $ echo JDK15
- * /opt/openjdk/java15
- * $ ./gradlew -Porg.gradle.java.installations.fromEnv=JDK11,JDK15 check
+ * $ echo JDK17
+ * /opt/openjdk/java17
+ * $ echo JDK18
+ * /opt/openjdk/java18
+ * $ ./gradlew -Porg.gradle.java.installations.fromEnv=JDK17,JDK18 check
  * }
  *
  * @author Brian Clozel
@@ -121,12 +121,12 @@ pluginManager.withPlugin("kotlin") {
 		// Fallback to JDK17
 		compileKotlin {
 			kotlinOptions {
-				jvmTarget = '1.8'
+				jvmTarget = '17'
 			}
 		}
 		compileTestKotlin {
 			kotlinOptions {
-				jvmTarget = '1.8'
+				jvmTarget = '17'
 			}
 		}
 	}
@@ -137,10 +137,9 @@ pluginManager.withPlugin("kotlin") {
 			languageVersion = testLanguageVersion
 		}
 		// See https://kotlinlang.org/docs/gradle.html#attributes-specific-for-jvm
-		def javaVersion = testLanguageVersion.toString() == '8' ? '1.8' : testLanguageVersion.toString()
 		compileTestKotlin {
 			kotlinOptions {
-				jvmTarget = javaVersion
+				jvmTarget = testLanguageVersion.toString()
 				jdkHome = compiler.get().metadata.installationPath.asFile.absolutePath
 			}
 		}

spring-core/src/test/kotlin/org/springframework/core/KotlinReactiveAdapterRegistryTests.kt

@@ -17,6 +17,7 @@
 package org.springframework.core
 
 import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.DelicateCoroutinesApi
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.async
 import kotlinx.coroutines.flow.Flow
@@ -32,6 +33,7 @@ import reactor.test.StepVerifier
 import java.time.Duration
 import kotlin.reflect.KClass
 
+@OptIn(DelicateCoroutinesApi::class)
 class KotlinReactiveAdapterRegistryTests {
 
 	private val registry = ReactiveAdapterRegistry.getSharedInstance()

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/method/annotation/CoroutinesIntegrationTests.kt

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation
 
 import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.DelicateCoroutinesApi
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.async
 import kotlinx.coroutines.delay
@@ -115,6 +116,7 @@ class CoroutinesIntegrationTests : AbstractRequestMappingIntegrationTests() {
 	@ComponentScan(resourcePattern = "**/CoroutinesIntegrationTests*")
 	open class WebConfig
 
+	@OptIn(DelicateCoroutinesApi::class)
 	@RestController
 	class CoroutinesController {