Upgrade to Kotlin 1.4 M2

- The compiler is configured to retain compatibility with Kotlin 1.3.
- Explicit API mode is not yet enabled but could be in the future.
- A workaround for Gradle build is required for now, see
  https://youtrack.jetbrains.com/issue/KT-39610 for more details.
- Some exceptions thrown by Kotlin have changed to NullPointerException,
  see https://youtrack.jetbrains.com/issue/KT-22275 for more details.

Closes gh-24171

build.gradle

@@ -1,6 +1,6 @@
 plugins {
 	id 'io.spring.dependency-management' version '1.0.9.RELEASE' apply false
-	id 'org.jetbrains.kotlin.jvm' version '1.3.72' apply false
+	id 'org.jetbrains.kotlin.jvm' version '1.4-M2' apply false
 	id 'org.jetbrains.dokka' version '0.10.1' apply false
 	id 'org.asciidoctor.jvm.convert' version '2.4.0'
 	id 'io.spring.nohttp' version '0.0.4.RELEASE'
@@ -28,8 +28,8 @@ configure(allprojects) { project ->
 			mavenBom "io.projectreactor:reactor-bom:2020.0.0-SNAPSHOT"
 			mavenBom "io.rsocket:rsocket-bom:1.0.1"
 			mavenBom "org.eclipse.jetty:jetty-bom:9.4.29.v20200521"
-			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.3.72"
-			mavenBom "org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.3.5"
+			mavenBom "org.jetbrains.kotlin:kotlin-bom:1.4-M2"
+			mavenBom "org.jetbrains.kotlinx:kotlinx-coroutines-bom:1.3.7-1.4-M2"
 			mavenBom "org.junit:junit-bom:5.6.2"
 		}
 		dependencies {
@@ -285,6 +285,8 @@ configure(allprojects) { project ->
 			mavenCentral()
 			maven { url "https://repo.spring.io/libs-spring-framework-build" }
 			maven { url "https://repo.spring.io/snapshot" } // Reactor
+			maven { url "https://dl.bintray.com/kotlin/kotlin-eap" } // Kotlin
+			maven { url "https://kotlin.bintray.com/kotlinx" } // Coroutines
 		}
 	}
 	configurations.all {
@@ -307,9 +309,16 @@ configure([rootProject] + javaProjects) { project ->
 
 	pluginManager.withPlugin("kotlin") {
 		apply plugin: "org.jetbrains.dokka"
+		// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+		configurations["kotlinCompilerPluginClasspath"].attributes.attribute(
+				org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+				objects.named(Usage.class, "java-runtime")
+		)
 		compileKotlin {
 			kotlinOptions {
 				jvmTarget = "1.8"
+				languageVersion = "1.3"
+				apiVersion = "1.3"
 				freeCompilerArgs = ["-Xjsr305=strict"]
 				allWarningsAsErrors = true
 			}

settings.gradle

@@ -2,6 +2,7 @@ pluginManagement {
 	repositories {
 		gradlePluginPortal()
 		maven { url 'https://repo.spring.io/plugins-release' }
+		maven { url "https://dl.bintray.com/kotlin/kotlin-eap" } // Kotlin
 	}
 }
 

spring-jdbc/src/test/kotlin/org/springframework/jdbc/core/JdbcOperationsExtensionsTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors
+ * Copyright 2002-2020 the original author or authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,7 +53,7 @@ class JdbcOperationsExtensionsTests {
 	@Test  // gh-22682
 	fun `queryForObject with nullable RowMapper-like function`() {
 		every { template.queryForObject(sql, any<RowMapper<Int>>(), 3) } returns null
-		assertThat(template.queryForObject(sql, 3) { _, _ -> null as Int? }).isNull()
+		assertThat(template.queryForObject<Int?>(sql, 3) { _, _ -> null }).isNull()
 		verify { template.queryForObject(eq(sql), any<RowMapper<Int?>>(), eq(3)) }
 	}
 
@@ -130,7 +130,7 @@ class JdbcOperationsExtensionsTests {
 
 	@Test
 	fun `query with RowMapper-like function`() {
-		val list = listOf(1, 2, 3)
+		val list = mutableListOf(1, 2, 3)
 		every { template.query(sql, ofType<RowMapper<*>>(), 3) } returns list
 		assertThat(template.query(sql, 3) { rs, _ ->
 			rs.getInt(1)

spring-messaging/spring-messaging.gradle

@@ -2,6 +2,12 @@ description = "Spring Messaging"
 
 apply plugin: "kotlin"
 
+// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+configurations["optional"].attributes.attribute(
+		org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+		objects.named(Usage.class, "java-runtime")
+)
+
 dependencies {
 	compile(project(":spring-beans"))
 	compile(project(":spring-core"))

spring-messaging/src/test/kotlin/org/springframework/messaging/simp/annotation/support/SimpAnnotationMethodMessageHandlerKotlinTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020  the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -132,7 +132,7 @@ class SimpAnnotationMethodMessageHandlerKotlinTests {
         messageHandler.registerHandler(testController)
         messageHandler.handleMessage(message)
         assertThat(testController.exception).isNotNull()
-		assertThat(testController.exception).isInstanceOf(IllegalArgumentException::class.java)
+		assertThat(testController.exception).isInstanceOf(NullPointerException::class.java)
     }
 
     private fun createMessage(destination: String, headers: Map<String, String?>): Message<ByteArray> {

spring-test/spring-test.gradle

@@ -2,6 +2,12 @@ description = "Spring TestContext Framework"
 
 apply plugin: "kotlin"
 
+// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+configurations["optional"].attributes.attribute(
+		org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+		objects.named(Usage.class, "java-runtime")
+)
+
 dependencies {
 	compile(project(":spring-core"))
 	optional(project(":spring-aop"))

spring-tx/spring-tx.gradle

@@ -2,6 +2,12 @@ description = "Spring Transaction"
 
 apply plugin: "kotlin"
 
+// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+configurations["optional"].attributes.attribute(
+		org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+		objects.named(Usage.class, "java-runtime")
+)
+
 dependencies {
 	compile(project(":spring-beans"))
 	compile(project(":spring-core"))

spring-web/src/test/kotlin/org/springframework/web/method/annotation/RequestParamMethodArgumentResolverKotlinTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -139,7 +139,7 @@ class RequestParamMethodArgumentResolverKotlinTests {
 
 	@Test
 	fun resolveNonNullableNotRequiredWithoutParameter() {
-		assertThatExceptionOfType(TypeCastException::class.java).isThrownBy {
+		assertThatExceptionOfType(NullPointerException::class.java).isThrownBy {
 			resolver.resolveArgument(nonNullableParamNotRequired, null, webRequest, binderFactory) as String
 		}
 	}
@@ -221,7 +221,7 @@ class RequestParamMethodArgumentResolverKotlinTests {
 		request.method = HttpMethod.POST.name
 		request.contentType = MediaType.MULTIPART_FORM_DATA_VALUE
 
-		assertThatExceptionOfType(TypeCastException::class.java).isThrownBy {
+		assertThatExceptionOfType(NullPointerException::class.java).isThrownBy {
 			resolver.resolveArgument(nonNullableMultipartParamNotRequired, null, webRequest, binderFactory) as MultipartFile
 		}
 	}

spring-webflux/spring-webflux.gradle

@@ -2,6 +2,12 @@ description = "Spring WebFlux"
 
 apply plugin: "kotlin"
 
+// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+configurations["optional"].attributes.attribute(
+		org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+		objects.named(Usage.class, "java-runtime")
+)
+
 dependencies {
 	compile(project(":spring-beans"))
 	compile(project(":spring-core"))