[BT] Add a test for KT-61206

compiler/build-tools/kotlin-build-tools-api/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
     kotlin("jvm")
     id("jps-compatible")
@@ -8,6 +10,8 @@ configureKotlinCompileTasksGradleCompatibility()
 
 dependencies {
     compileOnly(kotlinStdlib())
+    testApiJUnit5()
+    testImplementation(kotlinStdlib())
 }
 
 kotlin {
@@ -16,4 +20,14 @@ kotlin {
 
 publish()
 
-standardPublicJars()
+standardPublicJars()
+
+tasks.named<KotlinCompile>("compileTestKotlin") {
+    compilerOptions {
+        optIn.add("org.jetbrains.kotlin.buildtools.api.ExperimentalBuildToolsApi")
+    }
+}
+
+projectTest(jUnitMode = JUnitMode.JUnit5) {
+    useJUnitPlatform()
+}

compiler/build-tools/kotlin-build-tools-api/src/test/kotlin/SharedApiClassesClassLoaderTest.kt

@@ -0,0 +1,25 @@
+import org.jetbrains.kotlin.buildtools.api.CompilationService
+import org.jetbrains.kotlin.buildtools.api.SharedApiClassesClassLoader
+import org.junit.jupiter.api.DisplayName
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+class SharedApiClassesClassLoaderTest {
+    @DisplayName("No application classes are leaked into the `SharedApiClassesClassLoader`")
+    @Test
+    fun testNoLeakedApplicationClasses() {
+        val possibleLeakedClass = "org.junit.jupiter.api.Test"
+        ClassLoader.getSystemClassLoader()
+            .loadClass(possibleLeakedClass) // Ensure that the class is there. Otherwise, ClassNotFoundException will be thrown.
+        val sharedApiClassesClassLoader = SharedApiClassesClassLoader()
+        assertThrows<ClassNotFoundException>("`$possibleLeakedClass` is present in the system classloader, but must not be present in the custom classloader") {
+            sharedApiClassesClassLoader.loadClass(possibleLeakedClass)
+        }
+        sharedApiClassesClassLoader.loadClass(CompilationService::class.java.name) // The build tools API classes are still available
+    }
+}