inject Kotlin classes to javac classpath carefully

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -62,20 +62,23 @@ open class KotlinPlugin: Plugin<Project> {
                     val kotlinTask: KotlinCompile = project.getTasks().add(kotlinTaskName, javaClass<KotlinCompile>())!!
 
                     javaBasePlugin.configureForSourceSet(sourceSet, kotlinTask)
+                    // store kotlin classes in separate directory. They will serve as class-path to java compiler
+                    val kotlinOutputDir = File(project.getBuildDir(), "kotlin-classes/${sourceSetName}")
+                    // builtBy allows to correctly run kotlinTask as a transitive dependency on anything,
+                    // that wants to use sourceSet output (e.g., 'jar' task
+                    sourceSet.getOutput()?.dir(mapOf(Pair("builtBy", kotlinTask)), kotlinOutputDir)
+                    kotlinTask.setDestinationDir(kotlinOutputDir);
 
                     kotlinTask.setDescription("Compiles the $sourceSet.kotlin.")
                     kotlinTask.source(kotlinDirSet)
 
                     val javaTask = project.getTasks().findByName(sourceSet.getCompileJavaTaskName()) as AbstractCompile?
-                    javaTask?.dependsOn(kotlinTaskName)
 
-                    val sourceSetCompileConfigurationName = if (sourceSetName == SourceSet.MAIN_SOURCE_SET_NAME) {
-                        "compile"
-                    } else {
-                        "${sourceSetName}Compile"
+                    if (javaTask != null) {
+                        javaTask.dependsOn(kotlinTaskName)
+                        val javacClassPath = javaTask.getClasspath() + project.files(kotlinTask.getDestinationDir());
+                        javaTask.setClasspath(javacClassPath)
                     }
-
-                    project.getDependencies()?.add(sourceSetCompileConfigurationName, project.files(kotlinTask.getDestinationDir()))
                 }
             }
         })

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/KotlinGradlePluginIT.kt

@@ -38,7 +38,6 @@ class BasicKotlinGradleIT {
 
         val builder = ProcessBuilder(cmd)
         builder.directory(projectDir)
-
         builder.redirectErrorStream(true)
         val process = builder.start()
 
@@ -59,6 +58,32 @@ class BasicKotlinGradleIT {
         assertTrue(buildOutput.contains(":compileKotlin"), "Should contain ':compileKotlin'")
         assertTrue(buildOutput.contains(":compileTestKotlin"), "Should contain ':compileTestKotlin'")
         assertTrue(buildOutput.contains(":compileDeployKotlin"), "Should contain ':compileDeployKotlin'")
+
+        // Run the build second time, assert everything is up-to-date
+
+        val up2dateBuilder = ProcessBuilder(cmd)
+        up2dateBuilder.directory(projectDir)
+        up2dateBuilder.redirectErrorStream(true)
+        val up2dateProcess = up2dateBuilder.start()
+
+        val up2dateProcessScanner = Scanner(up2dateProcess.getInputStream()!!)
+        val up2dateText = StringBuilder()
+        while (up2dateProcessScanner.hasNextLine()) {
+            up2dateText append up2dateProcessScanner.nextLine()
+            up2dateText append "\n"
+        }
+        up2dateProcessScanner.close()
+
+        val up2dateResult = up2dateProcess.waitFor()
+        val up2dateBuildOutput = up2dateText.toString()
+
+        println(up2dateBuildOutput)
+
+        assertEquals(up2dateResult, 0)
+        assertTrue(up2dateBuildOutput.contains(":compileKotlin UP-TO-DATE"), "Should contain ':compileKotlin UP-TO-DATE'")
+        assertTrue(up2dateBuildOutput.contains(":compileTestKotlin UP-TO-DATE"), "Should contain ':compileTestKotlin UP-TO-DATE'")
+        assertTrue(up2dateBuildOutput.contains(":compileDeployKotlin UP-TO-DATE"), "Should contain ':compileDeployKotlin UP-TO-DATE'")
+        assertTrue(up2dateBuildOutput.contains(":compileJava UP-TO-DATE"), "Should contain ':compileJava UP-TO-DATE'")
     }