Add the compiler classpath of the KotlinCompile task as KaptTask's input

KaptTask was not declaring the compiler classpath (which it takes from
compileKotlin task for calling the compiler) as input. This commit fixes
that and adds a test that checks compiler classpath input checks for the
kapt tasks.

Issue #KT-23879 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/UpToDatenessIT.kt

@@ -72,7 +72,14 @@ class UpToDateIT : BaseGradleIT() {
         override fun initProject(project: Project) = with(project) {
             buildGradle.appendText("\nprintln 'compiler_cp=' + compileKotlin.getComputedCompilerClasspath\$kotlin_gradle_plugin()")
             build("clean") { originalCompilerCp = "compiler_cp=\\[(.*)]".toRegex().find(output)!!.groupValues[1].split(", ") }
-            buildGradle.appendText("\ncompileKotlin.compilerClasspath = files($originalPaths).toList()")
+            buildGradle.appendText("""${'\n'}
+                // Add Kapt to the project to test its input checks as well:
+                apply plugin: 'kotlin-kapt'
+                compileKotlin.compilerClasspath = files($originalPaths).toList()
+                afterEvaluate {
+                    kaptGenerateStubsKotlin.compilerClasspath = files($originalPaths).toList()
+                }
+            """.trimIndent())
         }
 
         override fun mutateProject(project: Project) = with(project) {
@@ -88,7 +95,7 @@ class UpToDateIT : BaseGradleIT() {
         }
 
         override fun checkAfterRebuild(compiledProject: CompiledProject) = with(compiledProject) {
-            assertTasksExecuted(listOf(":compileKotlin"))
+            assertTasksExecuted(listOf(":compileKotlin", ":kaptGenerateStubsKotlin", ":kaptKotlin"))
         }
     }
 

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

@@ -45,6 +45,9 @@ open class KaptTask : ConventionTask(), CompilerArgumentAwareWithInput<K2JVMComp
     val kaptClasspath: FileCollection
         get() = project.files(*kaptClasspathConfigurations.toTypedArray())
 
+    @get:Classpath @get:InputFiles
+    val compilerClasspath: List<File> get() = kotlinCompileTask.computedCompilerClasspath
+
     @get:Internal
     internal lateinit var kaptClasspathConfigurations: List<Configuration>
 
@@ -107,7 +110,7 @@ open class KaptTask : ConventionTask(), CompilerArgumentAwareWithInput<K2JVMComp
 
         val messageCollector = GradleMessageCollector(logger)
         val outputItemCollector = OutputItemsCollectorImpl()
-        val environment = GradleCompilerEnvironment(kotlinCompileTask.computedCompilerClasspath, messageCollector, outputItemCollector, args)
+        val environment = GradleCompilerEnvironment(compilerClasspath, messageCollector, outputItemCollector, args)
         if (environment.toolsJar == null && !isAtLeastJava9) {
             throw GradleException("Could not find tools.jar in system classpath, which is required for kapt to work")
         }