Fix handling of removed classpath folders

xtext-gradle-plugin/src/integTest/java/org/xtext/gradle/test/BuildingASimpleXtendProject.xtend

@@ -56,6 +56,26 @@ class BuildingASimpleXtendProject extends AbstractXtendIntegrationTest {
 
 		snapshot.assertChangedClasses("UpStream", "DownStream")
 	}
+
+	@Test
+	def generatorHandlesDeletionOfClasspathFolders() {
+		createFile('src/main/java/UpStream.xtend', '''
+			class UpStream {}
+		''')
+		createFile('src/test/java/DownStream.xtend', '''
+			class DownStream{
+			}
+		''')
+		build("build")
+		val snapshot = snapshot(projectDir)
+
+		//remove the main sources from the test classpath
+		buildFile << "sourceSets.test.compileClasspath = configurations.testCompileClasspath"
+		build("build")
+
+		snapshot.assertChangedClasses("DownStream")
+	}
+
 
 	@Test
 	def affectedResourcesAreDetectedAcrossXtendAndJava() {

xtext-gradle-plugin/src/main/java/org/xtext/gradle/tasks/XtextGenerate.xtend

@@ -127,10 +127,20 @@ abstract class XtextGenerate extends DefaultTask {
 				request.dirtyFiles += file
 			}
 		]
-		inputs.getFileChanges(classpath).forEach [ change |
-			// Gradle notifies us about individual .class files, but we only want their containing directory
-			request.dirtyClasspathEntries += classpath.files.findFirst[change.file.path.startsWith(it.path)]
-		]
+		val classpathRoots = classpath.files
+		inputs.getFileChanges(classpath)
+			.forEach[change |
+				if (change.normalizedPath.isEmpty) {
+					request.dirtyClasspathEntries += change.file
+				} else {
+					val root = classpathRoots.findFirst[change.file.path.startsWith(it.path)]
+					if (root !== null) {
+						request.dirtyClasspathEntries += root
+					} else {
+						request.incremental = false
+					}
+				}
+			]
 	}
 
 	def installDebugInfo(File classesDir) {