Add marker interface LibraryClassLoader

This mixes in a market interface with scalaVersion method into URLClassLoader.
We can use this later to distinguish new launcher vs old one.

launcher-implementation/src/main/scala/xsbt/boot/Launch.scala

@@ -328,7 +328,7 @@ class Launch private[xsbt] (val bootDirectory: File, val lockBoot: Boolean, val
 
     private object LoaderInit {
       val (library, other) = module.fullClasspath.partition(_.getName.startsWith("scala-library"))
-      val libraryLoader = new URLClassLoader(toURLs(library), parentLoader)
+      val libraryLoader = new LibraryClassLoader(toURLs(library), parentLoader, scalaVersion)
       val fullLoader = new URLClassLoader(toURLs(other), libraryLoader)
     }
 

launcher-implementation/src/main/scala/xsbt/boot/LibraryClassLoader.scala

@@ -0,0 +1,7 @@
+package xsbt.boot
+
+import java.net.{ URL, URLClassLoader }
+
+final class LibraryClassLoader(urls: Array[URL], parent: ClassLoader,
+  val scalaVersion: String) extends URLClassLoader(urls, parent) with xsbti.LibraryClassLoader {
+}

launcher-implementation/src/test/scala/ScalaProviderTest.scala

@@ -15,8 +15,10 @@ object ScalaProviderTest extends Specification {
     "provide ClassLoader for Scala 2.8.2" in { checkScalaLoader("2.8.2") }
     "provide ClassLoader for Scala 2.9.0" in { checkScalaLoader("2.9.0") }
     "provide ClassLoader for Scala 2.9.2" in { checkScalaLoader("2.9.2") }
-    "provide ClassLoader for Scala 2.10.4" in { checkScalaLoader("2.10.4") }
-    "provide ClassLoader for Scala 2.11.0" in { checkScalaLoader("2.11.0") }
+    "provide ClassLoader for Scala 2.10.7" in { checkScalaLoader("2.10.7") }
+    "provide ClassLoader for Scala 2.11.12" in { checkScalaLoader("2.11.12") }
+    // This requires the test to run in JDK 8
+    // "provide ClassLoader for Scala 2.12.4" in { checkScalaLoader("2.12.4") }
   }
 
   "Launch" should {
@@ -68,10 +70,20 @@ object ScalaProviderTest extends Specification {
       val provider = launcher.getScala(version)
       val loader = provider.loader
       // ensure that this loader can load Scala classes by trying scala.ScalaObject.
-      tryScala(loader)
+      tryScala(loader, loader.getParent)
       getScalaVersion(loader) must beEqualTo(versionValue)
+
+      val libraryLoader = provider.loader.getParent
+      // Test the structural type
+      libraryLoader match {
+        case x: ClassLoader with LibraryLoader => x.scalaVersion must be(version)
+      }
+      tryScala(libraryLoader, libraryLoader)
     }
-  private def tryScala(loader: ClassLoader) = Class.forName("scala.Product", false, loader).getClassLoader must be(loader)
+  private def tryScala(loader: ClassLoader, libraryLoader: ClassLoader) =
+    Class.forName("scala.Product", false, loader).getClassLoader must be(libraryLoader)
+
+  type LibraryLoader = { def scalaVersion: String }
 }
 object LaunchTest {
   def testApp(main: String): Application = testApp(main, Array[File]())

launcher-interface/src/main/java/xsbti/LibraryClassLoader.java

@@ -0,0 +1,11 @@
+package xsbti;
+
+import java.io.File;
+
+/**
+ * Marker interface for classloader with just scala-library.
+ */
+public interface LibraryClassLoader
+{
+  public String scalaVersion();
+}