Fix classpath detection when using the REPL

This was broken after I stopped appending
`System.getProperty("java.class.path")` to the constructed classpath.
Fixed with a hack, but this is still better than looking at
java.class.path.

Author: smarter

staging/src/scala/quoted/staging/QuoteDriver.scala

@@ -74,15 +74,32 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver {
    */
   private def classpathFromClassloader(cl: ClassLoader): String = {
     val classpathBuff = List.newBuilder[String]
-    def collectClassLoaderPaths(cl: ClassLoader): Unit = cl match {
-      case cl: URLClassLoader =>
-        collectClassLoaderPaths(cl.getParent)
-        // Parent classloaders are searched before their child, so the part of
-        // the classpath coming from the child is added at the _end_ of the
-        // classpath.
-        classpathBuff ++=
-          cl.getURLs.iterator.map(url => Paths.get(url.toURI).toAbsolutePath.toString)
-      case _ =>
+    def collectClassLoaderPaths(cl: ClassLoader): Unit = {
+      if (cl != null) {
+        cl match {
+          case cl: URLClassLoader =>
+            // This is wrong if we're in a subclass of URLClassLoader
+            // that filters loading classes from its parent ¯\_(ツ)_/¯
+            collectClassLoaderPaths(cl.getParent)
+            // Parent classloaders are searched before their child, so the part of
+            // the classpath coming from the child is added at the _end_ of the
+            // classpath.
+            classpathBuff ++=
+              cl.getURLs.iterator.map(url => Paths.get(url.toURI).toAbsolutePath.toString)
+          case _ =>
+            // HACK: We can't just collect the classpath from arbitrary parent
+            // classloaders since the current classloader might intentionally
+            // filter loading classes from its parent (for example
+            // BootFilteredLoader in the sbt launcher does this and we really
+            // don't want to include the scala-library that sbt depends on
+            // here), but we do need to look at the parent of the REPL
+            // classloader, so we special case it. We can't do this using a type
+            // test since the REPL classloader class itself is normally loaded
+            // with a different classloader.
+            if (cl.getClass.getName == classOf[AbstractFileClassLoader].getName)
+              collectClassLoaderPaths(cl.getParent)
+        }
+      }
     }
     collectClassLoaderPaths(cl)
     classpathBuff.result().mkString(java.io.File.pathSeparator)