Merge pull request #5166 from dotty-staging/fix-ide-java9

Fix the IDE always crashing due to an incorrect cast under Java 9+

Author: smarter

compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala

@@ -119,10 +119,14 @@ class InteractiveDriver(val settings: List[String]) extends Driver {
 
   private val (zipClassPaths, dirClassPaths) = currentCtx.platform.classPath(currentCtx) match {
     case AggregateClassPath(cps) =>
-      val (zipCps, dirCps) = cps.partition(_.isInstanceOf[ZipArchiveFileLookup[_]])
-      // This will be wrong if any other subclass of ClassPath is either used,
-      // like `JrtClassPath` once we get Java 9 support
-      (zipCps.asInstanceOf[Seq[ZipArchiveFileLookup[_]]], dirCps.asInstanceOf[Seq[JFileDirectoryLookup[_]]])
+      // FIXME: We shouldn't assume that ClassPath doesn't have other
+      // subclasses. For now, the only other subclass is JrtClassPath on Java
+      // 9+, we can safely ignore it for now because it's only used for the
+      // standard Java library, but this will change once we start supporting
+      // adding entries to the modulepath.
+      val zipCps = cps.collect { case cp: ZipArchiveFileLookup[_] => cp }
+      val dirCps = cps.collect { case cp: JFileDirectoryLookup[_] => cp }
+      (zipCps, dirCps)
     case _ =>
       (Seq(), Seq())
   }