Make pathTo operation more robust

Paths can contain non-tree elements. I observed a path that contained
an untpd.Mod instead of a tree.

Also, we should survive possible cyclic references when ccomputing the context
of a path. (this was also observed in the wild but I can't reproduce it).

Author: odersky

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

@@ -344,7 +344,7 @@ object Interactive {
     if (tree.pos.contains(pos)) {
       // FIXME: We shouldn't need a cast. Change NavigateAST.pathTo to return a List of Tree?
       val path = NavigateAST.pathTo(pos, tree, skipZeroExtent = true).asInstanceOf[List[untpd.Tree]]
-      path.dropWhile(!_.hasType).asInstanceOf[List[tpd.Tree]]
+      path.dropWhile(!_.hasType) collect { case t: tpd.Tree @unchecked => t }
     }
     else Nil
 
@@ -365,7 +365,7 @@ object Interactive {
     case nested :: encl :: rest =>
       import typer.Typer._
       val outer = contextOfPath(encl :: rest)
-      encl match {
+      try encl match {
         case tree @ PackageDef(pkg, stats) =>
           assert(tree.symbol.exists)
           if (nested `eq` pkg) outer
@@ -401,6 +401,9 @@ object Interactive {
         case _ =>
           outer
       }
+      catch {
+        case ex: CyclicReference => outer
+      }
   }
 
   /** The first tree in the path that is a definition. */