Fix #2743: Fix hover problem

Also add an assert in isRef to track down a stackoverflow going infinitely often
through this line. Hopefully the assert will track that down.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -125,7 +125,7 @@ object Types {
     def isRef(sym: Symbol)(implicit ctx: Context): Boolean = stripAnnots.stripTypeVar match {
       case this1: TypeRef =>
         this1.info match { // see comment in Namer#typeDefSig
-          case TypeAlias(tp) => tp.isRef(sym)
+          case TypeAlias(tp) => assert(tp ne this); tp.isRef(sym)
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedOrRecType => this1.parent.isRef(sym)

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -297,7 +297,7 @@ class DottyLanguageServer extends LanguageServer
     implicit val ctx = driver.currentCtx
 
     val pos = sourcePosition(driver, uri, params.getPosition)
-    val tp = Interactive.enclosingType(driver.openedTrees(uri), pos)
+    val tp = Interactive.enclosingType(driver.openedTrees.getOrElse(uri, Nil), pos)
     val tpw = tp.widenTermRefExpr
 
     if (tpw == NoType) new Hover