Make sure typer is a Typer

The previous cast can fail if applyOverloaded is used outside a compilation
run. This can happen in the IDE, when annotations are forced. Reconstructing
applications from Scala2 pickles calls applyOverloaded.

Author: odersky

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

@@ -131,7 +131,6 @@ object Contexts {
     private[this] var _typeAssigner: TypeAssigner = _
     protected def typeAssigner_=(typeAssigner: TypeAssigner) = _typeAssigner = typeAssigner
     def typeAssigner: TypeAssigner = _typeAssigner
-    def typer: Typer = _typeAssigner.asInstanceOf[Typer]
 
     /** The currently active import info */
     private[this] var _importInfo: ImportInfo = _

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -27,6 +27,11 @@ import reporting.diagnostic.messages._
 trait NamerContextOps { this: Context =>
   import NamerContextOps._
 
+  def typer = ctx.typeAssigner match {
+    case typer: Typer => typer
+    case _ => new Typer
+  }
+
   /** Enter symbol into current class, if current class is owner of current context,
    *  or into current scope, if not. Should always be called instead of scope.enter
    *  in order to make sure that updates to class members are reflected in