Search for classtag only during typer.

We do not want to do implicit search during tree checking.

Author: odersky

src/dotty/tools/dotc/typer/Typer.scala

@@ -431,7 +431,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         val tpt1 = typedTpt
         // special case for an abstract type that comes with a class tag
         tpt1.tpe.dealias match {
-          case tref: TypeRef if !tref.symbol.isClass =>
+          case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper =>
             inferImplicit(defn.ClassTagType.appliedTo(tref),
                EmptyTree, tpt1.pos)(ctx.retractMode(Mode.Pattern)) match {
               case SearchSuccess(arg, _, _) =>