SI-6338 fixes the unchecked warning in quick.comp

All those months when I thought it was yet another spurious error
in the new pattern matcher...

Author: xeno-by

src/compiler/scala/tools/nsc/interpreter/TypeStrings.scala

@@ -212,7 +212,11 @@ trait TypeStrings {
   }
 
   private def tparamString[T: ru.TypeTag] : String = {
-    def typeArguments: List[ru.Type] = ru.typeOf[T] match { case ru.TypeRef(_, _, args) => args; case _ => Nil }
+    def typeArguments: List[ru.Type] = {
+      import ru.TypeRefTag // otherwise the pattern match will be unchecked
+                           // because TypeRef is an abstract type
+      ru.typeOf[T] match { case ru.TypeRef(_, _, args) => args; case _ => Nil }
+    }
     // [Eugene to Paul] need to use not the `rootMirror`, but a mirror with the REPL's classloader
     // how do I get to it? acquiring context classloader seems unreliable because of multithreading
     def typeVariables: List[java.lang.Class[_]] = typeArguments map (targ => ru.rootMirror.runtimeClass(targ))