Generalize leak avoidance

Leaks can be avoided by following type aliases. Previously, this was only
done if the checked symbol is a term. But it is not clear why types should
not get the same treatment. This avoids a problem in pos/reference/exports.scala.

Author: odersky

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

@@ -499,11 +499,12 @@ object Checking {
           var tp1 =
             if (isLeaked(tp.symbol)) {
               errors =
-                (() => em"non-private $sym refers to private ${tp.symbol}\nin its type signature ${sym.info}") :: errors
+                (() => em"non-private ${sym.showLocated} refers to private ${tp.symbol}\nin its type signature ${sym.info}") ::
+                errors
               tp
             }
             else mapOver(tp)
-          if ((errors ne prevErrors) && !sym.isType && tp.info.isTypeAlias) {
+          if ((errors ne prevErrors) && tp.info.isTypeAlias) {
             // try to dealias to avoid a leak error
             val savedErrors = errors
             errors = prevErrors