ExtendedDefaultRules defaults to  for Foldable/Traversable
Extend -XExtendedDefaultRules to allow defaulting when one of the classes involved is Foldable or Traversable. (See the current rules here.) This will mean, essentially, that there will be two lists of default types, one list at kind * and one at kind * -> *. Both lists should be settable by the user by the default directive. (GHC can figure out which list is being set quite easily.) The default default list for * -> * will be .