private InconvertibleTypesChecker.LookForMutualSubclass getMutualSubclassMode() {
      if (!WARN_IF_NO_MUTUAL_SUBCLASS_FOUND) {
        return InconvertibleTypesChecker.LookForMutualSubclass.NEVER;
      }
      return isOnTheFly()
             ? InconvertibleTypesChecker.LookForMutualSubclass.IF_CHEAP
             : InconvertibleTypesChecker.LookForMutualSubclass.ALWAYS;
    }
private InconvertibleTypesChecker.LookForMutualSubclass getMutualSubclassMode()