Saver fares are actually "dual-inventory" fares. While they all book into X class, the Saver fare basis codes start with a Main class and there must be both inventory in X class and the Main class in order to book a particular Saver fare filing. As inventory in lower Main classes is zeroed out, it simultaneously moves the Main and Saver fares up to into higher priced fares. This is how they keep the price differentials between Saver and Main in sync no matter what Main fare is available for purchase. The underlying Main class of the Saver fare being booked should always be the same of whatever Main fare is available for purchase. It doesn't seem like this would have anything to do with ability to use discount.

Saver fares will have a 'B' in second to last character of fare basis code. While the matching Main fare will have an 'M'. You can see from below there is a $40 price differential between Saver and Main fares on SNA-SEA fares. While both fare basis codes start with G below, only the Main fare will book into G class and the Saver will book into X class. You can find fare basis codes of flights you are booking in the "Fare Rules" section on alaskaair.com.