You can have a sweet and salty taste. Sweet isn't the opposite of salty
I never said it was. But we're not talking about tastes in general, we're specifically talking about salt. That's "salt" as in the physical substance used for seasoning and food preservation, not "salty" as in the flavor.
Sure, you can have sweet and salty tasting
food. You can't, however, have sweet and salty tasting
salt. If your salt tastes sweet in addition to the expected saltiness, then it's got something other than salt mixed in. If your "salt" doesn't have a salty flavor at
all, then that's, by definition, not salt. It's some other white, powdery substance instead (likely sugar or a sugar substitute if it tastes sweet and doesn't immediately kill you/make you sick).
Salt, as in Sodium Chloride (NaCl), has only one flavor: Salty.