That's not a variant, it's just 2 optional fields: one called "foo" and one called "bar". A validating parser then has to know that they are semantically linked

That's not a variant, it's just 2 optional fields: one called "foo" and one called "bar". A validating parser then has to know that they are semantically linked. Admittedly this is what protocol buffers does in the binary wire format, because oneof's are just a frontend hack (iirc the latest value decoded gets kept), but it's not the same.