That's incredibly interesting, because I've been using union types a while and didn't know this. I think a lot of people will get introduced to them via function signatures, and var: ICat | IDog not meaning "var could be an ICat or an IDog" is a bit unintuitive given the boolean math.

But ultimately more useful? As in, code should never test whether this thing is an ICat or IDog, and I guess it's interesting that I don't remember ever encountering an issue where I attempted to access a member that was only present on one interface.

Perhaps it would be too overloaded, but "ICat + IDog" instead of "ICat & IDog" feels more obvious, I suppose a better convention for "ICat | IDog" is difficult though. "ICat /\ IDog" for intersection and "ICat \/ IDog" for union doesn't roll off the tongue.