The benefit is twofold: not only does the user not have to write the
stupid cases (imagine if Box had ten slots, would we want to write the
2^10 partial null cases?), but because we throw on the remainder, DA
can treat the switch as covering all boxes, and be assured there are
no leaks.
More on this last point. We don't yet have pattern assignment, but
totality and remainder are key to it. Suppose we have
Box<String> bs = ...
and we want to destructure:
Box(String s) = bs;
// s had better be DA here
The pattern on the LHS must be total, but can have remainder; the
statement is allowed to throw on the remainder, because the remainder
is, by definition, the "silly" matches. Here, the only remainder is
null; you can't destructure a null, so if you asked to destructure a
Box, you probably were assuming that it was a real box. So this
assignment NPEs on null (not unlike the corresponding switch).
