Next up for patterns: type patterns in switch

[Brian Goetz]

> If it is, the last clause is total too (well, unless it is an enum 
> switch that names all the constants, or a switch over a sealed type 
> that names all the sub-types but without a catch-all.) 

I want to drill into this comment because it's significant.

If we have

     enum E { A, B; }

then a switch

     switch (e) {
         case A: ...
         case B: ....
     }

is exhaustive but has no catch-all and will still NPE on null. (Which 
seems reasonable, there's a strong presumption that null enum values are 
a bug.) And we would not want to make the user explicitly say `case 
null`, just like we don't make them say `default` when all the cases are 
covered.

Presumably the same is true with:

     sealed interface E permits A, B { }

     switch (e) {
         case A a:
         case B b:
     }

and presumably this is also reasonable.

Which is to say: the nullity behaviors we've designed around pattern 
matching are specific to catch-all cases, which we presume are going to 
be common in pattern switches.  If null is not part of your domain, you 
won't notice; if it is part of your domain, you want the catch-all 
catching it.