[pattern-switch] Opting into totality

[Brian Goetz]

> It's unclear whether your "some remainder" is allowed to be empty. (There was some discussion earlier about outlawing 'default' in the equivalent of a sealed switch.) I hope full totality is fine—an expression switch, implicitly 'sealed', of course permits a 'default' clause.

Yes, remainder can be empty.  The key aspect of sealed switches is that 
we generate code to handle the remainder.  Of course, it could be 
handled explicitly too, in which case the synthetic handler is never 
matched.  For example:

     sealed switch (foo ) {
         case Foo(var x): ...
     }

is total on Foo with remainder { null }.  So sealing will cause us to 
throw on null.  OTOH, you could also say:

     sealed switch (foo ) {
         case null: haha(); break;
         case Foo(var x): ...
     }

The compiler does not need to analyze the cases that might match 
remainder; it just generates code at the end of the switch so that, if a 
previous case doesn't cover it, the catch-all does.


> And then note that, given the existence of 'sealed switch', the 'default Object o' feature is redundant. If you want to make sure you have a total case in your switch, just say 'sealed' at the top. All sealed switches (both statement and expression) guarantee either optimistic totality + NPE or that the last clause is total.

Yes, that's right.   If there's a way to ask for totality, you can 
safely use a total pattern without `default`.  So `default`, in this 
model, reverts to being `case _`, which is a fine role for default in 
this new world.