[pattern-switch] Opting into totality

[Brian Goetz]

I think this is the main open question at this point.  

We now have a deeper understanding of what this means, and the shape of the remainder.  Totality means not only “spot check me that I’m right”, but also “I know there might be some remainder, please deal with it.”   So totality is not merely about type checking, but about affirmative handling of the remainder.  

Expression switches automatically get  this treatment, and opting _out_ of that makes no sense for expression switches (expressions must be total), but statement switches make sense both ways (just like unbalanced and balanced if-else.)  Unfortunately the default has to be partial,  so the main question is, how  do we indicate the desire for totality in a way that is properly evocative for the user?  

We’ve talked about modifying switch (sealed switch), a hyphenated keyword (total-switch), a trailing modifier (switch case), and synthetic cases (“default: unreachable”).  Of course at this point it’s “just syntax”, but I think our goal should be picking something that  makes it obvious to users that what’s going on is not merely an assertion of totality, but also a desire to handle the remainder.  

>    - How does a switch opt into totality, other than by being an expression switch?

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