[pattern-switch] Totality

[Guy Steele]

> On Aug 22, 2020, at 2:06 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> . . .
>> 
>> Option 1: If the type of the switch expression is an enum or a sealed type, then it is a static error if the patterns are not at least optimistically total.  **This would be an incompatible change with respect to existing switches on enum types.**
> 
> Actually, as stated, this is not inconsistent -- for switch *expressions*.  We added switch expressions in Java 12 and required that they be total, and, when the target is an enum, we nodded to optimistic totality, by not requiring a `default` when the cases were optimistically total.  Where we don't have an equivalent story is for switch *statements*, which have always been partial -- and for which partiality is reasonable (just as an `if` without an `else` is reasonable.)  And the story for optimistic totality does not scale quite as well as we'd hoped to sealed types, for a few reasons:

Yes, this was an error on my part; I meant to write "Option 1: If the type of the switch statement is . . .”

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