Switch labels (null again), some tweaking

[Maurizio Cimadamore]

On 28/04/2021 17:29, Brian Goetz wrote:
> I assume that you are saying Box permits Soup only.  But your 
> assumptions about "where do the nulls go" here are not right. 
> Box(Soup) does not match Box(null); the set of patterns { Box(Soup) } 
> is total on Box(Lunch) _with remainder Box(null)_.  So the null paths 
> in this example are dead.  (Also missing break statements.)  So 
> rewriting, this switch is really equivalent to:
>
>     switch (lunch) {
>         case Box(Soup s):
>               System.err.println("Box of soup");
>               break;
>
>         case Bag(Soup s):
>              System.err.println("Bag of soup");
>              break;
>
>         /* implicit */
>         case Box(null), Bag(null): throw new NPE();
>     }
>
> and the switch is total on Container(Lunch) under the Lunch=Soup, 
> Container=Box|Bag assumptions. 

I have to admit that this is surprising.

So, if I have a sealed hierarchy that only permits one concrete type:

interface Foo permits Bar

doing:

Foo x = ...
if (x instanceof Bar)

is not considered a total instanceof?

Maurizio