[pattern-switch] Totality

[Brian Goetz]

> While i agree with the general idea, i disagree with the fact that the 
> compiler should handle the null and total cases, it can not handle the 
> total case, it has to be handled by the by the runtime, not the compiler.
>
> I will use '?' for the total type, by example this switch is "total 
> enough"
>   sealed interface Stuff permits Pixel, Car {}
>   record Pixel(int x, int y, Color color) implements Stuff {}
>   record Car(Color color) implements Stuff {}
>   switch(stuff) {
>     case Pixel(? x, ? y, Color color) -> color;
>     case Car(Color color) -> color
>   }
>
> We have agreed during one of our first meetings that we want that if 
> Pixel is changed to record Pixel(BigInteger x, BigInteger y, Color 
> color) {} with or without recompilation of the switch it should be Ok.

I don't recall agreeing to anything like this, so perhaps you can dig up 
a reference, and I can try to reconstruct what I thought I was agreeing to?

It seems that you are arguing that the use site of a pattern match 
should be unaffected by (possibly incompatible) changes in the 
declaration of the pattern.  But this can't possibly be what you are 
suggesting.

(If you want to have a conversation on what declaration changes are 
behaviorally compatible, that is a fine conversation!)

> That doesn't mean that '?' can not be spelt 'var', but it demonstrates 
> that the corner cases should be managed by the runtime, not the compiler.
>

I don't understand what you mean by this distinction.  The compiler 
identifies the corner cases, and generates runtime code to handle them, 
just like it does with expression enum switches today.
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