Exhaustiveness for record pattern with switch (JEP 440+441)

[David Alayachew]

I've CC'd the Amber Dev Team.

On Fri, Jul 7, 2023 at 1:52 PM Mikhail Pyltsin <pyltsinm at gmail.com> wrote:

> Hi!
> I investigated the algorithm, which Javac 21 (Build 30 (2023/7/6))
> ) uses to check exhaustiveness for record patterns (JEP 440+441), and
> found the strange behavior:
> - let's take compilable code
> ```
> class Test22 {
>
>   record Pair(I i1, I i2) {}
>
>   sealed interface I {}
>
>   record C() implements I {}
>
>   record D() implements I {}
>
>   void exhaustinvenessWithInterface(Pair pairI) {
>     switch (pairI) {
>       case Pair(C fst, D snd) -> {
>       }
>       case Pair(C fst, C snd) -> {
>       }
>       case Pair(I fst, C snd) -> {
>       }
>       case Pair(D fst, D snd) -> {
>       }
>     }
>   }
> }
> ```
> - If we swap types of components, it starts to produce the error "java:
> the switch statement does not cover all possible input values":
> ```
>   void exhaustinvenessWithInterface(Pair pairI) {
>     switch (pairI) {
>       case Pair(D fst, C snd) -> {
>       }
>       case Pair(C fst, C snd) -> {
>       }
>       case Pair(C fst, I snd) -> {
>       }
>       case Pair(D fst, D snd) -> {
>       }
>     }
>   }
> ```
> It happens because Javac tries to find the first distinguished type from
> the beginning, but I couldn't find any mention of it in JEP.
> Is this the expected behavior?
>
>
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