[patterns] Domination in guarded multiple pattern labels

[Tagir Valeev]

On Sat, Aug 19, 2023 at 4:00 PM Angelos Bimpoudis
<angelos.bimpoudis at oracle.com> wrote:
> It is a compile-time error if in a switch block there is a case label with case patterns p1,...,pn (n > 1) where one of the patterns pi (1 ≤ i < n) dominates another of the patterns pj (i < j ≤ n).

Ah, indeed! My bad, I haven't read until this point, thanks.

With best regards,
Tagir Valeev.

>
> Is this what you had in mind?
>
> Many thanks and best regards,
> Angelos
>
> ________________________________
> From: amber-spec-experts <amber-spec-experts-retn at openjdk.org> on behalf of Tagir Valeev <amaembo at gmail.com>
> Sent: 18 August 2023 11:48
> To: amber-spec-experts <amber-spec-experts at openjdk.org>
> Subject: [patterns] Domination in guarded multiple pattern labels
>
> Hello!
>
> Consider the following code:
>
> public class Test {
>     void test(Object obj) {
>        switch (obj) {
>           case Integer _, CharSequence _, String _ when obj.hashCode() > 0 -> {
>           }
>           default -> throw new IllegalStateException("Unexpected
> value: " + obj);
>        }
>     }
> }
>
> Javac compiles it, despite the `String _` pattern being unreachable.
> It looks like the spec [1] allows it. It speaks about domination of
> case labels, but not about the domination of individual patterns
> within the case label. I think this part should be improved. Namely,
> for several patterns under the single case label, domination rules
> must apply, and the guard must be ignored.
>
> It's interesting that if we remove the guard, the compilation fails:
>
> Test.java:4:45
> java: this case label is dominated by a preceding case label
>
> While formally according to the spec, this should be accepted, as spec
> in its current shape says us only about 'preceding case label', and
> there's no preceding case label at all, so the first label should not
> be dominated.
>
> With best regards,
> Tagir Valeev
>
> [1] https://cr.openjdk.org/~abimpoudis/unnamed/jep443-20230322/specs/unnamed-jls.html#jls-14.11.1