RFR: 8327368: javac crash when computing exhaustiveness checks. [v2]

[Jan Lahoda]

> Consider code like:
> 
> package test;
> 
> public class Test {
> 
>     sealed interface A permits T, U {}
> 
>     sealed interface B permits V, W {}
> 
>     static final class T implements A {}
> 
>     static final class U implements A {}
> 
>     static final class V implements B {}
> 
>     static final class W implements B {}
> 
>     final static record R(A a, B b) {}
> 
>     static int r(R r) {
>         return switch (r) {
>             case R(A a, V b) -> 1;
>             case R(T a, B b) -> 2;
>             case R(U a, W b) -> 3;
>         };
>     }
> }
> 
> 
> To properly evaluate exhaustiveness, we need to consider `R(T a, B b)` to act as `R(T a, W b)`, so that we can reduce `R(T a, W b)` and `R(U a, W b)` to `R(A a, W b)`. Which is then reduced together with `R(A a, V b)` to `R(A a, B b)` and consequently to just `R`.
> 
> This was done under JDK-8325215, by expanding `B` into all possible subtypes, from which we can pick patterns for reduction. Eventually, that patch may need a more complete revamp, but the idea herein is just to prevent a `ClassCastException` when the type of the binding pattern is not a class-based type - typically, when it is a type variable. This patch uses the type variable's bounds as the based from which the permitted subtypes are computed.

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Must handle type variables using their bounds.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19523/files
  - new: https://git.openjdk.org/jdk/pull/19523/files/e779ee2d..086f2520

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19523&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19523&range=00-01

  Stats: 87 lines in 2 files changed: 63 ins; 5 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/19523.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19523/head:pull/19523

PR: https://git.openjdk.org/jdk/pull/19523