RFR: 8262889: Compiler implementation for Record Patterns

[Jan Lahoda]

On Thu, 5 May 2022 15:17:11 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> You are right. It is the ii) which iteratively checks the component pattern list L.
>
> I now believe that the check is needed to properly classify patterns based on the type of the i-th component. That said, not sure this should be a subtyping check, or a type equality

A good question. Consider code like:

    private void test(R r) {
        switch (r) {
            case R(A a, A v) -> {}
            case R(B b, A v) -> {}
            case R(I i, B v) -> {}
        }
    }
    final class A implements I {}
    sealed interface I permits A, B {}
    final class B implements I {}
    record R(I i1, I i2) {}


The switch is exhaustive - all the possible combinations are covered. When checking the first component, the code will categorize the patterns like:

A -> R(A a, A v), R(I i, B v)
B -> R(B b, A v), R(I i, B v)
I   -> R(I i, B v)

this categorization is done using the subtype check, so that `R(I i, B v)` will appear in the list for `A`. When checking the second component, the possibility for `I` is not exhaustive (does not cover `A` in the second component), but the possibilities for `A` and `B` are exhaustive, and they together cover `I`.

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

PR: https://git.openjdk.java.net/jdk/pull/8516