8302202: Incorrect desugaring of null-allowed nested patterns

[lahodaj]

Original PR:
openjdk/jdk#12572

Original PR description:
Considering code like:

    private String test1b(Object i) {
        return switch (i) {
            case R1(Object o) when o == null -> "R1(null)";
            case R1(Object o) -> "R1(!null)";
            default -> "default";
        };
    }

javac will try to factor-out the common prefix, i.e. R1, and produce something along these lines:

        switch (i) {
            case R1 $r:
                Object c = $r.c();
                switch (c) {
                    case Object o when o == null: yield "R1(null)";
                    case Object o: yield "R1(!null)";
                }
            default -> "default";
        };

The problem with this code is that both cases in the nested switch must match null, but the bootstrap protocol only allows one case to match null (the SwitchBootstraps.typeSwitch method will return -1 for null). So this translation is broken, and will not match the first case if the component is null.

There are multiple ways to solve this problem, but the proposal here is change the way we accumulate the cases from which we factor out the common prefix, by stopping the accumulation after the first nullable case.

Another related issue is that when factoring out the common prefix, if the last case in the nested switch is has an unconditional pattern, but also has a guard, we need to generate a default to continue properly in the outer switch. Currently, the default is not generated when there's an unconditional pattern in the case, despite having a guard, and hence the case as a whole is not unconditional.

---

Progress

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Issue

• JDK-8302202: Incorrect desugaring of null-allowed nested patterns

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[bridgekeeper]

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[openjdk]

This backport pull request has now been updated with issue from the original commit.

[openjdk]

@lahodaj This change now passes all automated pre-integration checks.

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8302202: Incorrect desugaring of null-allowed nested patterns

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[mlbridge]

Webrevs

• 00: Full (5742aeb3)

[lahodaj]

/integrate

[openjdk]

Going to push as commit db260e5.
Since your change was applied there have been 3 commits pushed to the master branch:

• cc33209: 8301858: Verification error when compiling switch with record patterns
• a7d4a1a: 8295951: intermittent cmp_baseline task failures with CDS files
• c6a25f4: 8302879: doc/building.md update link to jtreg builds

Your commit was automatically rebased without conflicts.

[openjdk]

@lahodaj Pushed as commit db260e5.

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