Bug 309204 – Possible JIT miscompilation with array destructuring

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Philip Chimento]

Reproducer:
```js
const len = 500; // seems to happen most frequently between about 500-2000
const es = new Array(len);
const fs = new Array(len);
const as = [
  ['foo', [1]],
  ['foo', [1, 2]]
];
for (const [a, [b, c, d]] of as) {
  for (const e of es) {
    for (const f of fs) {}
  }
}
```
This throws:
```
Exception: TypeError: undefined is not a function (near '...[b, c, d]...')
module code@/path/to/bug.mjs:8:16
```
I've minimized the reproducer script as much as I can; everything in there seems to be necessary to trigger the error.

In particular, with a `len` of slightly less than 500 or slightly more than 2000, you may need to run the script a few times before getting the error. I've been using `for i in $(seq 1 100); do echo $i; ~/.esvu/bin/jsc bug.mjs || break; done` with jsc nightly installed using esvu. With a `len` farther outside that range, the error seems to not happen at all.

[Philip Chimento]

Forgot to mention the most important thing! The error doesn't occur with `--useJIT=0`

[Radar WebKit Bug Importer]

<rdar://problem/171766191>

[Yusuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/59942

[EWS]

Committed 308677@main (d9b3841d9b3f): <https://commits.webkit.org/308677@main>

Reviewed commits have been landed. Closing PR #59942 and removing active labels.