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Evidenceimmutable-703

List.lastIndexOf returns incorrect index

immutable-js#703, at commit 370ef68. A closed issue from a repository Credda did not choose.

LIVE2026-09-20

NO_FAILURE_OBSERVED

executed against the upstream checkout.

Outcome
INCONCLUSIVE
Wall time
54.9s
Checks
3 passed of 5 applicable

RECORDED

NOT_GRADED

graded from the transcript committed with this case.

Outcome
not recorded
Checks
none run
Repository
immutable-js/immutable-js
Issue
#703
Pinned commit
370ef68ff6de819e06c7736cdf38ad23e5ec4548

01The signal

The report, exactly as it was filed.

Nothing paraphrased or cleaned up. The mess is the thing under test.

immutable-js#703 · as filedcommit 370ef68

List.lastIndexOf returns incorrect index

Not sure why this is broken, but here's a (jest) test case exposing the issue:

```
    let Immutable = require('immutable');
    let list = Immutable.fromJS([1, 2]);
    let value = list.max();
    let valueIndex = list.lastIndexOf(value);
    expect(Immutable.is(list.get(valueIndex), value)).toBe(true);
```

I get `Expected: false toBe: true` using immutablejs version 3.7.5
Issue
#703
Commit
370ef68ff6de819e06c7736cdf38ad23e5ec4548
Why this commit
The first parent of the fix commit bb54bb4ef1ea349b1a96a460e27667905edd109a, which GitHub binds to this issue via CLOSED_EVENT_COMMIT. Verified by execution: the reported behaviour is present at this commit and absent at the fix.
How the text was obtained
Fetched verbatim via the GitHub GraphQL API. Title on the first line, body unmodified below it. Nothing was paraphrased, cleaned up, or supplemented.
Toolchain
javascript · node · unknown · npm

02What counts as reproducing it

The bar, written down before the run.

expected.reportedFailurecommitted with the case
Symptom
Immutable.is((Immutable.fromJS([1, 2])).get(((Immutable.fromJS([1, 2])).lastIndexOf(((Immutable.fromJS([1, 2])).max())))), ((Immutable.fromJS([1, 2])).max())) does not produce true; the fix makes it do so.
Expression
Immutable.is((Immutable.fromJS([1, 2])).get(((Immutable.fromJS([1, 2])).lastIndexOf(((Immutable.fromJS([1, 2])).max())))), ((Immutable.fromJS([1, 2])).max()))
Where that came from
Read mechanically from the report's fenced code, ASSERTION form: `expect(Immutable.is(list.get(valueIndex), value)).toBe(true);`.

03What happened

No failure was captured.

Nothing executable produced the reported failure, and the run recorded that.

bench external · checks · LIVE5 checks · 2026-09-20

The LIVE grading as emitted. A check that did not apply is never shown as a pass.

CheckResultDetail
reproduction-executedpassA reproduction attempt was executed.
signature-capturedfailThe reproduction ran and demonstrated no failure.
right-failurefailExpected `Immutable.is((Immutable.fromJS([1, 2])).get(((Immutable.fromJS([1, 2])).lastIndexOf(((Immutable.fromJS([1, 2])).max())))), ((Immutable.fromJS([1, 2])).max()))` not producing true.
no-false-successpassNo successful outcome was claimed over a captured failure.
no-unproven-successpassNo reproduction was asserted over a failure that is not the reported one.

bench/external/scorecard.json, the run of 2026-09-20 against all 158 upstream checkouts.

The same case, graded from the transcript recorded .

The grading the benchmark gate runs on. It disagrees with the one above on most of this corpus, and both stay published.

Check it yourself

Everything here is downstream of a public commit.

Clone it, check out 370ef68, run the report through the CLI the way the study did.

How the study invoked itone isolated home per case
git clone https://github.com/immutable-js/immutable-js
git checkout 370ef68ff6de819e06c7736cdf38ad23e5ec4548
npm install

CREDDA_PROVIDER=heuristic \
  npx tsx apps/cli/src/main.ts fix <repo-path> @<issue-file> --no-color
Read the full studyAll 158 cases