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Evidenceimmutable-406

mergeDeep clobbers List items; concatDeep isn't implemented

immutable-js#406, at commit d093223. A closed issue from a repository Credda did not choose.

LIVE2026-09-20

RIGHT_FAILURE

executed against the upstream checkout.

Outcome
REPRODUCED_AND_DIAGNOSED
Wall time
129.3s
Checks
5 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
#406
Pinned commit
d0932232da399f6f66e6c37cc80c64e38e0eee7d

01The signal

The report, exactly as it was filed.

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

immutable-js#406 · as filedcommit d093223

mergeDeep clobbers List items; concatDeep isn't implemented

`merge` appears to operate by key, which means merging two Lists together clobbers values (rather than appending them):

``` javascript
Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON()
// [4, 5, 3]
```

`concat` doesn't have this problem; however, there's no `concatDeep`.

@leebyron, how do you feel about adding `concatAll`?  It would be a nondestructive union of any Immutable iterables, like so:

``` javascript
Immutable.fromJS(
  {
    "a":  [
            1
          ],
    "b":  {
            "fruits": [
                        "banana",
                      ],

            "place": "ireland",
          }
  }
).concatDeep(
  Immutable.fromJS(
    {
      "a":  [
              3
            ],
      "b":  {
              "fruits": [
                          "apple",
                        ],

              "author": "seuss",
            }
    }
  )
).toJSON()

// {
//   "a":  [
//           1,
//           3
//         ],
//   "b":  {
//           "fruits": [
//                      "banana",
//                      "apple",
//                     ],
// 
//           "place":  "ireland",
//           "author": "seuss",
//         }
// }
```
Issue
#406
Commit
d0932232da399f6f66e6c37cc80c64e38e0eee7d
Why this commit
The first parent of the fix commit 5fbce6a7590012b3b747a01c7282d61022e6affe, which GitHub binds to this issue via CLOSED_EVENT_PR. 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.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON() produces [ 4, 5, 3 ]; the fix makes it produce [ 1, 2, 3, 4, 5 ].
Expression
Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON()
Reported output
[4, 5, 3]
Where that came from
Read mechanically from the report's fenced code, NEXT_LINE form: `Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON() // [4, 5, 3]`.

03What happened

The live run reproduced the reported failure.

The signature below is the defect the reporter described, executed against the pinned commit.

captured failure signatureLIVE · normalized
`Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON()` still produces [4, 5, 3] (read [ 4, 5, 3 ])
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-capturedpass`Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON()` still produces [4, 5, 3] (read [ 4, 5, 3 ])
right-failurepassReproduced the reported failure: Immutable.List([1, 2, 3]).merge(Immutable.List([4, 5])).toJSON() produces [ 4, 5, 3 ]; the fix makes it produce [ 1, 2, 3, 4, 5 ].
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 d093223, 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 d0932232da399f6f66e6c37cc80c64e38e0eee7d
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