operator.iconcat ignores list subclass __iadd__ override

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: "Same as a += b, for a and b sequences." — supplied public API and iconcat docstring.

Expected: Return the fresh plain list [99], leaving both original operands unchanged.

Actual: Returns the fresh plain list [1, 2], leaving both original operands unchanged.

import operator

class L(list):
    def __iadd__(self, other):
        return [99]

a, ref, b, rb = L([1]), L([1]), [2], [2]
data = {"mode": "fresh99", "initial": list(a), "rhs": list(b)}

def snapshot(result, original, rhs):
    return {
        "result": list(result),
        "result_type": type(result).__name__,
        "original_after": list(original),
        "rhs_after": list(rhs),
        "result_is_original": result is original,
    }

if not (isinstance(a, list) and type(b) is list
        and type(a.__iadd__(b)) is list and a == [1] and b == [2]):
    print("REFUTATION REJECTED:", "input does not satisfy the claimed domain")
else:
    actual = snapshot(operator.iconcat(a, b), a, b)
    original = ref
    ref += rb
    expected = snapshot(ref, original, rb)
    if actual != expected:
        print("REFUTATION CONFIRMED:", data, actual, expected)
    else:
        print("REFUTATION REJECTED:", "iconcat agrees with independently executed +=")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library operator:

REFUTATION CONFIRMED: {'mode': 'fresh99', 'initial': [1], 'rhs': [2]} {'result': [1, 2], 'result_type': 'list', 'original_after': [1], 'rhs_after': [2], 'result_is_original': False} {'result': [99], 'result_type': 'list', 'original_after': [1], 'rhs_after': [2], 'result_is_original': False}

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/operator-20261003-044103-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).