dict.update(<variant>) SEGFAULTED

• Type: bug (NilPy, crash) — Track N
• Found: 2026-08-09, running a realistic config reader against CPython.
• Status: FIXED the same session.

DEFAULTS = {"host": "localhost"}

def merged(sec):
    m = dict(DEFAULTS)
    m.update(sec)        # SIGSEGV
    return m

m.update({"k": v}) with a LITERAL was always fine. That is what kept this out of every API sweep: it needs the value to arrive through a PARAMETER, and merged = dict(DEFAULTS); merged.update(section) is the ordinary shape.

Cause

TPyDict.update has two typed overloads — update(TPyList) for an iterable of pairs and update(TPyDict) for a mapping. An unannotated parameter is a VARIANT, which matches neither, and it resolved to the TPyList arm: a TPyDict was then walked as a list.

Fix

A third update(const v: Variant) overload that dispatches on the runtime tag and delegates to whichever typed arm the value actually is, raising TypeError for anything else (CPython's own answer for a non-mapping, non-iterable).

Both argument kinds are asserted THROUGH A PARAMETER, since a fix that sent both down one arm would still pass a mapping-only test.

Verified

test/test_nilpy_dict_update_variant.{npy,expected} (.expected from CPython): a mapping and an iterable of pairs through a parameter, an empty mapping, a literal, the loop shape it was found in, and the statically-typed spellings that always worked as controls. gate.sh quick GREEN; the dict test family re-diffed against CPython.

Noted while fixing

The same realistic program then hit [[bug-nilpy-set-augmented-union-does-nothing]] (a |= set(...) silently does nothing), filed separately.