s.split(None) does not split — the whole string comes back as one field

• Type: bug (NilPy; valid CPython, silently wrong answer) — Track N
• Found: 2026-08-09, realistic-program sweep (a text-wrapping utility that used line.split(None, 1) to peel the first word).

s = "a b  c d"
s.split()            # ['a', 'b', 'c', 'd']   — always worked
s.split(None)        # CPython ['a','b','c','d'];  pxx ['a b  c d']
s.split(None, 1)     # CPython ['a', 'b  c d'];    pxx ['a b  c d']
s.rsplit(None, 1)    # CPython ['a b  c', 'd'];    pxx ['a b  c d']

Silent: a wrong split still answers a list of strings, so the caller keeps going with one field where it expected several.

Cause

CPython's signature is split(sep=None, maxsplit=-1) — None IS the default separator, not a degenerate one. PyStrMethodFinish routed by ARITY alone, so one explicit argument always meant pystr_split_sep, and the None was coerced to a string separator that matches nothing.

Fix

Two parts, both needed — the arity table cannot express this on its own:

• PyArgIsNoneLiteral recognises the None literal (a no-argument pynone call, what PyMakeNone builds) as the first argument of split/rsplit, and drops it from the chain so the existing no-separator arity logic applies. It matches the VALUE, not the spelling: s.split(sep) where sep happens to hold None is a run-time fact and is left alone.
• split(None, maxsplit) has no separator-based equivalent — the separator is a whitespace RUN of any width, leading whitespace is skipped rather than producing an empty first field, and an all-whitespace remainder contributes no field at all. So pystr_split_ws_max / pystr_rsplit_ws_max join pylib beside pystr_split_ws.

pylib.pas is a NilPy PROGRAM's runtime and the compiler does not use it, so this needs no re-pin to gate (see the scope note on [[project_builtin_change_needs_repin_for_gate_fixedpoint]]); a pin is still what carries the new runtime to $(PXX_STABLE) consumers.

Verified

test/test_nilpy_split_none_separator.npy — 19 rows (every maxsplit including 0 and -1, both directions, leading/trailing whitespace, all-whitespace and empty receivers, plus the exact-separator forms as controls) diff clean against .expected, which is CPython's own output. make compiler/pascal26 fixedpoint + tools/gate.sh quick GREEN.

Log

• 2026-08-09 — resolved, commit 2726abe5b.