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nilpy: f(None) where the parameter is Optional[str]

Repro

from typing import Optional
def g(x: Optional[str]) -> str:
    if x is None:
        return "none"
    return x
print(g("a"), g(None))



error: no overload of g matches these arguments
  argument types: (Pointer)

str | None behaves identically, by design — unions get Optional's exact treatment. RETURNING None from such a function works; only passing it in fails.

Why

An Optional[str] parameter is typed AnsiString (Optional widens str to AnsiString), and None arrives as a nil Pointer, which does not match. The return path works because a return annotation widens further, to a real variant, so None and a legitimate value stay distinct.

Shape

Type an Optional PARAMETER the way an Optional RETURN is typed — a variant, so None is VT_EMPTY rather than a nil pointer of the wrong type — or accept a nil Pointer argument for an Optional-annotated string parameter. The first is more consistent with what the return path already does.

Gate

make test-nilpy green with a .npy case passing None and a real value to Optional params of str / int / a class type, diffed against CPython, + --tier quick + self-host byte-identical.

Recon 2026-07-31 — the repro has changed shape; still broken, differently

Re-ran the ticket's own repro on current HEAD. It no longer hits the compile error ("no overload of g matches these arguments") — g(None) now COMPILES. But the runtime answer is wrong in a new way:

print(g("a"), g(None))   # CPython: a none     pxx: a None

g(None) is being called (not rejected), but inside g, x is None does not take the True branch — the function falls through to return x, and PRINTING x renders the text "None" (four characters, capital N), not the identity check's own None marker. So whatever coercion now lets the call through is not the pynone() VT_EMPTY-boxing fix IRLowerCallArg already has for a VARIANT parameter (ir.inc ~line 2208, "Python's None reaches here as the nil POINTER literal... f(None) then printed 0") — that block is gated on the PARAMETER being tyVariant, and Optional[str] maps to tyAnsiString (PyAnnTypeAt), not variant, so it never fires here. Something ELSE now accepts a nil-pointer argument against an AnsiString parameter and turns it into the text "None" rather than a nil string handle — not yet located. IRLowerCallArg is a very large, historically fragile function (each of its many special cases documents a specific past regression); finding the exact coercion without measuring it directly (PXXDBG a.ir on the call site, or bisecting which recent commit changed this from a compile error to a silent wrong value) risks a guess, which the project's own debugging playbook warns against. Left for the next session with time to measure it properly rather than patch blind.

Not fixed. Recon only — see above for what changed and what still needs locating.