fix: NoneType parameters lose their default and their nullability

[lesnik512]

A creator parameter annotated None with a default raises instead of using it:

class Svc:
    def __init__(self, hook: None = None) -> None: ...   # -> ArgumentResolutionError

class Svc2:
    def __init__(self, hook: str = "d") -> None: ...     # -> resolves, hook == "d"

SignatureItem.from_type (types_parser.py:21) early-returns a bare cls() for types.NoneType, discarding both the default it was handed and the nullability the annotation implies. Every other branch threads default through via result = {"default": default}. absent_disposition then sees default is UNSET and is_nullable is False, and classifies the parameter UNWIRABLE.

x: None = None and x: None were indistinguishable after parsing — the tell that cls() was dropping state rather than encoding a decision.

The fix

Return the degenerate nullable instead of an empty record. NoneType is a union with zero non-None members, and this is exactly what the union branch below would produce for it:

if type_ is types.NoneType:
    return cls(default=default, is_nullable=True)

The branch still has to exist: without it, NoneType falls through to isinstance(type_, type) and becomes arg_type=NoneType, which would then be looked up in the registry.

Behavior

None now takes the same two branches as X | None, which were already correct:

creator	before	after
x: None = None	UNWIRABLE (raises)	OMIT (default applies)
x: None	UNWIRABLE (raises)	NULL (injects None)
x: str | None = None	OMIT	OMIT (unchanged)
x: str | None	NULL	NULL (unchanged)

Row 2 goes beyond the reported defect: bare x: None currently errors and will now inject None. That is the annotation's only legal value, so injecting it is what the type says to do and the error was a false positive.

In the return position (-> None, a void creator) the new nullability is simply unread — factory.py:75 consults only .arg_type to derive bound_type, so a void creator still gets bound_type=None exactly as before. Three existing test_parse_creator expectations are updated for the return-position record; no behavior rides on them.

Design: planning/changes/2026-07-14.07-nonetype-param-drops-default.md.

Not in scope

The card this came from also flagged that UnsupportedCreatorParameterError is raised from two owners — types_parser.py (positional-only) and factory.py (parameterized generic) — one policy, two homes. Deliberately left alone: it is churn without a forcing function, and the revisit trigger on decisions/2026-07-14-signatureitem-opacity-superseded.md (a new duplicated raw-field decode idiom) has not fired.

Testing

TDD, 5 failing tests written first.

• test_signature_item_parser[NoneType] — from_type(None) is nullable.
• test_nonetype_threads_its_default — parametrized over None, 3, "x". Exercised through from_type rather than a real creator, because only None is assignable to a None-annotated parameter, so a non-None default cannot be spelled in a signature (ty correctly rejects it).
• test_nonetype_params_keep_defaults_through_parse_creator — end-to-end through the parser.
• test_nonetype_param_with_default_uses_the_default / ..._without_default_injects_none — resolution-level, the behavior a user actually hits.

just test-ci: 386 passed, 100% coverage. just lint-ci: clean.

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