Closes #272.

Problem

_invoke_with_optional_kwargs (added in #268 to forward validate() keyword arguments to callable Optional defaults) calls inspect.signature(f) unconditionally. For many C-implemented built-ins — dict, int, and others — inspect.signature raises ValueError: no signature found for builtin type, so those callables can no longer be used as defaults:

>>> from schema import Schema, Optional
>>> Schema({Optional("k", default=dict): dict}).validate({})
ValueError: no signature found for builtin type <class 'dict'>

(Exactly which built-ins are affected varies by Python version — on 3.13 dict and int still raise, on older versions more do — but the breakage has been present since 0.7.5.)

Fix

Catch the introspection failure and fall back to calling the default with no arguments. That is the only sensible call for a callable whose signature cannot be inspected, and it restores the behaviour these defaults had before keyword-argument forwarding was introduced. Callables that do expose a signature are untouched, so kwarg forwarding still works:

>>> Schema({Optional("k", default=dict): dict}).validate({})
{'k': {}}

Notes

• Added a regression test (test_optional_callable_default_builtin_c_callables) that also asserts validate() kwargs are still forwarded to introspectable callables.
• Source + test only, leaving CHANGELOG.md to gitchangelog (matching recent fixes such as Fix TypeError building key error message for a tuple key (#253) #347).
• Full suite passes locally (pytest, 124 tests); ruff and ruff-format (v0.4.3) clean.
• This is a smaller, focused alternative to the now-closed Fix Optional failing when default is some builtins #276, deliberately avoiding the kwargs-forwarding redesign that stalled that PR.