stubtest: Recognize __ as indicating positional-only arg at runtime

[JelleZijlstra]

I got this error on typing-extensions:

error: typing_extensions.override is inconsistent, stub argument "__arg" should be positional or keyword (remove leading double underscore)
Stub: in file stdlib\typing_extensions.pyi:366
def [_F <: def (*Any, **Any) -> Any] (_F`-1) -> _F`-1
Runtime: in file C:\Users\RUNNER~1\AppData\Local\Temp\tmp769672by\Lib\site-packages\typing_extensions.py:2403
def (__arg: ~_F) -> ~_F

Since typing_extensions.override uses __arg at runtime, I think stubtest should recognize it as positional-only.

[JelleZijlstra]

It also complains that error: typing_extensions.override is inconsistent, stub argument "__arg" differs from runtime argument "__arg", which is a rather enigmatic issue to resolve.

[AlexWaygood]

It also complains that error: typing_extensions.override is inconsistent, stub argument "__arg" differs from runtime argument "__arg", which is a rather enigmatic issue to resolve.

That second issue is a duplicate of #14560

[jorenham]

This issue is also affecting NumPy.

At runtime, numpy.testing._private.utils.check_support_sve (also re-exported in numpy.testing) has the following signature:

def check_support_sve(__cache=[]): ...

I tried annotating it in 3 ways; all of them resulted in a stubtest error. This forces us to put it in the allowist.

attempt 1: pos-only cache

def check_support_sve(cache: list[_T_or_bool] = [], /) -> _T: ...

$ uv run tool/stubtest.py
error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "cache" differs from runtime argument "__cache"
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "cache" should be positional or keyword (remove "/")
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

Found 2 errors (checked 324 modules)

attempt 2: pos-only __cache

def check_support_sve(__cache: list[_T_or_bool] = [], /) -> _T_or_bool: ...

$ uv run tool/stubtest.py
error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "__cache" differs from runtime argument "__cache"
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "__cache" should be positional or keyword (remove "/")
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

Found 2 errors (checked 324 modules)

attempt 3: pos-or-kw __cache

def check_support_sve(__cache: list[_T_or_bool] = []) -> _T_or_bool: ...  # noqa: PYI063

$ uv run tool/stubtest.py
error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "__cache" differs from runtime argument "__cache"
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

error: numpy.testing._private.utils.check_support_sve is inconsistent, stub argument "__cache" should be positional or keyword (remove "/")
Stub: in file src/numpy-stubs/testing/_private/utils.pyi:435
def [_T_or_bool = bool] (list[_T_or_bool`-1 = bool] =) -> _T_or_bool`-1 = bool
Runtime: in file .venv/lib/python3.13/site-packages/numpy/testing/_private/utils.py:1413
def (__cache=[])

Found 2 errors (checked 324 modules)