stubtest: False-positives with parameter name mangling outside of classes

[Avasam]

Bug Report

It seems that when the implementation defines a function (not inside a class) with a parameter that has double leading underscore, it can become impossible to type for stubtest. See the following example in SQLAlchemy: python/typeshed#9596 (comment)

(A clear and concise description of what the bug is.)

To Reproduce

In typeshed: python .\tests\stubtest_third_party.py sqlalchemy

with the following stub definitions:

# sqlalchemy.testing.util
def resolve_lambda(__fn, **kw): ...

# sqlalchemy.orm.collections
class MappedCollection(dict[_KT, _VT]):
    # [...]
    # This function is actually defined dynamically outside the class
    @overload
    def update(self, __other: SupportsKeysAndGetItem[_KT, _VT], **kwargs: _VT) -> None: ...
    @overload
    def update(self, __other: Iterable[tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
    @overload
    def update(self, __other: symbol | _symbol = ..., **kwargs: _VT) -> None: ...

Expected Behavior

No errors

Actual Behavior

error: sqlalchemy.orm.collections.MappedCollection.update is inconsistent, stub argument "__other" differs from runtime argument "__other"
Stub: at line 127 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\orm\collections.pyi
Overload(def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], _typeshed.SupportsKeysAndGetItem[_KT`1, _VT`2], **kwargs: _VT`2), def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], typing.Iterable[Tuple[_KT`1, _VT`2]], **kwargs: _VT`2), def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], Union[sqlalchemy.util.langhelpers.symbol, sqlalchemy.util.langhelpers._symbol] =, **kwargs: _VT`2))
Inferred signature: def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], __other: Union[_typeshed.SupportsKeysAndGetItem[_KT`1, _VT`2], typing.Iterable[Tuple[_KT`1, _VT`2]], sqlalchemy.util.langhelpers.symbol, sqlalchemy.util.langhelpers._symbol] = ..., **kwargs)
Runtime: at line 1402 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\orm\collections.py
def (self, __other=symbol('Unspecified'), **kw)

error: sqlalchemy.orm.collections.MappedCollection.update is inconsistent, stub argument "__other" should be positional or keyword (remove leading double underscore)
Stub: at line 127 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\orm\collections.pyi
Overload(def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], _typeshed.SupportsKeysAndGetItem[_KT`1, _VT`2], **kwargs: _VT`2), def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], typing.Iterable[Tuple[_KT`1, _VT`2]], **kwargs: _VT`2), def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], Union[sqlalchemy.util.langhelpers.symbol, sqlalchemy.util.langhelpers._symbol] =, **kwargs: _VT`2))
Inferred signature: def (self: sqlalchemy.orm.collections.MappedCollection[_KT`1, _VT`2], __other: Union[_typeshed.SupportsKeysAndGetItem[_KT`1, _VT`2], typing.Iterable[Tuple[_KT`1, _VT`2]], sqlalchemy.util.langhelpers.symbol, sqlalchemy.util.langhelpers._symbol] = ..., **kwargs)
Runtime: at line 1402 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\orm\collections.py
def (self, __other=symbol('Unspecified'), **kw)

error: sqlalchemy.testing.resolve_lambda is inconsistent, stub argument "__fn" differs from runtime argument "__fn"
Stub: at line 28 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\testing\__init__.pyi
None
Runtime: at line 322 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\testing\util.py
def (__fn, **kw)

error: sqlalchemy.testing.resolve_lambda is inconsistent, stub argument "__fn" should be positional or keyword (remove leading double underscore)
Stub: at line 28 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\testing\__init__.pyi
None
Runtime: at line 322 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\testing\util.py
def (__fn, **kw)

error: sqlalchemy.testing.util.resolve_lambda is inconsistent, stub argument "__fn" differs from 
runtime argument "__fn"
Stub: at line 28 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\testing\util.pyi
None
Runtime: at line 322 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\testing\util.py
def (__fn, **kw)

error: sqlalchemy.testing.util.resolve_lambda is inconsistent, stub argument "__fn" should be positional or keyword (remove leading double underscore)
Stub: at line 28 in file E:\Users\Avasam\Documents\Git\typeshed\stubs\sqlalchemy\sqlalchemy\testing\util.pyi
None
Runtime: at line 322 in file C:\Users\Avasam\AppData\Local\Temp\tmpfa40k4ao\lib\site-packages\sqlalchemy\testing\util.py
def (__fn, **kw)

Found 6 errors (checked 228 modules)

Your Environment

• Mypy version used: mypy 0.991 (compiled: yes)
• Mypy command-line flags: (from typeshed, see above)
• Mypy configuration options from mypy.ini (and other config files): (from typeshed, see above)
• Python version used: Python 3.9.13