generics_defaults.py mandates that type checkers should allow a class that raises TypeError at runtime #2211
Description
This test mandates that type checkers should allow classes that have ParamSpecs-with-defaults following TypeVarTuples:
typing/conformance/tests/generics_defaults.py
Lines 191 to 200 in 22d9392
But these fail at runtime. Using Python 3.11 syntax:
% uvx python3.14
Python 3.14.0 (main, Oct 10 2025, 12:54:13) [Clang 20.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import *
>>> Ts = TypeVarTuple("Ts")
>>> P = ParamSpec("P", default=[int, str])
>>> class A(Generic[*Ts, P]): ...
...
Traceback (most recent call last):
File "<python-input-3>", line 1, in <module>
class A(Generic[*Ts, P]): ...
~~~~~~~^^^^^^^^
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 402, in inner
return func(*args, **kwds)
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 1157, in _generic_class_getitem
return _GenericAlias(cls, args)
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 1334, in __init__
self.__parameters__ = _collect_type_parameters(
~~~~~~~~~~~~~~~~~~~~~~~~^
args,
^^^^^
enforce_default_ordering=enforce_default_ordering,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 291, in _collect_type_parameters
raise TypeError('Type parameter with a default'
' follows TypeVarTuple')
TypeError: Type parameter with a default follows TypeVarTupleUsing Python <=3.10 syntax:
>>> class B(Generic[Unpack[Ts], P]): ...
...
Traceback (most recent call last):
File "<python-input-4>", line 1, in <module>
class B(Generic[Unpack[Ts], P]): ...
~~~~~~~^^^^^^^^^^^^^^^
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 402, in inner
return func(*args, **kwds)
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 1157, in _generic_class_getitem
return _GenericAlias(cls, args)
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 1334, in __init__
self.__parameters__ = _collect_type_parameters(
~~~~~~~~~~~~~~~~~~~~~~~~^
args,
^^^^^
enforce_default_ordering=enforce_default_ordering,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 291, in _collect_type_parameters
raise TypeError('Type parameter with a default'
' follows TypeVarTuple')
TypeError: Type parameter with a default follows TypeVarTupleAnd using Python 3.13+ syntax:
>>> class C[*Ts, **P = [int, str]]: ...
...
Traceback (most recent call last):
File "<python-input-5>", line 1, in <module>
class C[*Ts, **P = [int, str]]: ...
File "<python-input-5>", line 1, in <generic parameters of C>
class C[*Ts, **P = [int, str]]: ...
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 1334, in __init__
self.__parameters__ = _collect_type_parameters(
~~~~~~~~~~~~~~~~~~~~~~~~^
args,
^^^^^
enforce_default_ordering=enforce_default_ordering,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 291, in _collect_type_parameters
raise TypeError('Type parameter with a default'
' follows TypeVarTuple')
TypeError: Type parameter with a default follows TypeVarTupleSince it seems that the tests are quoting the spec here, is this a bug in the runtime implementation of the typing module? Or should we update the spec and the conformance suite to allow type checkers to catch this runtime error?