`generics_defaults.py` mandates that type checkers should allow a class that raises `TypeError` at runtime

This test mandates that type checkers should allow classes that have ParamSpecs-with-defaults following TypeVarTuples: https://github.com/python/typing/blob/22d9392a5ead302cc02c47ecbbcf3aa40106fd9f/conformance/tests/generics_defaults.py#L191-L200

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 TypeVarTuple

Using 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 TypeVarTuple

And 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 TypeVarTuple

Since 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?

Comienza con conformance/tests/generics_defaults.py en las líneas 191-200 y compara su requisito con la especificación de typing. Después, inspecciona typing.py en _collect_type_parameters y reproduce los ejemplos en las versiones de sintaxis de Python citadas. Se considera terminado cuando se haya decidido si el runtime o la especificación y la prueba de conformance son inconsistentes, con cobertura de regresión para la corrección elegida.