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

[AlexWaygood]

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

	# > It is allowed to have a ``ParamSpec`` with a default following a
	# > ``TypeVarTuple`` with a default, as there can be no ambiguity between a
	# > type argument for the ``ParamSpec`` and one for the ``TypeVarTuple``.
	P = ParamSpec("P", default=[float, bool])
	class Foo6(Generic[*Ts, P]):
	x: tuple[*Ts]
	y: Callable[P, None]

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?

[JelleZijlstra]

I think this is a bug in the runtime, yes.

[JelleZijlstra]

The relevant spec section is https://typing.python.org/en/latest/spec/generics.html#defaults-following-typevartuple, deriving from PEP 696 https://peps.python.org/pep-0696/#defaults-following-typevartuple