subscripting a class with a TypeVarTuple followed by a defaulted ParamSpec reports "Can only replace ParamSpec" and infers Any #22375

Bug Report

PEP 696 and the typing spec (section "Defaults following TypeVarTuple") allow a ParamSpec with a default after a TypeVarTuple. The spec's example also gives the TypeVarTuple a default; the result below is the same with and without it. mypy accepts the class definition, but subscripting it gives an error: the last type argument is taken as the argument for the ParamSpec, and the result is Foo[*tuple[Any, ...], Any].

To Reproduce

from collections.abc import Callable
from typing import assert_type


class Foo[*Ts, **P = [float, bool]]:
    def args(self) -> tuple[*Ts]:
        raise NotImplementedError

    def fn(self) -> Callable[P, None]:
        raise NotImplementedError


foo = Foo[int, str]()
assert_type(foo.args(), tuple[int, str])
assert_type(foo.fn(), Callable[[float, bool], None])

bar = Foo[int, str, [bytes]]()
assert_type(bar.args(), tuple[int, str])
assert_type(bar.fn(), Callable[[bytes], None])

The old-style spelling (Ts = TypeVarTuple("Ts"), P = ParamSpec("P", default=[float, bool]), class Foo(Generic[Unpack[Ts], P]), a parameter annotated Foo[int, str]) gives the same message on the subscript.

Actual Behavior

t.py:13: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
t.py:14: error: Expression is of type "tuple[Any, ...]", not "tuple[int, str]"  [assert-type]
t.py:15: error: Expression is of type "def (*Any, **Any) -> None", not "Callable[[float, bool], None]"  [assert-type]
t.py:17: error: Can only replace ParamSpec with a parameter types list or another ParamSpec, got "str"  [misc]
t.py:18: error: Expression is of type "tuple[Any, ...]", not "tuple[int, str]"  [assert-type]
t.py:19: error: Expression is of type "def (*Any, **Any) -> None", not "Callable[[bytes], None]"  [assert-type]

With a default on the TypeVarTuple too (class Foo[*Ts = *tuple[int, str], **P = [float, bool]], the example from the spec) the result is the same. This is also what the typing conformance suite shows for mypy on generics_defaults.py, lines 203 to 205.

Expected Behavior

No errors. Foo[int, str] is Ts = (int, str), P = [float, bool] and Foo[int, str, [bytes]] is Ts = (int, str), P = [bytes], as in the spec. A ParamSpec argument has to be a list, ... or another ParamSpec, so str cannot be one and there is no ambiguity with the TypeVarTuple. pyright 1.1.414 and ty 0.0.97 accept the program.

Note: CPython 3.13.16, 3.14.8 and 3.15.0 raise TypeError: Type parameter with a default follows TypeVarTuple when this class is created (python/typing#2211, a runtime bug), so the snippet is for type checking only.

My Environment

• Mypy version used: 2.4.0, 2.4.1 and master 2.5.0+dev.879da562
• Mypy command-line flags: --python-version 3.13 --no-incremental
• Python version used: 3.13.16
• OS: Linux

mypy-play: https://mypy-play.net/?gist=250c90e8db7f8b3279d619ca6d36c422