Allow TypeVarTuple as type argument for subclasses of generic TypedDict

Bug Report

I have a TypedDict class which has TypeVarTuple type argument(s), and that works as I would expect. However, when I try to make a class derived from that base class, I get unexpected errors from mypy. Different errors, actually, depending on whether I use Unpack or the new "star" unpacking syntax.

This all ought to work, though, right?

To Reproduce

# typeddict_typevartuple_310.py

from collections.abc import Callable
from typing import Generic, Tuple, TypedDict, TypeVar
from typing_extensions import TypeVarTuple, Unpack

ATs = TypeVarTuple('ATs')
RT = TypeVar('RT')


class BaseDict(TypedDict, Generic[Unpack[ATs], RT]):
    callback: Callable[[Unpack[ATs]], RT]
    arguments: Tuple[Unpack[ATs]]


class DerivedDict(BaseDict[Unpack[ATs], RT]):  # E: Unpack is only valid in a variadic position
    returned: RT

# typeddict_typevartuple_311.py

from collections.abc import Callable
from typing import Generic, TypedDict, TypeVar, TypeVarTuple

ATs = TypeVarTuple('ATs')
RT = TypeVar('RT')


class BaseDict(TypedDict, Generic[*ATs, RT]):
    callback: Callable[[*ATs], RT]
    arguments: tuple[*ATs]


class DerivedDict(BaseDict[*ATs, RT]):  # E: Invalid TypedDict type argument
    returned: RT

Expected Behavior

I would have expected mypy to accept both these versions as correct...

Actual Behavior

... But it fails both of them, with distinct messages (see inline comments).

Your Environment

• Mypy version used: mypy 1.10.0+dev.3c87af272cbf7c49699b7508c7f51365da139c05 (compiled: no)
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.10 and 3.11

Start by running the two reproductions, typeddict_typevartuple_310.py and typeddict_typevartuple_311.py, with the reported mypy behavior. Trace the TypedDict generic type-argument validation reached by these examples, then add focused coverage for both Unpack and star-unpacking forms. Done means mypy accepts both derived TypedDict definitions without the reported errors.