Zero-long TypeVarTuple special case doesn't work

[TeamSpen210]

Bug Report

It seems the experimental variadic generic classes support doesn't handle having a zero-long tuple of types correctly. This is useful when handling a callback which takes no arguments. Both explicitly parametrizing with [()] and inferring from a function doesn't seem to work.

To Reproduce

from typing import Generic, Callable, TypeVarTuple, Unpack

T = TypeVarTuple("T")

def no_args() -> None: ...
def one_arg(a: int, /) -> None: ...

class MyClass(Generic[Unpack[T]]):
    func: Callable[[Unpack[T]], object]

    def __init__(self, func: Callable[[Unpack[T]], object]) -> None:
        self.func = func

def check(explicit: MyClass[()]) -> None:
    # note: Revealed type is "MyClass[Unpack[builtins.tuple[Any, ...]]]"
    reveal_type(explicit)
    # error: Incompatible types in assignment (expression has type "Callable[[], None]",
    # variable has type "Callable[[VarArg(Any)], object]")  [assignment]
    explicit.func = no_args
    implicit = MyClass(no_args)  # Inference doesn't work either.
    reveal_type(implicit)  # note: Revealed type is "MyClass"
    reveal_type(implicit.func) # note: Revealed type is "def (*Unpack[T`1]) -> builtins.object"
    implicit.func()  # error: Too few arguments  [call-arg]
    implicit.func(1)  # error: Argument 1 has incompatible type "int"; expected "Unpack[T]"  [arg-type]
    reveal_type(MyClass(one_arg))  # note: Revealed type is "MyClass[builtins.int]"

Expected Behavior

MyClass[()] should set T to be zero-long, thus accepting only functions with no args.

Actual Behavior

It looks like the TypeVarTuple is being set to tuple[Any, ...], which is not what is intended.

Your Environment

• Mypy version used: master = 181cbe8
• Mypy command-line flags: --new-type-inference --enable-incomplete-feature=Unpack --enable-incomplete-feature=TypeVarTuple
• Python version used: 3.11.4 (tags/v3.11.4:d2340ef, Jun 7 2023, 05:45:37) [MSC v.1934 64 bit (AMD64)]