Support unpacked arbitrary-length tuple in type argument lists

[mrahtz]

Pyre Bug

PEP 646 specifies that the following behaviour should be valid:

T = TypeVar('T')
Ts = TypeVarTuple('Ts')
class C(Generic[T, *Ts]): pass
C[*tuple[int, ...]]

I think Pyre doesn't handle this correctly yet. For example:

from typing import Generic, TypeVar
from pyre_extensions import Unpack, TypeVarTuple

T = TypeVar('T')
Ts = TypeVarTuple('Ts')

class C(Generic[T, Unpack[Ts]]): ...

x: C[Unpack[tuple[int, ...]]] = C()
reveal_type(x)

ƛ Found 2 type errors!
foo.py:9:3 Invalid type parameters [24]: Generic type `C` expects at least 1 type parameter, received 1.
foo.py:10:0 Revealed type [-1]: Revealed type for `x` is `C[typing.Any, *Tuple[typing.Any, ...]]`.

Similarly, the following should be valid with type aliases:

Alias = tuple[T, *Ts]
Alias[*tuple[int, ...]]

With Pyre:

from typing import Generic, TypeVar
from pyre_extensions import Unpack, TypeVarTuple

T = TypeVar('T')
Ts = TypeVarTuple('Ts')

Alias = tuple[T, Unpack[Ts]]

x: Alias[Unpack[tuple[int, ...]]] = (1, 2, 3)
reveal_type(x)

foo2.py:9:3 Invalid type variable [34]: The type variable `Variable[T]` can only be used to annotate generic classes or functions.
foo2.py:9:3 Invalid type variable [34]: The type variable `Ts` can only be used to annotate generic classes or functions.
foo2.py:10:0 Revealed type [-1]: Revealed type for `x` is `typing.Tuple[typing_extensions.Literal[1], typing_extensions.Literal[2], typing_extensions.Literal[3]]`.

@pradeep90 I think this is yours too?