Empty variadic tuple subtype not assignable to tuple[int, ...]

[jorenham]

class Shape[*Ts](tuple[*Ts]): ...
class Shape0(Shape[()]): ...
class Shape1(Shape[int]): ...

def rank(shape: tuple[int, ...]) -> int:
    return len(shape)

_ = rank(Shape())       # ✅
_ = rank(())            # ✅
_ = rank(Shape0())      # ❌  Argument 1 to "rank" has incompatible type "Shape0"; expected "tuple[int, ...]"
_ = rank(Shape1([1]))   # ✅

playground

Perhaps this is related to #19105, which also involves empty tuple subtypes.

[jorenham]

I confirmed that #19100 does not solve this

[jorenham]

I'm guessing that it's a consequence of this issue, but it might be worth noting that these variadic tuple subtypes ignore the constraints of typevars:

class Shape[*Ts](tuple[*Ts]): ...

class Shape0(Shape[()]): ...
class Shape1(Shape[int]): ...
class ShapeN(Shape[*tuple[int, ...]]): ...


def upcast[T: (tuple[()], tuple[int], tuple[int, ...])](shape: T) -> T:
    return shape

reveal_type(upcast(Shape0()))  # ❌ Shape0
reveal_type(upcast(Shape1()))  # ✅ tuple[builtins.int]
reveal_type(upcast(ShapeN()))  # ❌ ShapeN

full output:

main.py:11: error: Value of type variable "T" of "upcast" cannot be "Shape0"  [type-var]
main.py:11: note: Revealed type is "__main__.Shape0"
main.py:12: note: Revealed type is "tuple[builtins.int]"
main.py:13: error: Value of type variable "T" of "upcast" cannot be "ShapeN"  [type-var]
main.py:13: note: Revealed type is "__main__.ShapeN"
Found 2 errors in 1 file (checked 1 source file)

playground

For reference, pyright reveals these as tuple[()], tuple[int], tuple[int, ...] (playground).

Let me know in case I should raise this separately.

[sterliakov]

Wow, you opened a can of worms with your typevartuple experiments! This is definitely a bug.