Recursive sequence type of `Sequence[NotAString]` should not accept `str`

[jorenham]

This is a (p)repost from https://github.com/python/mypy/issues/18184, which also applies here:

The false negative can be reproduced with:

from collections.abc import Sequence
from typing import TypeAlias

class Scalar: ...

Vector: TypeAlias = Sequence[Scalar]
Tensor: TypeAlias = Vector | Sequence["Tensor"]

accepted_vector: Vector = [Scalar()]     # true negative
rejected_vector: Vector = "duck"         # true positive: ducks aren't vectors

accepted_tensor: Tensor = [[[Scalar()]]] # true negative
rejected_tensor1: Tensor = object()      # true positive: objects aren't tensors
rejected_tensor2: Tensor = "duck"        # false negative: ducks are tensors...?

mypy-play

[jorenham]

oh woops, this should be labeled as bug

[KotlinIsland]

oh woops, this should be labeled as bug

feel free to update it 😁

[KotlinIsland]

is this the same thing?

class C[T]: ...
type A = C[A] | int
type B = C[B]

b: B
a: A = b

[KotlinIsland]

• related #651

[jorenham]

class C[T]: ...
type A = C[A] | int
type B = C[B]

b: B
a: A = b

If T is covariant, and mypy treats str like a recursive type, then it might indeed be a more general example.

---

edit it's actually a different, but similar, issue. Because this Sequence issue actually works in as expected pyright, but your A, B example is accepted in both mypy and pyright:

• https://basedpyright.com/?pyrightVersion=1.22.0&typeCheckingMode=all&code=MYGwhgzhAEDCDaAVAugLmgOiwWAFB4BcBPABwFNoBBaAXjnkuUNIoCFb7Wn9cATMgGbQBACgBG6VgBpoAegCU0ALQA%2BKqjzQt0AE5kCAVx0A7aGLxA

• https://basedpyright.com/?pyrightVersion=1.22.0&typeCheckingMode=all&code=GYJw9gtgBAxmA28CmMAuBLMA7AzgOgEMAjGKdCABzBFSgGUkBHAVySxiQFgAoUSKVAE8K6LAHMylarQAqwpAEF46Ajh48Y8VTnowCWkAC4oeU%2Bu4A1FKmrG5FRctVQAvPSat2SANp09BgF0eGTYcWyh7RxUdNys0aigAH3cWNg5vACIQ3GoMoO4eAhgOClQkABMAfQA3a3C4mxBXKF9-AhAACgBKAKg%2BvoBiARBWKCwkMQIMWp4QJAArawqauqMoBoS3DPLmGABrDP6jweHRqhx0aaRjHf2ddrYAclpa%2BJA1Au4ikrKqspy1tkwk03N4wX59O1ugEYVAhqgRkgxhMpugZtw5os0Mt-sCAIx2UKbKBgIhY1DdY5w05I86XNHXElk6z3OZYZ4CInvWYLJZ-LkAJkJAOa212BypJ2A%2BhwSPGkyuN3FrKRuOo%2BFMAH4gA

[jorenham]

is this the same thing?

class C[T]: ...
type A = C[A] | int
type B = C[B]

b: B
a: A = b

What's the | int for btw 🤔 ?

[KotlinIsland]

What's the | int for btw 🤔 ?

just for being closer to the original egg

[jorenham]

this seems relevant

https://github.com/KotlinIsland/basedmypy/blob/0935784ab77e91c869bae67be01ba0f420678620/mypy/subtypes.py#L170-L189