specialtypes_never_2 error

Never type compatibility violations

Detects type compatibility errors involving the Never bottom type:

1. Assigning a parameter typed ContainerNever to a local annotated ContainerT where T is not Never or Any (invariant violation) 2. Returning ClassCNever() from a function annotated -> ClassCU where the class's type parameter is invariant (not covariant)

from typing import Never, Any, Generic, TypeVar

T = TypeVar("T")
U = TypeVar("U")

def func(c: list[Never]):
    v: list[int] = c  # E0070 — list is invariant, list[Never] != list[int]

class ClassC(Generic[T]):
    pass

def func2(x: U) -> ClassC[U]:
    return ClassC[Never]()  # E0070 — ClassC is invariant

How to handle it

Every rule is on by default — strict is the default, not a cage. You can dial specialtypes_never_2 down per-file or per-path from your editor or pyproject.toml, or fix the code so it type-checks. See the Type System rules and the complete diagnostic reference.

Canonical URL: https://www.basilisk-python.dev/errors/specialtypes_never_2