TypeIs narrows constrained type variable to Never

[InSyncWithFoo]

Minimal reproducible example (playground):

from typing_extensions import TypeIs

def is_str(v: object) -> TypeIs[str]:
    ...

def foo[T: (str, int)](v: T) -> T:
    if is_str(v):
        reveal_type(v)  # Never
		return v
    ...

Is $(str, int) \land str$ really $\emptyset$?

[erictraut]

I agree this is a bug. Converting to a bug report so it's tracked accordingly.

[erictraut]

Here's a link to the issue: #7760

[minmax]

IMHO version 1.1.361 does not match the description of PEP 742 for else branches.

Is it possible to add support for these in isinstance, so that the new version of TypeIs will also support them?

From PEP: "TypeIs can narrow the type in both the if and else branches of a conditional."

"When a TypeIs function returns False, type checkers can narrow the type of the variable to exclude the TypeIs type."

[erictraut]

Please file a new issue.

[minmax]

Here's a link to the issue: #7837