Type inference returns a type different from the declared type.

[sasano8]

In the following example,
passing func3 will return str instead of the declared Literal["running"].
It should return the declared type as is.

from typing import Literal, Tuple, List, Sequence, TypeVar, Iterable

T = TypeVar("T")

def func1(val: Iterable[T]) -> T:
    ...

def func2(val: Sequence[T]) -> T:
    ...

def func3(val: Tuple[T]) -> T:
    ...


status: Tuple[Literal["running"]] = ("running",)

a = func1(status)  # Literal["running"] to Literal["running"]
b = func2(status)  # Literal["running"] to Literal["running"]
c = func3(status)  # Literal["running"] to str why?

[erictraut]

Are you seeing a behavior that leads to a false positive or false negative type error, or are you just curious about the behavior you're seeing? If you are seeing an actual bug, please provide a sample that demonstrates the problem. If you're just curious, then posting a question in the discussion section would be more appropriate.

[sasano8]

I see this as a false positive type error.

I have only given it the type Literal["running"].
So, TypeVar T is Literal["running"].
And, str does not belong to Literal["running"].
This is because Literal["running"] is narrower than str.

So it contradicts the next sentence.

Return Type Inference
The returned type is assumed to be the union of all types returned from all return statements.

The str that should not have belonged to Literal["running"] has returned.

I can't indicate the problem any further, and I'm not interested in it, so if it's not a problem, please close it.

kind regards.

[erictraut]

I agree that this is an inconsistency. I investigated and found the root cause. This inconsistency will be eliminated in the next release.

[sasano8]

Thank you!
I love VSCODE and I wish VSCODE further development.

[jakebailey]

This issue has been fixed in version 2021.9.2, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202192-16-september-2021