Issue with type-narrowing inside while loop

[bijij]

Describe the bug
Pyright seemingly resets type-narrowing in specific circumstances. however adding boilerplate code fixes this?

To Reproduce
See the Screenshots/Code section below.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots or Code
With the original code:

from typing_extensions import Self
from collections.abc import Generator

class A:
    parent: Self | None

class B:...

def foo(v: A | B | None) -> Generator[A, None, None]:
    reveal_type(v)
    if not isinstance(v, B):
        reveal_type(v)
        while v is not None:
            reveal_type(v)
            yield v
            v = v.parent
            reveal_type(v)

Pyright outputs the following:

| test.py:10:17 - information: Type of "v" is "A | B | None"
| test.py:12:21 - information: Type of "v" is "A | None"
| test.py:14:25 - information: Type of "v" is "A | B" ???
- test.py:15:19 - error: Expression of type "A | B" cannot be assigned to yield type "A"
-   Type "A | B" cannot be assigned to type "A"
-     "B" is incompatible with "A" (reportGeneralTypeIssues)
- test.py:16:19 - error: Cannot access member "parent" for type "B"
-   Member "parent" is unknown (reportGeneralTypeIssues)
| test.py:17:25 - information: Type of "v" is "A | B | None"

Moving the None check inside the loop

from typing_extensions import Self
from collections.abc import Generator

class A:
    parent: Self | None

class B:...

def foo(v: A | B | None) -> Generator[A, None, None]:
    reveal_type(v)
    if not isinstance(v, B):
        reveal_type(v)
        while True:
            if v is None:
                break
            reveal_type(v)
            yield v
            v = v.parent
            reveal_type(v)

Pyright outputs the following

| test.py:10:17 - information: Type of "v" is "A | B | None"
| test.py:12:21 - information: Type of "v" is "A | None"
| test.py:16:25 - information: Type of "v" is "A | B"  ???
- test.py:17:19 - error: Expression of type "A | B" cannot be assigned to yield type "A"
-   Type "A | B" cannot be assigned to type "A"
-     "B" is incompatible with "A" (reportGeneralTypeIssues)
- test.py:18:19 - error: Cannot access member "parent" for type "B"
-   Member "parent" is unknown (reportGeneralTypeIssues)
| test.py:19:25 - information: Type of "v" is "A | B | None"

adding a reveal_type above if v is None:

from typing_extensions import Self, reveal_type
from collections.abc import Generator

class A:
    parent: Self | None

class B:...

def foo(v: A | B | None) -> Generator[A, None, None]:
    reveal_type(v)
    if not isinstance(v, B):
        reveal_type(v)
        while True:
            reveal_type(v)
            if v is None:
                break
            reveal_type(v)
            yield v
            v = v.parent
            reveal_type(v)

Pyright outputs the following

| test.py:10:17 - information: Type of "v" is "A | B | None"
| test.py:12:21 - information: Type of "v" is "A | None"
| test.py:14:25 - information: Type of "v" is "A | None"
| test.py:17:25 - information: Type of "v" is "A"
| test.py:20:25 - information: Type of "v" is "A | None"

why does adding reveal_type here cause this to type-check???

VS Code extension or command-line
pyright 1.1.231

Additional context

mypy, once remoiving reliance on Self since it is currently unsupported for the original snippet outputs the following

test.py:10: note: Revealed type is "Union[test.A, test.B, None]"
test.py:12: note: Revealed type is "Union[test.A, None]"
test.py:14: note: Revealed type is "test.A"
test.py:17: note: Revealed type is "Union[test.A, None]"

which is what I would expect pyright to do.

---

Interesting tidbit

When using pylance (v2022.3.3-pre.1) moving the position of the cursor and saving will randomly cause the file to pass/fail type-checking. Not sure what to make of that.

Code_HPU5zYAuZ4.mp4

Would it be wise to make an issue on the pylance repo for this?

[erictraut]

Thanks for the bug report. I'm able to repro the problem. This is a core type checker issue, so no need to file a bug in the pylance repo.

This bug is related to another one that is currently open: #2983. I suspect that both have a similar root cause.

[erictraut]

This will be addressed in the next release.

[erictraut]

This is included in pyright 1.1.233, which I just published. It will also be included in the next release of pylance.