cannot narrow some-type|Any to some-type by isinstance

[pinterior]

Describe the bug

Unions can be narrowed to it's element type by isinstance as described in https://github.com/microsoft/pyright/blob/main/docs/type-concepts.md#type-narrowing .

but, for example Union[str,Any] cannot be narrowed to str by isinstance(..., str).

To Reproduce

from typing import Any, Union
from typing_extensions import reveal_type

def t0() -> Any: ...
def t1() -> str: ...
def t2() -> Union[str, int]: ...
def t3() -> Union[str, Any]: ...
def t4() -> Union[str, int, Any]: ...

r0 = t0()
r1 = t1()
r2 = t2()
r3 = t3()
r4 = t4()

reveal_type(r0)
if isinstance(r0, str):
    reveal_type(r0)

reveal_type(r1)
if isinstance(r1, str):
    reveal_type(r1)

reveal_type(r2)
if isinstance(r2, str):
    reveal_type(r2)

reveal_type(r3)  # str|Any
if isinstance(r3, str):
    reveal_type(r3)  # str expeted, but str|Any

reveal_type(r4)  # str|int|Any
if isinstance(r4, str):
    reveal_type(r4)  # str expeted, but str|Any

  bb.py:16:13 - information: Type of "r0" is "Any"
  bb.py:18:17 - information: Type of "r0" is "str"
  bb.py:20:13 - information: Type of "r1" is "str"
  bb.py:22:17 - information: Type of "r1" is "str"
  bb.py:24:13 - information: Type of "r2" is "str | int"
  bb.py:26:17 - information: Type of "r2" is "str"
  bb.py:28:13 - information: Type of "r3" is "str | Any"
  bb.py:30:17 - information: Type of "r3" is "str | Any"
  bb.py:32:13 - information: Type of "r4" is "str | int | Any"
  bb.py:34:17 - information: Type of "r4" is "str | Any"

Expected behavior

in above example, all reveal_type in if isinstance(..., str): should output str.

VS Code extension or command-line

command-line tool, 1.1.290

[erictraut]

I agree this is inconsistent. I've updated the isinstance type narrowing logic to be more consistent in this case. This will be included in the next release.

[erictraut]

I needed to back out this change for now because it introduced a regression in some other isinstance narrowing cases. I'll need to see if I can make both cases work.

[erictraut]

I found a way to make both cases work. This will be included in the next release.

[erictraut]

This is included in pyright 1.1.291, which I just published. It will also be included in a future release of pylance.