Potential regression in type narrowing logic

[brearleyt]

Describe the bug
I believe there has been a regression in the type narrowing logic for the below example. The error is raised when using 1.1.399, but was not raised when using 1.1.398.

Code or Screenshots

import subprocess
import typing as t


def foo(handle: subprocess.Popen[str] | t.Callable[[str], None]) -> str:
    assert isinstance(handle, subprocess.Popen)
    t.reveal_type(handle)
    stdout, _ = handle.communicate()
    return stdout

Pyright output:

example.py:7:19 - information: Type of "handle" is "Popen[str] | Popen[Unknown]"
example.py:8:5 - error: Type of "stdout" is partially unknown
    Type of "stdout" is "str | Unknown" (reportUnknownVariableType)
example.py:8:13 - error: Type of "_" is partially unknown
    Type of "_" is "str | Unknown" (reportUnknownVariableType)
example.py:8:17 - error: Type of "communicate" is partially unknown
    Type of "communicate" is "((input: str | None = None, timeout: float | None = None) -> tuple[str, str]) | ((input: Unknown | None = None, timeout: float | None = None) -> tuple[Unknown, Unknown])" (reportUnknownMemberType)
example.py:9:12 - error: Return type, "str | Unknown", is partially unknown (reportUnknownVariableType)
4 errors, 0 warnings, 1 information 

VS Code extension or command-line
Python: 3.13
Pyright: 1.1.399 via the commandline.

[erictraut]

This change was intentional, so I don't consider it a regression.

I'll note that no error is reported by default with this code. This error is reported only if you enable strict-mode type checking. There are situations (like this) where a bug fix can change a behavior that affects strict-mode checks.

This change was needed to fix bugs #10171 and #10179. The Popen class is not marked @final, so it can be subclassed, and a subclass can provide a __call__ method override, which makes it callable. Therefore the type guard isn't sufficient to eliminate the Callable[[str], None]. If you change your assert to assert not callable(handle), you will no longer see the error.