Type widens in nested function

[randolf-scholz]

Bug Report

Despite an if x is not None check, mypy suddenly believes x could be None inside a function definition.

To Reproduce

from functools import wraps
from collections.abc import Callable
from typing import TypeVar, ParamSpec, Optional

P = ParamSpec("P")
R = TypeVar("R")


def foo(func: Callable[P, R], before_func: Optional[Callable[P, None]] = None) -> Callable[P, R]:

    if before_func is not None:

        reveal_type(before_func)  # <--  Revealed type is "def (*Any, **Any) -> Any"

        @wraps(func)  # problem persists when removing @wraps
        def wrapper(*args, **kwargs):
            reveal_type(before_func)  # Revealed type is "Union[def (*Any, **Any) -> Any, None]"
            before_func(*args, **kwargs)  # error: "None" not callable
            return func(*args, **kwargs)

        return wrapper

    return func

Your Environment

• Mypy version used: mypy 0.961 (compiled: yes)
• Python version used: Python 3.10.5
• Operating system and version: Ubuntu 20.04.4 LTS

Mypy configuration

[tool.mypy]
allow_redefinition = true
color_output = true
error_summary = true
files = ["src/", "test/"]
plugins = "numpy.typing.mypy_plugin"
pretty = true
python_version = "3.10"
check_untyped_defs = true
show_column_numbers = true
show_error_codes = true
warn_unused_ignores = true
#disallow_untyped_defs = true
disallow_incomplete_defs = true

[AlexWaygood]

Duplicate of #2608