None check is not respected in nested function

[maksbotan]

Bug Report

mypy does not respect None check in nested functions defined after the check.

To Reproduce

Typecheck this code with Python 3.10 and mypy 0.971:

from typing import Optional

def foo(arg: Optional[int]) -> int:
    reveal_type(arg)
    
    if arg is None:
        return 0
    
    reveal_type(arg)
    
    def sub() -> int:
        reveal_type(arg)
        return arg + 5

    return sub()

Expected Behavior

Since arg is checked against None before sub() is even defined, we can't get to that definition if arg is None

Actual Behavior

mypy thinks that arg is still Optional[int] inside sub body:

main.py:4: note: Revealed type is "Union[builtins.int, None]"
main.py:9: note: Revealed type is "builtins.int"
main.py:12: note: Revealed type is "Union[builtins.int, None]"
main.py:13: error: Unsupported operand types for + ("None" and "int")
main.py:13: note: Left operand is of type "Optional[int]"
Found 1 error in 1 file (checked 1 source file)

Compare messages for lines 9 and 12.

Your Environment

• Mypy version used: 0.971
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10
• Operating system and version: mypy-play playground, also on macOS with python 3.9 and mypy 0.961.

Pyright in VSCode gets it right:

[hauntsaninja]

https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions
See #2608 for ways forward