Type narrowing not applied to a generator

[spagh-eddie]

Bug Report

An argument typed as None | str gets correctly narrowed to str after an if argument:, but the narrowing is not applied inside the if block.

To Reproduce

words = ["this", "that", "other"]


def fizz(mandatory_letters: None | str):
    filters = []
    reveal_type(mandatory_letters)
    if mandatory_letters:
        reveal_type(mandatory_letters)
        filters.append(lambda word: all(c in word for c in mandatory_letters))
    return filter(lambda word: all(f(word) for f in filters), words)

Expected Behavior

I expected no error.
I expected the type narrowing to apply to the if block and its generator.

Actual Behavior

% mypy file.py
file.py:6: note: Revealed type is "Union[None, builtins.str]"
file.py:8: note: Revealed type is "builtins.str"
file.py:9: error: Item "None" of "Optional[str]" has no attribute "__iter__" (not iterable)
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used:

% mypy --version
mypy 0.961 (compiled: yes)

• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.1
• Operating system and version: macOS Monterey 12.0.1 (21A559)

[JelleZijlstra]

This is an instance of #2608.