Typechecking differs between one-line def of "return expr" vs. "r = expr; return r"

[drvink]

mypy 0.931

#!/usr/bin/env python3

from collections.abc import Iterable
from typing import Optional
import os

def min_direntry_good(
 fs: 'Iterable[os.DirEntry[str]]'
) -> 'Optional[os.DirEntry[str]]':
 r = min(fs, default=None, key=lambda x: x.stat().st_mtime)
 return r

def min_direntry_bad(
 fs: 'Iterable[os.DirEntry[str]]'
) -> 'Optional[os.DirEntry[str]]':
 return min(fs, default=None, key=lambda x: x.stat().st_mtime)

hober.py:16: error: Item "None" of "Optional[DirEntry[str]]" has no attribute "stat"
Found 1 error in 1 file (checked 1 source file)

[drvink]

This is still broken in mypy 0.940:

hober.py:16: error: Item "None" of "Optional[DirEntry[str]]" has no attribute "stat" [union-attr]
hober.py:16: error: Invalid self argument "stat_result" to attribute function "st_mtime" with type "Callable[[stat_result], float]" [misc]
Found 2 errors in 1 file (checked 1 source file)

[JelleZijlstra]

This is likely an issue with mypy's "type context", which infers different types based on the "expected" type. Here it obviously goes wrong, but chances are it's hard to fix it without breaking some other use case.