wrongly reported typing error in min with key lambda function and default value when result is Optional

[jan-spurny]

Bug Report

When using key function in min builtin function, default None value and returning it from a function which has an Optional type, mypy reports a problem where I believe there is none.

To Reproduce

from dataclasses import dataclass
from typing import List

@dataclass
class X:
    x: int

def get_min(vals: List[X]) -> X | None:
    return min(vals, key = lambda tsync: tsync.x, default=None)

result = get_min([X(1), X(2), X(3)])

Expected Behavior

No errors reported.

Actual Behavior

$ mypy b.py --pretty
b.py:11: error: Item "None" of "X | None" has no attribute "x"  [union-attr]
        return min(vals, key = lambda tsync: tsync.x, default=None)
                                             ^~~~~~~
Found 1 error in 1 file (checked 1 source file)

The error goes away if I remove the default=None or when the min is not in a function with X | None return type:

from dataclasses import dataclass
from typing import List

@dataclass
class X:
    x: int

def get_min_1(vals: List[X]) -> X | None:
    return min(vals, key = lambda tsync: tsync.x, default=None)

def get_min_2(vals: List[X]) -> X | None:
    return min(vals, key = lambda tsync: tsync.x) # <- this is fine

vals = [X(1), X(2), X(3)]

result1 = get_min_1(vals)
result2 = get_min_2(vals)
result3 = min(vals, key = lambda tsync: tsync.x, default=None) # <- this is fine

Here, mypy complains only about get_min_1:

$ mypy a.py --pretty
a.py:11: error: Item "None" of "X | None" has no attribute "x"  [union-attr]
        return min(vals, key = lambda tsync: tsync.x, default=None)
                                             ^~~~~~~
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: none required for reproducting the bug
• Mypy configuration options from mypy.ini (and other config files): (no config)
• Python version used: 3.11