Unexpected comparison-overlap error for identity comparison of optionals

[cebtenzzre]

Bug Report

mypy generates "error: Non-overlapping identity check" for identity comparison between optionals, even though None has the same identity as None. This is especially confusing for the x is y is None pattern, which I would expect to be valid as long as x and y are optional.

To Reproduce

def foo(x: int | None, y: str | None) -> None:
    if x is y:
        print('x and y have same identity!')

foo(None, None)

Actual Behavior

$ python repr.py
x and y have same identity!

$ mypy --strict repr.py
repr.py:2: error: Non-overlapping identity check (left operand type: "Optional[int]", right operand type: "Optional[str]")  [comparison-overlap]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10.8

[chnmasta05]

It was decided when --strict-equality was implemented that (according to #6370) "Optional[X] and Optional[Y] are non-overlapping if X and Y are non-overlapping".

In my opinion, the documented reason "although technically None is overlap, it is most likely an error" is not very convincing, and we would all benefit from the change if the corner case was removed.

I'm not sure if I understand correctly, but this restriction was originally added to account for type promotions? Now that we don't ignore promotions anymore (#6674), we really should change this.