Hashable and None have overlapping type in mypy >= 0.981

[headtr1ck]

Bug Report

mypy 0.981 and 0.982 consider Hashable and None to be overlapping.
I know that technically it is correct, since None is indeed hashable, but the way that None is used in python for default values makes this overlap inconvenient.

I could not find any hint in the changelog that indicates that this has changed.

To Reproduce

from typing import Hashable

@overload
def x(a: Hashable) -> Hashable: ...

@overload
def x(a: None = None) -> None: ...

def x(a: Hashable | None = None) -> Hashable | None:
    return a

Expected Behavior

Like mypy <= 0.971 no issue here.

Actual Behavior

error: Overloaded function signatures 1 and 2 overlap with incompatible return types [misc]

Other

In case the new behavior is intended, is there a way to specify a "hashable but not None" type?

Your Environment

• Mypy version used: 0.981 (and 0.982, 0.971 to compare)
• Mypy command-line flags: /
• Mypy configuration options from mypy.ini (and other config files): show_error_codes = True
• Python version used: 3.9.7

[hauntsaninja]

Side effect of #13303.

Like you say, these overloads do overlap, and in fact you likely want to switch the order of them (and then type ignore the overlap lint, mypy will still understand what's going on).

For more type system features, maybe follow python/typing#801. But Hashable is just a tricky type.