Make None compatible with SupportsBool protocol

[blablatdinov]

None had special case for __str__ and __hash__, but it can be extened for __bool__ as well for supporting next code:

from typing import Protocol

class SupportsBool(Protocol):
    def __bool__(self) -> bool:
        ...
        
x: SupportsBool = None

>>> bool(None)
False

https://mypy-play.net/?mypy=latest&python=3.11&gist=42d8731eb61fd5b1e9150b693375015f

[TeamSpen210]

Does this check the return type? None should probably be incompatible with -> Literal[True], but compatible with bool or Literal[False].

[blablatdinov]

@TeamSpen210 Hi! No, this changes only add supports "Boolable" protocols for None

[TeamSpen210]

I mean if you create a __bool__ protocol with a literal return type, that would be useful to distinguish between types that are always falsey. None shouldn’t match def __bool__(self) -> Literal[True]:.

[blablatdinov]

__bool__ method not support Literal[False] return type.

note:     Expected:
note:         def __bool__(self) -> Literal[False]
note:     Got:
note:         def __bool__(self) -> bool

[sobolevn]

Please, add two more protocols to the test: def __bool__(self) -> Literal[True]: ... and def __bool__(self) -> Literal[False]: ...

Other than that - it looks good to me.

[blablatdinov]

Thanks for review! I add tests with Literal return types, take a look, please

[sobolevn]

I am not sure that this is correct. https://github.com/python/typeshed/blob/ef758b66c0947a73f6225cac0568f3f068cd61d4/stdlib/types.pyi#L627-L629

[blablatdinov]

Yes, you're right, it's not correct behavior, but tests passed. How I can fix it? Maybe we have other bug?

[sobolevn]

None is quite complicated, because mypy uses pre-types.NoneType logic for None.
Check out checkmember and NoneType handling there.

[blablatdinov]

I'm implemented check into visit_none_type method, because __bool__ of Protocol not handled by analyze_none_member_access

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sobolevn]

I think that instead we should try changing None to have def __bool__(self) -> Literal[False] signature.

[blablatdinov]

You mean the mypy.types.NoneType?

[sobolevn]

Yes, the member __bool__ of NoneType