Add truthy-callable-bool check

[ikonst]

Feature

Using a method as a property is a common bug that can be detected by a type checker:

if my_obj.my_method:
   ...

I'd like to introduce a way to enable a check like truthy-bool (#10666) but limited to callable types.

Pitch

Users users might be hesitant to enable truthy-bool due to false positives. For callable types, there's a much higher degree of confidence that evaluating a callable as a boolean is unintentional.

• The error message should be specific, e.g.

  - error: Member "my_method" has type "function" which does not implement __bool__ or __len__ so it could always be true in boolean context 
  + error: Member "my_method" is a method (are you trying to use it as a property?)

  (happy to wordsmith this more, only trying to illustrate that I want it to be friendlier and more actionable)
• The feature could be a separate error-code (thus splitting "callable evaluated in boolean context" from truthy-bool), or perhaps a way to configure truthy-bool to have a reduced scope? (Is there a common idiom for that?)

[ikonst]

@hauntsaninja what do you think?

[erictraut]

FWIW, I recently added this check to pyright, and it was well received. This is a common programming error, and it can be detected with a negligible false positive rate. It would be great if mypy could detect and report this error.

[hauntsaninja]

I'm in favour, the reduced scope means we could likely have it on by default.