Summary

Fixes #13829.

Replacing an instance method definition with an operator-module function doesn't work, because instance methods need to be callable objects with __get__ methods ("descriptors"), and while user-defined functions are descriptors, this is not true for the operator-module functions. We already avoid flagging user-defined methods that are defined using def statements, but we currently incorrectly flag user-defined functions that are defined by assigning lambdas to variables in class bodies. I.e., this is a perfectly valid definition of an instance method, and FURB118's suggestion to replace the lambda with operator.eq function would break it:

class Spam:
    foo = lambda self, other: self == other

As a demonstration in the REPL:

>>> class Spam:
...     foo = lambda self, other: self == other
...     
>>> Spam().foo(Spam())
False
>>> from operator import eq
>>> class Spam:
...     foo = eq
...     
>>> Spam().foo(Spam())
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    Spam().foo(Spam())
    ~~~~~~~~~~^^^^^^^^
TypeError: eq expected 2 arguments, got 1

This PR fixes things so that lambda assignments in class bodies are considered off-limits for the rule in the same way as def statements in class bodies. It also makes the fix-safety docs more expansive, so that they're clearer that there's a wider range of reasons why the rule might be unsafe than just the keyword-argument issue.

Test Plan

I added a new fixture that confirms that all function definitions in class bodies are ignored by the rule.