Fix __post_init__() internal error

[ikonst]

Fixes #16057.

[ikonst]

👋 @sobolevn as author of #15503

[AlexWaygood]

Maybe add a test?

[ikonst]

This comment was added in 2018 in #4438.

FakeInfo was added in 2017, and the following comment seems to be at odds:

mypy/mypy/nodes.py

Lines 3366 to 3370 in fdc7914

	# 'None' is not used as a dummy value for two reasons:
	# 1. This will require around 80-100 asserts to make 'mypy --strict-optional mypy'
	# pass cleanly.
	# 2. If NOT_READY value is accidentally used somewhere, it will be obvious where the value
	# is from, whereas a 'None' value could come from anywhere.

[ikonst]

btw, repro is even simpler 🤦

def __post_init__() -> None:
    pass

[github-actions]

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

[sobolevn]

Thanks!