Currently, attrs will delete any non-None class attributes when _ClassBuilder._delete_attribs is True (i.e. when attributes are specified via class attributes, not directly via these field).
According to comment on _make.py:502, this is intended to remove attr.ib instances from the class.
But it does not play well with attributes declared using PEP 484 type annotations and auto_attribs=True argument to attr.s, because any attribute with non-None default value specified will be pruned from the class, while attributes with None value are retained.
This leads to unexpected behaviour in many cases, in particular when generating documentation using Sphinx autosummary extension which relies on class introspection.
I think we should remove only attr.ib instances, not any not-None fields. At least in auto_attribs mode. Or at least fix the comment.

Attrs version checked: 19.1.0
Code to reproduce:

import attr
import typing

@attr.s(auto_attribs=True)
class Foo:
    bar: int = 42
    baz: typing.Any = None

print([f for f in dir(Foo) if not f.startswith('_')])  # ['baz']
print(Foo.baz)  # 42
print(Foo.bar)  # AttributeError