builds fails when used with attrs classes containing private attributes

[AdrianSosic]

Hi, I think I may have found a bug that occurs when using builds with automatic attribute inference from attrs-built classes that have required but "private" attributes. The bug is probably due to the fact that the name exposed via the constructor differs from the attribute name in these cases, since attrs by default strips the leading underscore.

Here a minimal example:

from attrs import define
from hypothesis import given, strategies as st


@define
class MyClass:
    _x: int


@given(st.builds(MyClass))
def test_class(my_obj):
    assert isinstance(my_obj, MyClass)

The error you get:

AttributeError: 'MyClassAttributes' object has no attribute 'x'

[AdrianSosic]

Actually, the problem is not restricted to required arguments but also appears when explicitly requesting strategies for default argument via the ... syntax, since the same name matching machinery is used and fails.

To be concrete, if the class was instead

@define
class MyClass:
    _x: int = 0

then neither builds(MyClass, x=...) nor builds(MyClass, _x=...) would work.