Ensure that bare attributes with default None are removed too

Fixes #523

src/attr/_make.py

@@ -42,6 +42,9 @@
 
 _empty_metadata_singleton = metadata_proxy({})
 
+# Unique object for unequivocal getattr() defaults.
+_sentinel = object()
+
 
 class _Nothing(object):
     """
@@ -504,7 +507,7 @@ def _patch_original_class(self):
             for name in self._attr_names:
                 if (
                     name not in base_names
-                    and getattr(cls, name, None) is not None
+                    and getattr(cls, name, _sentinel) != _sentinel
                 ):
                     try:
                         delattr(cls, name)

tests/test_annotations.py

@@ -265,3 +265,20 @@ class C(Base):
             x: int
 
         assert 1 == C(1).x
+
+    def test_removes_none_too(self):
+        """
+        Regression test for #523: make sure defaults that are set to None are
+        removed too.
+        """
+
+        @attr.s(auto_attribs=True)
+        class C:
+            x: int = 42
+            y: typing.Any = None
+
+        with pytest.raises(AttributeError):
+            C.x
+
+        with pytest.raises(AttributeError):
+            C.y