Add end-to-end test for effects of cells and __init_subclass__

src/attr/_compat.py

@@ -6,6 +6,7 @@
 
 
 PY2 = sys.version_info[0] == 2
+PY36PLUS = sys.version_info[0:2] >= (3, 6)
 PYPY = platform.python_implementation() == "PyPy"
 
 

tests/test_dark_magic.py

@@ -14,7 +14,7 @@
 
 import attr
 
-from attr._compat import TYPE
+from attr._compat import TYPE, PY36PLUS
 from attr._make import Attribute, NOTHING
 from attr.exceptions import FrozenInstanceError
 
@@ -374,3 +374,24 @@ class E(D):
             z = attr.ib(default=4)
 
         assert "E(c=100, b=23, a=42, x=2, d=3.14, y=3, z=4)" == repr(E())
+
+    @pytest.mark.skipif(
+        not PY36PLUS,
+        reason="__init__subclass__ requires Python 3.6+"
+    )
+    @pytest.mark.parametrize("slots", [True, False])
+    def test_init_subclass(self, slots):
+        """
+        super().__init_subclass__ can be called. This is problematic due to
+        certain cell intricancies around static and class methods.
+        """
+        @attr.s(slots=slots)
+        class Base:
+            def __init_subclass__(cls, param, **kw):
+                super().__init_subclass__(**kw)
+                cls.param = param
+
+        class Vanilla(Base, param="foo"):
+            pass
+
+        assert "foo" == Vanilla().param