Fix crash on inference of __len__

ChangeLog

@@ -12,6 +12,10 @@ What's New in astroid 2.8.5?
 ============================
 Release date: TBA
 
+* Fix crash on inference of ``__len__``
+
+  Closes PyCQA/pylint#5244
+
 * Added missing ``kind`` (for ``Const``) and ``conversion`` (for ``FormattedValue``) fields to repr.
 
 

astroid/helpers.py

@@ -258,7 +258,7 @@ def object_len(node, context=None):
     if (
         isinstance(node_frame, scoped_nodes.FunctionDef)
         and node_frame.name == "__len__"
-        and inferred_node is not None
+        and hasattr(inferred_node, "_proxied")
         and inferred_node._proxied == node_frame.parent
     ):
         message = (

tests/unittest_brain.py

@@ -3126,5 +3126,50 @@ def __len__(self) -> int:
         node.inferred()
 
 
+def test_inference_on_outer_referential_length_check() -> None:
+    """
+    Regression test for https://github.com/PyCQA/pylint/issues/5244
+    """
+    node = astroid.extract_node(
+        """
+    class A:
+        def __len__(self) -> int:
+            return 42
+
+    class Crash:
+        def __len__(self) -> int:
+            a = A()
+            return len(a)
+
+    len(Crash()) #@
+    """
+    )
+    inferred = node.inferred()
+    assert len(inferred) == 1
+    assert isinstance(inferred[0], nodes.Const)
+    assert inferred[0].value == 42
+
+
+def test_no_attributeerror_on_self_referential_length_check() -> None:
+    """
+    Regression test for https://github.com/PyCQA/pylint/issues/5244
+    """
+    with pytest.raises(InferenceError):
+        node = astroid.extract_node(
+            """
+        class MyClass:
+            def some_func(self):
+                return lambda: 42
+
+            def __len__(self):
+                return len(self.some_func())
+
+        len(MyClass()) #@
+        """
+        )
+        assert isinstance(node, nodes.NodeNG)
+        node.inferred()
+
+
 if __name__ == "__main__":
     unittest.main()