Add InferenceContext to ClassModel (#1257)

ChangeLog

@@ -12,6 +12,10 @@ What's New in astroid 2.8.6?
 ============================
 Release date: TBA
 
+* Fix crash on inference of subclasses created from ``Class().__subclasses__``
+
+  Closes PyCQA/pylint#4982
+
 * Fix bug with Python 3.7.0 / 3.7.1 and ``typing.NoReturn``.
 
   Closes #1239

astroid/interpreter/objectmodel.py

@@ -403,6 +403,12 @@ def attr___ne__(self):
 
 
 class ClassModel(ObjectModel):
+    def __init__(self):
+        # Add a context so that inferences called from an instance don't recurse endlessly
+        self.context = InferenceContext()
+
+        super().__init__()
+
     @property
     def attr___module__(self):
         return node_classes.Const(self._instance.root().qname())
@@ -485,7 +491,7 @@ def attr___subclasses__(self):
         classes = [
             cls
             for cls in root.nodes_of_class(scoped_nodes.ClassDef)
-            if cls != self._instance and cls.is_subtype_of(qname)
+            if cls != self._instance and cls.is_subtype_of(qname, context=self.context)
         ]
 
         obj = node_classes.List(parent=self._instance)

tests/unittest_regrtest.py

@@ -24,7 +24,7 @@
 
 import pytest
 
-from astroid import MANAGER, Instance, nodes, test_utils
+from astroid import MANAGER, Instance, nodes, parse, test_utils
 from astroid.builder import AstroidBuilder, extract_node
 from astroid.const import PY38_PLUS
 from astroid.exceptions import InferenceError
@@ -379,5 +379,25 @@ def fu(self, objects):
     assert inferred.qname() == "builtins.dict.__delitem__"
 
 
+def test_regression_crash_classmethod() -> None:
+    """Regression test for a crash reported in https://github.com/PyCQA/pylint/issues/4982"""
+    code = """
+    class Base:
+        @classmethod
+        def get_first_subclass(cls):
+            for subclass in cls.__subclasses__():
+                return subclass
+            return object
+
+
+    subclass = Base.get_first_subclass()
+
+
+    class Another(subclass):
+        pass
+    """
+    parse(code)
+
+
 if __name__ == "__main__":
     unittest.main()