Restore setting a Call as a base for classes using six.with_metaclass (#2049)

Harden support for using enums as metaclasses.

Fixes the crash in pylint-dev/pylint#5935 by adopting the check
for not-none bases as in ClassDef._inferred_bases without recausing
the false positive reported in pylint-dev/pylint#7506, which requires
correct bases.

Author: jacobtylerwalls

ChangeLog

@@ -12,6 +12,12 @@ What's New in astroid 2.15.1?
 =============================
 Release date: TBA
 
+* Restore behavior of setting a Call as a base for classes created using ``six.with_metaclass()``,
+  and harden support for using enums as metaclasses in this case.
+
+  Reverts #1622
+  Refs PyCQA/pylint#5935
+  Refs PyCQA/pylint#7506
 
 
 What's New in astroid 2.15.0?

astroid/brain/brain_six.py

@@ -219,7 +219,6 @@ def transform_six_with_metaclass(node):
     """
     call = node.bases[0]
     node._metaclass = call.args[0]
-    node.bases = call.args[1:]
     return node
 
 

astroid/nodes/scoped_nodes/scoped_nodes.py

@@ -1701,7 +1701,15 @@ def infer_call_result(self, caller=None, context: InferenceContext | None = None
             metaclass = next(caller.args[0].infer(context), None)
             if isinstance(metaclass, ClassDef):
                 try:
-                    class_bases = [next(arg.infer(context)) for arg in caller.args[1:]]
+                    class_bases = [
+                        # Find the first non-None inferred base value
+                        next(
+                            b
+                            for b in arg.infer(context=context.clone())
+                            if not (isinstance(b, Const) and b.value is None)
+                        )
+                        for arg in caller.args[1:]
+                    ]
                 except StopIteration as e:
                     raise InferenceError(node=caller.args[1:], context=context) from e
                 new_class = ClassDef(name="temporary_class")

tests/brain/test_six.py

@@ -110,8 +110,7 @@ class B(six.with_metaclass(A, C)):
         inferred = next(ast_node.infer())
         self.assertIsInstance(inferred, nodes.ClassDef)
         self.assertEqual(inferred.name, "B")
-        self.assertIsInstance(inferred.bases[0], nodes.Name)
-        self.assertEqual(inferred.bases[0].name, "C")
+        self.assertIsInstance(inferred.bases[0], nodes.Call)
         ancestors = tuple(inferred.ancestors())
         self.assertIsInstance(ancestors[0], nodes.ClassDef)
         self.assertEqual(ancestors[0].name, "C")
@@ -131,7 +130,7 @@ class Foo(six.with_metaclass(FooMeta, Enum)):  #@
             bar = 1
         """
         klass = astroid.extract_node(code)
-        assert list(klass.ancestors())[-1].name == "Enum"
+        assert next(klass.ancestors()).name == "Enum"
 
     def test_six_with_metaclass_with_additional_transform(self) -> None:
         def transform_class(cls: Any) -> ClassDef:

tests/test_scoped_nodes.py

@@ -1403,6 +1403,20 @@ class WithMeta(six.with_metaclass(type, object)): #@
         self.assertEqual(["object"], [base.name for base in klass.ancestors()])
         self.assertEqual("type", klass.metaclass().name)
 
+    @unittest.skipUnless(HAS_SIX, "These tests require the six library")
+    def test_metaclass_generator_hack_enum_base(self):
+        """Regression test for https://github.com/PyCQA/pylint/issues/5935"""
+        klass = builder.extract_node(
+            """
+            import six
+            from enum import Enum, EnumMeta
+
+            class PetEnumPy2Metaclass(six.with_metaclass(EnumMeta, Enum)): #@
+                DOG = "dog"
+        """
+        )
+        self.assertEqual(list(klass.local_attr_ancestors("DOG")), [])
+
     def test_add_metaclass(self) -> None:
         klass = builder.extract_node(
             """