gh-117266: Fix crashes on user-created AST subclasses (GH-117276)

Fix crashes on user-created AST subclasses

Lib/test/test_ast.py

@@ -2916,6 +2916,47 @@ class ASTConstructorTests(unittest.TestCase):
         self.assertEqual(node.name, 'foo')
         self.assertEqual(node.decorator_list, [])
 
+    def test_custom_subclass(self):
+        class NoInit(ast.AST):
+            pass
+
+        obj = NoInit()
+        self.assertIsInstance(obj, NoInit)
+        self.assertEqual(obj.__dict__, {})
+
+        class Fields(ast.AST):
+            _fields = ('a',)
+
+        with self.assertWarnsRegex(DeprecationWarning,
+                                   r"Fields provides _fields but not _field_types."):
+            obj = Fields()
+        with self.assertRaises(AttributeError):
+            obj.a
+        obj = Fields(a=1)
+        self.assertEqual(obj.a, 1)
+
+        class FieldsAndTypes(ast.AST):
+            _fields = ('a',)
+            _field_types = {'a': int | None}
+            a: int | None = None
+
+        obj = FieldsAndTypes()
+        self.assertIs(obj.a, None)
+        obj = FieldsAndTypes(a=1)
+        self.assertEqual(obj.a, 1)
+
+        class FieldsAndTypesNoDefault(ast.AST):
+            _fields = ('a',)
+            _field_types = {'a': int}
+
+        with self.assertWarnsRegex(DeprecationWarning,
+                                   r"FieldsAndTypesNoDefault\.__init__ missing 1 required positional argument: 'a'\."):
+            obj = FieldsAndTypesNoDefault()
+        with self.assertRaises(AttributeError):
+            obj.a
+        obj = FieldsAndTypesNoDefault(a=1)
+        self.assertEqual(obj.a, 1)
+
 
 @support.cpython_only
 class ModuleStateTests(unittest.TestCase):