gh-161849: Fix crash compiling a class that binds __classdict__

Lib/test/test_type_annotations.py

@@ -505,6 +505,32 @@ class C:
         """)
         self.assertEqual(ns["C"].__annotations__, {"__classdict__": int})
 
+    def test_class_binds_dunder_classdict(self):
+        # The class body binding __classdict__ must not hide the cell that
+        # annotation and type parameter scopes use to see the class namespace.
+        for binding in (
+            "__classdict__ = 1",
+            "__classdict__: str = 1",
+            "for __classdict__ in (1,): pass",
+            "global __classdict__",
+        ):
+            with self.subTest(binding=binding):
+                ns = run_code(f"""
+                    class C:
+                        {binding}
+                        y = int
+                        def f(x: y): pass
+                        class D[T: y]: pass
+                        class E[T]: pass
+                        type A[T: y] = y
+                """)
+                C = ns["C"]
+                self.assertEqual(C.f.__annotations__, {"x": int})
+                self.assertIs(C.D.__type_params__[0].__bound__, int)
+                self.assertEqual(C.E.__type_params__[0].__name__, "T")
+                self.assertIs(C.A.__type_params__[0].__bound__, int)
+                self.assertIs(C.A.__value__, int)
+
     def test_future_annotations(self):
         code = """
         from __future__ import annotations

Misc/NEWS.d/next/Core_and_Builtins/2026-11-13-08-43-30.gh-issue-161849.m5BmNR.rst

@@ -0,0 +1,4 @@
+Fix a crash and a :exc:`SystemError` when compiling a class whose body binds
+``__classdict__`` (for example ``__classdict__ = 1`` or ``global
+__classdict__``) and also contains an annotated function, a generic class or
+function, or a generic type alias.

Python/symtable.c

@@ -753,7 +753,9 @@ analyze_name(PySTEntryObject *ste, PyObject *scopes, PyObject *name, long flags,
     // namespace and the globals (not the namespace providing the bound).
     // Similarly, if the name is explicitly global in the class namespace (through the
     // global statement), we want to also treat it as a global in this scope.
-    if (class_entry != NULL) {
+    // __classdict__ is the exception: these scopes load it from the cell that
+    // the class creates for them, whatever the class body does with that name.
+    if (class_entry != NULL && !_PyUnicode_Equal(name, &_Py_ID(__classdict__))) {
         long class_flags = _PyST_GetSymbol(class_entry, name);
         if (class_flags < 0) {
             return 0;