gh-154902: Add an intrinsic for conditional annotations

Author: deadlovelll

Include/internal/pycore_intrinsics.h

@@ -30,8 +30,9 @@
 #define INTRINSIC_TYPEVAR_WITH_CONSTRAINTS       3
 #define INTRINSIC_SET_FUNCTION_TYPE_PARAMS       4
 #define INTRINSIC_SET_TYPEPARAM_DEFAULT          5
+#define INTRINSIC_ADD_CONDITIONAL_ANNOTATION     6
 
-#define MAX_INTRINSIC_2                          5
+#define MAX_INTRINSIC_2                          6
 
 typedef PyObject *(*intrinsic_func1)(PyThreadState* tstate, PyObject *value);
 typedef PyObject *(*intrinsic_func2)(PyThreadState* tstate, PyObject *value1, PyObject *value2);

Lib/test/test_dis.py

@@ -395,12 +395,12 @@ def wrap_func_w_kwargs():
                STORE_NAME               1 (x)
                LOAD_NAME                0 (__conditional_annotations__)
                LOAD_SMALL_INT           0
-               SET_ADD                  1
+               CALL_INTRINSIC_2         6 (INTRINSIC_ADD_CONDITIONAL_ANNOTATION)
                POP_TOP
 
    3           LOAD_NAME                0 (__conditional_annotations__)
                LOAD_SMALL_INT           1
-               SET_ADD                  1
+               CALL_INTRINSIC_2         6 (INTRINSIC_ADD_CONDITIONAL_ANNOTATION)
                POP_TOP
 
    4           LOAD_SMALL_INT           1

Lib/test/test_type_annotations.py

@@ -896,3 +896,34 @@ class Generic:
                 mod = build_module(code)
                 annos = mod.__annotations__
                 self.assertEqual(annos, {"annotated_name": 0})
+
+    # gh-154902
+    def test_conditional_annotations_rebound(self):
+        # user code can rebind __conditional_annotations__ to any object
+        for value in ("0", "{}", "[]", "''", "object()"):
+            with self.subTest(value=value):
+                code = f"""
+                    __conditional_annotations__ = {value}
+                    x: int
+                """
+                with self.assertRaises(TypeError):
+                    run_code(code)
+
+    # gh-154902
+    def test_conditional_annotations_rebound_via_globals(self):
+        code = """
+            globals()["__conditional_annotations__"] = 0
+            x: int
+        """
+        with self.assertRaises(TypeError):
+            run_code(code)
+
+    # gh-154902
+    def test_conditional_annotations_rebound_to_frozenset(self):
+        marker = frozenset()
+        code = """
+            __conditional_annotations__ = marker
+            x: int
+        """
+        with self.assertRaises(TypeError):
+            run_code(code, {"marker": marker})

Misc/NEWS.d/next/Core_and_Builtins/2026-08-01-15-07-10.gh-issue-154902.DIi6sf.rst

@@ -0,0 +1,2 @@
+Fix a crash when ``__conditional_annotations__`` is rebound to a non-set
+object.

Python/codegen.c

@@ -5807,7 +5807,9 @@ codegen_annassign(compiler *c, stmt_ty s)
                         ADDOP_NAME(c, loc, LOAD_NAME, &_Py_ID(__conditional_annotations__), names);
                     }
                     ADDOP_LOAD_CONST_NEW(c, loc, conditional_annotation_index);
-                    ADDOP_I(c, loc, SET_ADD, 1);
+                    // gh-154902: change SET_ADD to new INTRINSIC_ADD_CONDITIONAL_ANNOTATION intrinsic
+                    ADDOP_I(c, loc, CALL_INTRINSIC_2,
+                            INTRINSIC_ADD_CONDITIONAL_ANNOTATION);
                     ADDOP(c, loc, POP_TOP);
                 }
             }

Python/intrinsics.c

@@ -270,6 +270,23 @@ make_typevar_with_constraints(PyThreadState* Py_UNUSED(ignored), PyObject *name,
     return _Py_make_typevar(name, NULL, evaluate_constraints);
 }
 
+static PyObject *
+add_conditional_annotation(PyThreadState* tstate, PyObject *conditional_annotations,
+                           PyObject *index)
+{
+    // gh-154902: user code can rebind __conditional_annotations__ to any object
+    if (!PySet_Check(conditional_annotations)) {
+        _PyErr_Format(tstate, PyExc_TypeError,
+                      "__conditional_annotations__ must be a set, not %T",
+                      conditional_annotations);
+        return NULL;
+    }
+    if (PySet_Add(conditional_annotations, index) < 0) {
+        return NULL;
+    }
+    Py_RETURN_NONE;
+}
+
 const intrinsic_func2_info
 _PyIntrinsics_BinaryFunctions[] = {
     INTRINSIC_FUNC_ENTRY(INTRINSIC_2_INVALID, no_intrinsic2)
@@ -278,6 +295,7 @@ _PyIntrinsics_BinaryFunctions[] = {
     INTRINSIC_FUNC_ENTRY(INTRINSIC_TYPEVAR_WITH_CONSTRAINTS, make_typevar_with_constraints)
     INTRINSIC_FUNC_ENTRY(INTRINSIC_SET_FUNCTION_TYPE_PARAMS, _Py_set_function_type_params)
     INTRINSIC_FUNC_ENTRY(INTRINSIC_SET_TYPEPARAM_DEFAULT, _Py_set_typeparam_default)
+    INTRINSIC_FUNC_ENTRY(INTRINSIC_ADD_CONDITIONAL_ANNOTATION, add_conditional_annotation)
 };
 
 #undef INTRINSIC_FUNC_ENTRY