gh-158490: Don't freeze a shared set in the frozenset() fast path

Lib/test/test_builtin.py

@@ -329,6 +329,29 @@ async def f_set():
                 with self.assertRaises(TypeError):
                     run_yielding_async_fn(f)
 
+    def test_frozenset_optimization_shared_set(self):
+        # gh-158490: frozenset({... for ...}) must not freeze a set that
+        # something else still references, here a trace function
+        kept = []
+        def tracer(frame, event, arg):
+            if event == 'return' and type(arg) is set:
+                kept.append(arg)
+            return tracer
+
+        class C:
+            x: frozenset({i for i in range(3)})
+
+        old_trace = sys.gettrace()
+        sys.settrace(tracer)
+        try:
+            fs = C.__annotations__['x']
+        finally:
+            sys.settrace(old_trace)
+        self.assertEqual(fs, frozenset({0, 1, 2}))
+        self.assertEqual(len(kept), 1)
+        self.assertIs(type(kept[0]), set)
+        self.assertIsNot(kept[0], fs)
+
     def test_ascii(self):
         self.assertEqual(ascii(''), '\'\'')
         self.assertEqual(ascii(0), '0')

Misc/NEWS.d/next/Core_and_Builtins/2026-11-03-17-53-21.gh-issue-158490.U63vs5.rst

@@ -0,0 +1,2 @@
+Fix :class:`frozenset` of a set comprehension in a class annotation or type
+alias freezing a set that a trace function or debugger still references.

Python/codegen.c

@@ -4029,6 +4029,20 @@ maybe_optimize_function_call(compiler *c, expr_ty e, jump_target_label end)
 
     if (_PyUnicode_EqualToASCIIString(func->v.Name.id, "frozenset")
         && (arg_expr->kind == Set_kind || arg_expr->kind == SetComp_kind)) {
+        if (arg_expr->kind == SetComp_kind) {
+            /* A comprehension that is not inlined returns its set from a
+               call, so a trace function may still hold a reference to it. */
+            PySTEntryObject *entry = _PySymtable_Lookup(SYMTABLE(c),
+                                                        (void *)arg_expr);
+            if (entry == NULL) {
+                return ERROR;
+            }
+            int is_inlined = (entry->ste_type == InlinedComprehensionBlock);
+            Py_DECREF(entry);
+            if (!is_inlined) {
+                return 0;
+            }
+        }
         NEW_JUMP_TARGET_LABEL(c, skip_optimization);
 
         ADDOP_I(c, loc, COPY, 1);