fix: use PyUnstable_IsImmortal

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

Author: henryiii

tests/pybind11_tests.cpp

@@ -75,6 +75,15 @@ const char *cpp_std() {
 #endif
 }
 
+bool is_immortal(py::handle object) {
+    // If Python doesn't support immortal objects, this returns False
+#if PY_VERSION_HEX >= 0x030E0000
+    return PyUnstable_IsImmortal(object.ptr()) != 0;
+#else
+    return Py_REFCNT(object.ptr()) == UINT32_MAX;
+#endif
+}
+
 PYBIND11_MODULE(pybind11_tests, m, py::mod_gil_not_used()) {
     m.doc() = "pybind11 test module";
 
@@ -90,7 +99,6 @@ PYBIND11_MODULE(pybind11_tests, m, py::mod_gil_not_used()) {
     m.attr("cpp_std") = cpp_std();
     m.attr("PYBIND11_INTERNALS_ID") = PYBIND11_INTERNALS_ID;
     // Free threaded Python uses UINT32_MAX for immortal objects.
-    m.attr("PYBIND11_REFCNT_IMMORTAL") = UINT32_MAX;
     m.attr("PYBIND11_SIMPLE_GIL_MANAGEMENT") =
 #if defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
         true;
@@ -106,6 +114,8 @@ PYBIND11_MODULE(pybind11_tests, m, py::mod_gil_not_used()) {
     m.attr("detailed_error_messages_enabled") = false;
 #endif
 
+    m.def("is_immortal", &is_immortal, "Returns true if Python supports it and the object is immortal");
+
     py::class_<UserType>(m, "UserType", "A `py::class_` type for testing")
         .def(py::init<>())
         .def(py::init<int>())

tests/test_class.py

@@ -6,7 +6,7 @@
 import pytest
 
 import env
-from pybind11_tests import PYBIND11_REFCNT_IMMORTAL, ConstructorStats, UserType
+from pybind11_tests import is_immortal, ConstructorStats, UserType
 from pybind11_tests import class_ as m
 
 
@@ -398,7 +398,7 @@ class PyDog(m.Dog):
 
         assert refcount_1 == refcount_3
         assert (refcount_2 > refcount_1) or (
-            refcount_2 == refcount_1 == PYBIND11_REFCNT_IMMORTAL
+            is_immortal(refcount_2) and is_immortal(refcount_1)
         )