From 5657ab11de67d6a8eb6a25e2d869b0fd343eda0d Mon Sep 17 00:00:00 2001
From: gentlegiantJGC <gentlegiantJGC@users.noreply.github.com>
Date: Tue, 18 Feb 2025 12:27:03 +0000
Subject: [PATCH] Don't allow keep_alive or call_guard on properties

The def_property family blindly ignore the keep_alive and call_guard arguments passed to them making them confusing to use.
This adds a static_assert if either is passed to make it clear it doesn't work.
I would prefer this to be a compiler warning but I can't find a way to do that. Is that even possible?
---
 include/pybind11/attr.h     | 6 ++++++
 include/pybind11/pybind11.h | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/include/pybind11/attr.h b/include/pybind11/attr.h
index 1044db94d..742a2fc40 100644
--- a/include/pybind11/attr.h
+++ b/include/pybind11/attr.h
@@ -670,6 +670,12 @@ struct process_attributes {
     }
 };
 
+template <typename T>
+struct is_keep_alive : std::false_type { };
+
+template <size_t Nurse, size_t Patient>
+struct is_keep_alive<keep_alive<Nurse, Patient>> : std::true_type { };
+
 template <typename T>
 using is_call_guard = is_instantiation<call_guard, T>;
 
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 4387c2754..e2dddb9f0 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1849,6 +1849,10 @@ public:
                                 const Extra &...extra) {
         static_assert(0 == detail::constexpr_sum(std::is_base_of<arg, Extra>::value...),
                       "Argument annotations are not allowed for properties");
+        static_assert(0 == detail::constexpr_sum(detail::is_call_guard<Extra>::value...),
+                      "def_property family does not currently support call_guard. Use a py::cpp_function instead.");
+        static_assert(0 == detail::constexpr_sum(detail::is_keep_alive<Extra>::value...),
+                      "def_property family does not currently support keep_alive. Use a py::cpp_function instead.");
         auto rec_fget = get_function_record(fget), rec_fset = get_function_record(fset);
         auto *rec_active = rec_fget;
         if (rec_fget) {