Don't allow keep_alive or call_guard on properties (#5533)

* 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?

* style: pre-commit fixes

* Re-run tests

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: gentlegiantJGC

include/pybind11/attr.h

@@ -702,6 +702,12 @@ 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>;
 

include/pybind11/pybind11.h

@@ -2430,6 +2430,12 @@ class class_ : public detail::generic_type {
                                 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) {