Don't allow keep_alive or call_guard on properties

[gentlegiantJGC]

Description

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?

This does not fix - #4236 or #5046 but makes it clear that they don't work.

Workaround

If your property returns a reference, pybind11 will automatically link the lifespan of self to the return value.
See return_value_policy::reference_internal doc for more info.

.def_property("foo", &Obj::getFoo, &Obj::setFoo)

.def_readwrite("foo", &Obj::foo)

If you need a custom keep_alive or call_guard these can be passed into py::cpp_function

.def_property(
    "foo", 
    py::cpp_function(&Obj::getFoo, py::call_guard<py::gil_scoped_release>()),
    py::cpp_function(&Obj::setFoo, py::call_guard<py::gil_scoped_release>()))

Suggested changelog entry:

Stop keep_alive and call_guard being passed to def_property functions.

[rwgk]

(random timing, I'm cleaning up my email backlog)

Looks good, thanks! — 'll get back here a few days after #5542 is merged, but before the v3.0.0 release.

[gentlegiantJGC]

Looks like this got missed

[rwgk]

Oh, sorry. Could you please update this branch and tag me if you see that the CI passed?

[gentlegiantJGC]

I think the android runner is broken. This is also an issue in #5896

[rwgk]

Ignoring, after two unsuccessful reruns:

CIBW / Android wheel ubuntu-latest (pull_request) (pull_request)Failing after 10m

[ax3l]

@gentlegiantJGC Thank you so much for the guard rails! Just to check the update path for this: should we use a syntax with py::keep_alive to getters, like this?

[gentlegiantJGC]

@gentlegiantJGC Thank you so much for the guard rails! Just to check the update path for this: should we use a syntax with py::keep_alive to getters, like this?

def_property and family still automatically apply return_value_policy::reference_internal to link the lifespan of self to the return value of the getter.
If you need to use py::keep_alive in another way you can pass it into the py::cpp_function constructor. The first is the getter and the second is the setter. The same applies for call_guard.

The only cases this change should break are cases where keep_alive and call_guard were passed into def_property and family. Before, pybind11 just silently ignored these arguments. Now it asserts at compile time to warn the programmer that it does not work.