Broken usage of pybind11 "keep_alive" annotation

[StefanBruens]

What version of OR-Tools and what language are you using?
Version: main/v9.15
Language: Python

Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)
n/a

What operating system (Linux, Windows, ...) and version?

What did you do?
Compile it

What did you expect to see
no errors

What did you see instead?

[  762s]  2870 |     .def_property_readonly(
[  762s]       |     ~~~~~~~~~~~~~~~~~~~~~~^
[  762s]  2871 |         "tails",
[  762s]       |         ~~~~~~~~           
[  762s]  2872 |         [](operations_research::sat::CircuitConstraintProto* self) { return self->mutable_tails(); },
[  762s]       |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[  762s]  2873 |         py::return_value_policy::reference, py::keep_alive<0, 1>())
[  762s]       |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[  762s] /usr/include/pybind11/pybind11.h:2659:25: error: static assertion failed: def_property family does not currently support keep_alive. Use a py::cpp_function instead.

While this error message is specific to current pybind11 versions, the actual error is inside the or-tools code, and the generated code is also wrong with the bundled pybind11.

For details, see pybind/pybind11#5533

[StefanBruens]

@Mizux - this is likely the reason a current pybind11 does not work.

[cielavenir]

@Mizux I saw bdcf172 .

Is reference_internal safe to replace py::keep_alive<1, 0>, not py::keep_alive<0, 1>?