[BUG]: Returning a reference to an element within a vector will be invalidated.

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What version (or hash if on master) of pybind11 are you using?

2.6.2

Problem description

Lets say I have a Camera class and a vector of Cameras. I bind the vector and I bind the Camera class. Everything is working fine until the user wants to reference a single element within the vector rather than taking a reference to the vector as a whole. Well since the heap memory within a std::vector can change/move depending on whether it shrinks or grows, the reference(or pointer) to that single element will be completely invalidated (specifically we're encountering a "heap-use-after-free" error from AddressSanitizer). Does pybind provide a way to avoid this? I've been searching through the forums, docs and issues tab on github but they seem to only talk about binding stl containers as a whole rather than keeping track of a single element within a container. I don't see anything within the return value policies that address this particular use case either. Any advice would be great. Thanks :)

Reproducible example code

// assume Camera and vector<Camera> is opaquely binded.

// c++ code
class CameraContainer{
public:
// ...
// various other non related methods


Camera& add(const int id) {
    cameras.emplace_back(Camera(id));
    return cameras.back();
}
private:
std::vector<Camera> cameras
};

py::class_<CameraContainer>(m)
.def(
          "add",
          &CameraContainer::add,
          py::arg("id") = -1,
          py::return_value_policy::reference_internal); // this is the problem

// python code
// add one camera at a time and return each camera individually.
cameras = [add_camera(id) for id in range(num_views_per_frame)]

Is this a regression? Put the last known working version here if it is.

Not a regression

Start with the CameraContainer::add example and its reference_internal return policy, then reproduce the list-comprehension case with AddressSanitizer as described. Read the pybind11 documentation and issue discussion referenced in the report to determine whether element lifetime can be supported; done means the invalid reference behavior is resolved or the supported limitation is clearly established.