[BUG]: `py::cast()` from `std::shared_ptr` throws with custom holder

### Required prerequisites - [x] Make sure you've read the [documentation](https://pybind11.readthedocs.io). Your issue may be addressed there. - [x] Search the [issue tracker](https://github.com/pybind/pybind11/issues) and [Discussions](https:/pybind/pybind11/discussions) to verify that this hasn't already been reported. +1 or comment there if it has. - [ ] Consider asking first in the [Gitter chat room](https://gitter.im/pybind/Lobby) or in a [Discussion](https:/pybind/pybind11/discussions/new). ### What version (or hash if on master) of pybind11 are you using? 3.0.4 ### Problem description The *libcamera* project provides python bindings via pybind11. One particular type has a private destructor and is managed via `std::shared_ptr`. It has a custom holder to work around the need for a public destructor. This has worked so far, but now it does work not with pybind11 3.0.4. Specifically, calling `py::cast()` on an `std::shared_ptr` of that type in question throws: ``` >>> import libcamera >>> cm = libcamera.CameraManager.singleton() >>> cm.cameras RuntimeError: Unable to convert std::shared_ptr<T> to Python when the bound type does not use std::shared_ptr or py::smart_holder as its holder type ``` As far as I can see this has been added by #6008. So I'm wondering, is this exception diagnosing a real issue, or the above PR missed some cases? It seems to work with `py::smart_holder`, but this code predates that, so it wasn't used. <details> <summary>holder type</summary> ```cpp template<typename T> class PyCameraSmartPtr { public: using element_type = T; PyCameraSmartPtr() { } explicit PyCameraSmartPtr(T *) { throw std::runtime_error("invalid SmartPtr constructor call"); } explicit PyCameraSmartPtr(std::shared_ptr<T> p) : ptr_(std::move(p)) { } T *get() const { return ptr_.get(); } operator std::shared_ptr<T>() const { return ptr_; } private: std::shared_ptr<T> ptr_; }; PYBIND11_DECLARE_HOLDER_TYPE(T, PyCameraSmartPtr<T>) // ... auto pyCamera = py::class_<Camera, PyCameraSmartPtr<Camera>>(m, "Camera"); ``` </details> ### Reproducible example code ```text ``` ### Is this a regression? Put the last known working version here if it is. 3.0.2 (?)