Weird behavior with sending derived-in-Python classes back to C++

[ZealanL]

I am working on a bindings project and encountered some bizarre behavior when deriving a C++ class in Python, and then sending that Python class back to C++. I'm not sure if this is a bug with pybind11, a limitation with Python, or me doing something wrong.

Here is a minimal example that reproduces my problem:

namespace pyb = pybind11;

// Inheritable base class A
class A {
public:
	virtual void say() {
		std::cout << "say() called from A" << std::endl;
	}
};
class PyA : public A {
public:
	void say() override { PYBIND11_OVERRIDE(void, A, say); }
};
pyb::class_<A, PyA, std::shared_ptr<A>>(m, "A")
	.def(pyb::init<>());

// Container that holds an A reference
struct AHolder {
	std::shared_ptr<A> aRef = NULL;
};
pyb::class_<AHolder>(m, "AHolder")
	.def(pyb::init<>())
	.def_readwrite("a_ref", &AHolder::aRef);

// C++ function that use an AHolder
m.def("use_holder",
	[](const AHolder& holder) {
		holder.aRef->say();
	}
);

Then, in python:

class B(A):
	def say(self):
		print("say() called from B")

holder = AHolder()
holder.a_ref = B()
use_holder(holder)

This outputs say() called from A, meaning the derived class was lost in the transfer back to C++.

However, if I edit the Python code to assign B() to a variable first:

holder = AHolder()
-holder.a_ref = B()
+b = B()
+holder.a_ref = b
use_holder(holder)

Then, it correctly outputs say() called from B.
My guess is that my B instance is losing its connection to pybind when constructed and assigned to the C++ class, but will maintain that connection when assigned separately as a variable (meaning the holder just gets a shared reference).

Is this a bug? If not, can I do something about it?

[ZealanL]

I managed to find someone's solution that completely fixes this problem: #2839
The issue is that pybind normally does not keep the derived python object alive, which is what that PR fixes.