Can't implicit cast to pointer of bound type?

[d0t2]

It seems you aren't able to implicitly cast types from a source type to a pointer of the target type:

#include "nanobind/nanobind.h"
#include <cstdint>

namespace nb = nanobind;
using namespace nb::literals;

class IntWrapper {
public:
    int64_t i;
    IntWrapper(int64_t i) : i(i) {}
};

NB_MODULE(thing, m) {
    nb::class_<IntWrapper>(m, "IntWrapper").def(nb::init_implicit<int64_t>(), "i"_a);
    m.def("cast_to_value", [](nb::object obj) { return nb::cast<IntWrapper>(obj); }, "obj"_a);
    m.def("cast_to_ptr", [](nb::object obj) { return nb::cast<IntWrapper *>(obj); }, "obj"_a);
}

Running the following test:

import thing

x, y = thing.IntWrapper(1), 1
print(f"cast IntWrapper to value: {x} -> {thing.cast_to_value(x)}")
print(f"cast IntWrapper to ptr: {x} -> {thing.cast_to_ptr(x)}")
print(f"cast int to value: {y} -> {thing.cast_to_value(y)}")
print(f"cast int to ptr: {y} -> {thing.cast_to_ptr(y)}")

Results in this:

cast IntWrapper to value: <thing.IntWrapper object at 0x104fad8f0> -> <thing.IntWrapper object at 0x104faf450>
cast IntWrapper to ptr: <thing.IntWrapper object at 0x104fad8f0> -> <thing.IntWrapper object at 0x104fad8f0>
cast int to value: 1 -> <thing.IntWrapper object at 0x104faf450>
Traceback (most recent call last):
  File ".../test.py", line 7, in <module>
    print(f"cast int to ptr: {y} -> {thing.cast_to_ptr(y)}")
                                     ~~~~~~~~~~~~~~~~~^^^
RuntimeError: std::bad_cast

While an equivalent setup in pybind11:

#include "pybind11/pybind11.h"
#include <cstdint>

namespace py = pybind11;
using namespace py::literals;

class IntWrapper {
public:
    int64_t i;
    IntWrapper(int64_t i) : i(i) {}
};

PYBIND11_MODULE(thing, m) {
    py::class_<IntWrapper>(m, "IntWrapper").def(py::init<int64_t>(), "i"_a);
    py::implicitly_convertible<int64_t, IntWrapper>();
    m.def("cast_to_value", [](py::object obj) { return py::cast<IntWrapper>(obj); }, "obj"_a);
    m.def("cast_to_ptr", [](py::object obj) { return py::cast<IntWrapper *>(obj); }, "obj"_a);
}

seems to work fine:

cast IntWrapper to value: <thing.IntWrapper object at 0x10533ff70> -> <thing.IntWrapper object at 0x104d684f0>
cast IntWrapper to ptr: <thing.IntWrapper object at 0x10533ff70> -> <thing.IntWrapper object at 0x10533ff70>
cast int to value: 1 -> <thing.IntWrapper object at 0x10533fef0>
cast int to ptr: 1 -> <thing.IntWrapper object at 0x104d684f0>

I suppose since you are create a new object, ownership is more ambiguous if you return a pointer, but I don't think that is unique to implicit casting, and pybind11 seems to handle the issue fine? Is there some reason why nanobind can't handle this case? I'm not sure this is necessarily a bug, so I didn't post it under issues. But it should probably be mentioned in the porting guide section of the docs, if it's not fixable.