[BUG]: std::vector<std::optional<std::string>> behaviour has changed in 2.2.0

[daveah]

Problem description

When binding a std::vector<std::optional<std::string>> Nones in python become empty strings in C++ in 2.2.0; whereas in 2.1.0 Nones become empty optionals. I suspect this is related to #675

Reproducible example code

C++:

#include <nanobind/nanobind.h>
#include <nanobind/stl/optional.h>
#include <nanobind/stl/string.h>
#include <nanobind/stl/vector.h>

namespace nb = nanobind;

using namespace nb::literals;

std::string vector_optional_string(std::vector<std::optional<std::string>> strs) {
    std::string result;
    for (const auto& str : strs) {
        if (!str.has_value()) {
            result += "NONE, ";
        } else {
            result += str.value() + ", ";
        }
    }
    return result;
}

NB_MODULE(nanobind_example_ext, m) {
    m.def("vector_optional_string", &vector_optional_string);
}

Python3:

from .nanobind_example_ext import vector_optional_string
vector_optional_string(["foo", "bar", "baz", None])

In 2.1.0 we get:
'foo, bar, baz, NONE, '

In 2.2.0 we get:
'foo, bar, baz, , '

[daveah]

To reproduce this, I downloaded nanobind_example from github, hacked it for the above functions, compiled in Python 3.11; nanobind 2.1.0 and nanobind 2.2.0

[wojdyr]

Ooops. I was thinking that we don't need to explicitly set nullopt in from_python. Here is a patch with a fix:

--- a/include/nanobind/stl/detail/nb_optional.h
+++ b/include/nanobind/stl/detail/nb_optional.h
@@ -23,5 +23,6 @@ struct optional_caster {
     bool from_python(handle src, uint8_t flags, cleanup_list* cleanup) noexcept {
-        if (src.is_none())
-            // default-constructed value is already empty
+        if (src.is_none()) {
+            value.reset();
             return true;
+        }

I'll add a test and make a PR.

EDIT:
actually I copied the comment about default-constructed value being empty from pybind11:
https://github.com/pybind/pybind11/blob/7e418f49243bb7d13fa92cf2634af1eeac386465/include/pybind11/stl.h#L523-L529
Is it that resetting the value is not needed in pybind11, but is needed in nanobind?