test: add regression test for PYBIND11_MAKE_OPAQUE with std::array (issue #5988)

[Copilot]

Description

pybind11 3.0.2 crashes on Windows (Python 3.11/MSVC) during module initialization when PYBIND11_MAKE_OPAQUE is used with std::array types. The crash trace shows a null pointer access violation inside pybind11::detail::get_internals() → Thrd_yield (MSVCP140.dll), triggered at import time.

This PR adds a regression test exercising PYBIND11_MAKE_OPAQUE with std::array<double, 3> and the nested std::vector<std::array<double, 3>> — types not currently covered by any test and safe from ODR conflicts (unlike std::array<int, 3> which is used as a converted type in test_stl.cpp):

// Reproduces the reported usage pattern from issue #5988
PYBIND11_MAKE_OPAQUE(std::array<double, 3>)
PYBIND11_MAKE_OPAQUE(std::vector<std::array<double, 3>>)

• tests/test_opaque_types.cpp: Adds PYBIND11_MAKE_OPAQUE declarations for std::array<double, 3> and std::vector<std::array<double, 3>>, with py::class_ bindings exposing indexing, mutation, and length operations.
• tests/test_opaque_types.py: Adds test_issue_5988_opaque_std_array() covering element access, bounds checking (IndexError), and vector-of-array operations.

Suggested changelog entry:

• Added regression test for issue FOR TESTING ONLY: Combine PRs #5879, #5887, #5960, #5986 #5988: PYBIND11_MAKE_OPAQUE with std::array and nested std::vector<std::array<...>> types.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[BUG]: 3.0.2 is crashing when binding with std containers</issue_title>
<issue_description>### Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and 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 or in a Discussion.

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

3.0.2

Problem description

I updated from 3.0.1 to 3.0.2 and now pybind11 is crashing when the compiled and loaded in Python. I am currently trying it in windows with VS2026 and intel oneAPI compiler. I haven't tried Linux or g++ yet. I also have not narrowed down which container binding is crashing.
version 3.0.1 is working fine. I have attached the crash report. It is coming from Blender. Blender is using python 3.11. The crash is from python.

blender.crash.txt

Reproducible example code

C++

#include <pybind11/pybind11.h>
namespace py = pybind11;

PYBIND11_MAKE_OPAQUE(std::array<double, 3>)
PYBIND11_MAKE_OPAQUE(std::array<int, 3>)
PYBIND11_MAKE_OPAQUE(std::vector<double>)
PYBIND11_MAKE_OPAQUE(std::vector<int>)
PYBIND11_MAKE_OPAQUE(std::vector<uint8_t>)
PYBIND11_MAKE_OPAQUE(std::vector<std::array<double, 3>>)
PYBIND11_MAKE_OPAQUE(std::vector<std::array<int, 3>>)
PYBIND11_MAKE_OPAQUE(std::vector<std::string>)

PYBIND11_MODULE(TestAPI, m, py::mod_gil_not_used())
{
}

Python:

if (os.name == "nt"):
    try:
        ctypes.CDLL(os.path.join(addon_dir, "cAPI\SimulationAPI.cp311-win_amd64.pyd"))
    except OSError as e:
        print(f"Error loading .pyd file: {e}")

from .cAPI import TestAPI

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

3.0.1</issue_description>

<agent_instructions>Make a test case with the example to see if CI does crash with it.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [BUG]: 3.0.2 is crashing when binding with std containers #5993

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[henryiii]

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