pybind11 v3.1.0

Seamless operability between C++11 and Python

Release notes

New Features:

- Support for Python 3.8 was removed. The minimum supported version is now Python 3.9. [#6110](https://github.com/pybind/pybind11/pull/6110)

- Support for MSVC 2017 has been dropped. [#6110](https://github.com/pybind/pybind11/pull/6110)

- Changed strict-mode numeric conversions for PEP 484 compatibility: `float` now accepts `int`, and `complex` now accepts `int` and `float`. [#5879](https://github.com/pybind/pybind11/pull/5879)

- Small extra features added to the pybind11 command line tool based on python-config. [#4272](https://github.com/pybind/pybind11/pull/4272)

- Added `subinterpreter_thread_state`, an RAII wrapper that owns a reusable `PyThreadState` for a sub-interpreter, together with a `subinterpreter_scoped_activate` overload that activates it. This lets an OS thread re-enter one or more sub-interpreters without creating and destroying a `PyThreadState` on every activation. [#6073](https://github.com/pybind/pybind11/pull/6073)

- Add `py::mod_gil_used()` as replacement spelling to `py::mod_gil_not_used(false)`. [#5797](https://github.com/pybind/pybind11/pull/5797)

Bug fixes:

- Fixed a crash in `py::subinterpreter::create()` when called without a current `PyThreadState`, which its documentation explicitly allows — for example from an embedder that ended initialization with `PyEval_SaveThread()`, or from a worker thread that has never touched Python. `error_scope` is now constructed after a thread state has been attached instead of before. [#6127](https://github.com/pybind/pybind11/pull/6127)

- Make `py::print` delegate to the current frame/interpreter built-ins `print` entry, fixing handling of `sys.stdout = None`, following the active runtime's stream, keyword, and error semantics, and remaining a no-op if the entry is unavailable during teardown. [#6121](https://github.com/pybind/pybind11/pull/6121)

- Include the builtin `complex` type in the input annotation of the `std::complex<T>` type caster. [#6113](https://github.com/pybind/pybind11/pull/6113)

- String views (e.g. `std::string_view`) are now kept alive only when loaded from a transient source (such as a generator), fixing both a use-after-free and a regression where casting a view from a durable object outside a bound function would throw. [#6096](https://github.com/pybind/pybind11/pull/6096)

- Drop Python 2 prepend to evaluated source, causing errors to be off by one. [#6089](https://github.com/pybind/pybind11/pull/6089)

- Fix data race on `last_storage_ptr_` cache in `gil_safe_call_once_and_store`. [#6087](https://github.com/pybind/pybind11/pull/6087)

- Allow user defined `__str__` on `enum_`. [#6078](https://github.com/pybind/pybind11/pull/6078)

- Fix generated `Callable` type annotations for Python callbacks passed into C++ by inverting the callback's argument/return I/O context relative to the enclosing function signature. [#6055](https://github.com/pybind/pybind11/pull/6055)

- Correct `__delitem__` for negative-step slices and re-enable contiguous erase fast path. [#6088](https://github.com/pybind/pybind11/pull/6088)

- Do not pass `-fno-fat-lto-objects` to GCC on macOS, which made all LTO probes fail and silently disabled LTO. [#6114](https://github.com/pybind/pybind11/pull/6114)

- Fixed cross-compilation to Emscripten/Pyodide with CMake ≥ 4.1 by no longer overriding an explicitly set `PYBIND11_USE_CROSSCOMPILING` when `CMAKE_CROSSCOMPILING_EMULATOR` is defined. [#6094](https://github.com/pybind/pybind11/pull/6094)

- Unset stale `PYTHON_MODULE_DEBUG_POSTFIX` and correct `USE_PYTHON_INCLUDE_DIR` variable. [#6086](https://github.com/pybind/pybind11/pull/6086)

- Apply `-undefined dynamic_lookup` to all Apple platforms. [#6075](https://github.com/pybind/pybind11/pull/6075)

Internal:

- Improved `py::enum_` operator performance by restoring type-specific implementations. [#5887](https://github.com/pybind/pybind11/pull/5887)

- Optimized internal `std::unordered_map`/`std::unordere…

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