* Insert type_caster_odr_guard<> (an empty struct to start with).

* Add odr_guard_registry() used in type_caster_odr_guard() default constructor.

* Add minimal_real_caster (from PR #3862) to test_async, test_buffers

* VERY MESSY SNAPSHOT of WIP, this was the starting point for cl/454658864, which has more changes on top.

* Restore original test_async, test_buffers from current smart_holder HEAD

* Copy from cl/454991845 snapshot Jun 14, 5:08 PM

* Cleanup of tests. Systematically insert `if (make_caster<T>::translation_unit_local) {`

* Small simplification of odr_guard_impl()

* WIP

* Add PYBIND11_SOURCE_FILE_LINE macro.

* Replace PYBIND11_TYPE_CASTER_UNIQUE_IDENTIFIER with PYBIND11_TYPE_CASTER_SOURCE_FILE_LINE, baked into PYBIND11_TYPE_CASTER macro.

* Add more PYBIND11_DETAIL_TYPE_CASTER_ACCESS_TRANSLATION_UNIT_LOCAL; resolves "unused" warning when compiling test_custom_type_casters.cpp

* load_type fixes & follow-on cleanup

* Strip ./ from source_file_line

* Add new tests to CMakeLists.txt, disable PYBIND11_WERROR

* Replace C++17 syntax. Compiles with Debian clang 13 C++11 mode, but fails to link. Trying GitHub Actions anyway to see if there are any platforms that support https://en.cppreference.com/w/cpp/language/tu_local before C++20. Note that Debian clang 13 C++17 works locally.

* Show C++ version along with ODR VIOLATION DETECTED message.

* Add source_file_line_basename()

* Introduce PYBIND11_TYPE_CASTER_ODR_GUARD_ON (but not set automatically).

* Minor cleanup.

* Set PYBIND11_TYPE_CASTER_ODR_GUARD_ON automatically.

* Resolve clang-tidy error.

* Compatibility with old compilers.

* Fix off-by-one in source_file_line_basename()

* Report PYBIND11_INTERNALS_ID & C++ Version from pytest_configure()

* Restore use of PYBIND11_WERROR

* Move cpp_version_in_use() from cast.h to pybind11_tests.cpp

* define PYBIND11_DETAIL_ODR_GUARD_IMPL_THROW_DISABLED true in test_odr_guard_1,2.cpp

* IWYU cleanup of detail/type_caster_odr_guard.h

* Replace `throw err;` to resolve clang-tidy error.

* Add new header filename to CMakeLists.txt, test_files.py

* Experiment: Try any C++17 compiler.

* Fix ifdef for pragma GCC diagnostic.

* type_caster_odr_guard_impl() cleanup

* Move type_caster_odr_guard to type_caster_odr_guard.h

* Rename test_odr_guard* to test_type_caster_odr_guard*

* Remove comments that are (now) more distracting than helpful.

* Mark tu_local_no_data_always_false operator bool as explicit (clang-tidy). See also: https://stackoverflow.com/questions/39995573/when-can-i-use-explicit-operator-bool-without-a-cast

* New PYBIND11_TYPE_CASTER_ODR_GUARD_STRICT option (current on by default).

* Add test_type_caster_odr_registry_values(), test_type_caster_odr_violation_detected_counter()

* Report UNEXPECTED: test_type_caster_odr_guard_2.cpp prevailed (but do not fail).

* Apply clang-tidy suggestion.

* Attempt to handle valgrind behavior.

* Another attempt to handle valgrind behavior.

* Yet another attempt to handle valgrind behavior.

* Trying a new direction: show compiler info & std for UNEXPECTED: type_caster_odr_violation_detected_count() == 0

* compiler_info MSVC fix. num_violations == 0 condition.

* assert pybind11_tests.compiler_info is not None

* Introduce `make_caster_intrinsic<T>`, to be able to undo the 2 changes from `load_type` to `load_type<T>`. This is to avoid breaking 2 `pybind11::detail::load_type()` calls found in the wild (Google global testing).

One of the breakages in the wild was: https://github.com/google/tensorstore/blob/0f0f6007670a3588093acd9df77cce423e0de805/python/tensorstore/subscript_method.h#L61

* Add test for stl.h / stl_bind.h mix.

Manually verified that the ODR guard detects the ODR violation:

```
C++ Info: Debian Clang 13.0.1 C++17 __pybind11_internals_v4_clang_libstdcpp_cxxabi1002_sh_def__
=========================================================== test session starts ============================================================
platform linux -- Python 3.9.12, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
...
================================================================= FAILURES =================================================================
_____________________________________________ test_type_caster_odr_violation_detected_counter ______________________________________________

    def test_type_caster_odr_violation_detected_counter():
        ...
        else:
>           assert num_violations == 1
E           assert 2 == 1
E             +2
E             -1

num_violations = 2

test_type_caster_odr_guard_1.py:51: AssertionError
========================================================= short test summary info ==========================================================
FAILED test_type_caster_odr_guard_1.py::test_type_caster_odr_violation_detected_counter - assert 2 == 1
======================================================= 1 failed, 5 passed in 0.08s ========================================================
```

* Eliminate need for `PYBIND11_DETAIL_TYPE_CASTER_ACCESS_TRANSLATION_UNIT_LOCAL` macro.

Copying code first developed by @amauryfa. I tried this at an earlier stage, but by itself this was insufficient. In the meantime I added in the TU-local mechanisms: trying again.

Passes local testing:
```
DISABLED std::system_error: ODR VIOLATION DETECTED: pybind11::detail::type_caster<mrc_ns::type_mrc>: SourceLocation1="/usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_odr_guard_1.cpp:18", SourceLocation2="/usr/local/google/home/rwgk/forked/pybind11/tests/test_type_caster_odr_guard_2.cpp:19"
C++ Info: Debian Clang 13.0.1 C++17 __pybind11_internals_v4_clang_libstdcpp_cxxabi1002_sh_def__
=========================================================== test session starts ============================================================
platform linux -- Python 3.9.12, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/google/home/rwgk/forked/pybind11/tests, configfile: pytest.ini
collected 6 items

test_type_caster_odr_guard_1.py::test_type_mrc_to_python PASSED
test_type_caster_odr_guard_1.py::test_type_mrc_from_python PASSED
test_type_caster_odr_guard_1.py::test_type_caster_odr_registry_values PASSED
test_type_caster_odr_guard_1.py::test_type_caster_odr_violation_detected_counter PASSED
test_type_caster_odr_guard_2.py::test_type_mrc_to_python PASSED
test_type_caster_odr_guard_2.py::test_type_mrc_from_python PASSED

============================================================ 6 passed in 0.01s =============================================================
```

* tu_local_descr with src_loc experiment

* clang-tidy suggested fixes

* Use source_file_line_from_sloc in type_caster_odr_guard_registry

* Disable type_caster ODR guard for __INTEL_COMPILER (see comment). Also turn off printf.

* Add missing include (discovered via google-internal testing).

* Work `scr_loc` into `descr`

* Use `TypeCasterType::name.sloc` instead of `source_file_line.sloc`

Manual re-verification:

```
+++ b/tests/test_type_caster_odr_guard_2.cpp
-    // m.def("pass_vector_type_mrc", mrc_ns::pass_vector_type_mrc);
+    m.def("pass_vector_type_mrc", mrc_ns::pass_vector_type_mrc);
```

```
>           assert num_violations == 1
E           assert 2 == 1

num_violations = 2

test_type_caster_odr_guard_1.py:51: AssertionError
```

* Fix small oversight (src_loc::here() -> src_loc{nullptr, 0}).

* Remove PYBIND11_DETAIL_TYPE_CASTER_ACCESS_TRANSLATION_UNIT_LOCAL macro completely.

* Remove PYBIND11_TYPE_CASTER_SOURCE_FILE_LINE macro completely. Some small extra cleanup.

* Minor tweaks looking at the PR with a fresh eye.

* src_loc comments

* Add new test_descr_src_loc & and fix descr.h `concat()` `src_loc` bug discovered while working on the test.

* Some more work on source code comments.

* Fully document the ODR violations in the ODR guard itself and introduce `PYBIND11_TYPE_CASTER_ODR_GUARD_ON_IF_AVAILABLE`

* Update comment (incl. mention of deadsnakes known to not work as intended).

* Use no-destructor idiom for type_caster_odr_guard_registry, as suggested by @laramiel

* Fix clang-tidy error: 'auto reg' can be declared as 'auto *reg' [readability-qualified-auto,-warnings-as-errors]

* WIP

* Revert "WIP" (tu_local_no_data_always_false_base experiment).

This reverts commit 31e8ac562f91a2e3d480a82feaa76bf2142c9ce7.

* Change `PYBIND11_TYPE_CASTER_ODR_GUARD_ON` to `PYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD`, based on a suggestion by @rainwoodman

* Improved `#if` determining `PYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD`, based on suggestion by @laramiel

* Make `descr::sloc` `const`, as suggested by @rainwoodman

* Rename macro to `PYBIND11_DETAIL_TYPE_CASTER_ODR_GUARD_IMPL_DEBUG`, as suggested by @laramiel

* Tweak comments some more (add "white hat hacker" analogy).

* Bring back `PYBIND11_CPP17` in determining `PYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD`, to hopefully resolve most if not all of the many CI failures (89 failing, 32 successful: https://github.com/pybind/pybind11/runs/7430295771).

* Try another workaround for `__has_builtin`-related breakages (https://github.com/pybind/pybind11/runs/7430720321).

* Remove `defined(__has_builtin)` and subconditions.

* Update "known to not work" expectation in test and comment.

* `pytest.skip` `num_violations == 0` only `#ifdef __NO_INLINE__` (irrespective of the compiler)

* Systematically change all new `#ifdef` to `#if defined` (review suggestion).

* Bring back MSVC comment that got lost while experimenting.

* Add test_namespace_visibility

To probe environment/toolchain/platform-specific behavior under the exact same conditions as normal tests.

(An earlier version of this code was used to inform PR #4043.)

* Disable flake8 in ubench/holder_comparison_*.py, to suppress new & useless diagnostics.

* Disable namespace_visibility_1s.cpp (tosee if that resolves the MSVC and CUDA `test_cross_module_exception_translator` failures).

* Turn off flake8 completely for ubench (the Strip unnecessary `# noqa`s action un-helpfully removed the added noqa).

* Disable test_namespace_visibility completely. Just keep the two .cpp files, only setting the module docstring and doing nothing else.

* Rename test_namespace_visibility.py to test_exc_namespace_visibility.py, so that it is imported by pytest before test_exceptions.py

* Add `set_property(SOURCE namespace_visibility_1s.cpp PROPERTY LANGUAGE CUDA)`

* Add reference to PR #4054

* Complete the documentation (comments in test_exc_namespace_visibility.py).

* Rename namespace_visibility.h to namespace_visibility.inl, as suggested by @charlesbeattie

updates:
- [github.com/asottile/pyupgrade: v2.34.0 → v2.37.1](https://github.com/asottile/pyupgrade/compare/v2.34.0...v2.37.1)
- [github.com/hadialqattan/pycln: v1.3.5 → v2.0.1](https://github.com/hadialqattan/pycln/compare/v1.3.5...v2.0.1

)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

To suppress these new errors (apparently after a flake8 upgrade):

```
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

ubench/holder_comparison.py:96:38: B023 Function definition does not bind loop variable 'nb1'.
                    assert int(round(nb1.sum())) == data_size
                                     ^
ubench/holder_comparison.py:96:53: B023 Function definition does not bind loop variable 'data_size'.
                    assert int(round(nb1.sum())) == data_size
                                                    ^
ubench/holder_comparison.py:99:25: B023 Function definition does not bind loop variable 'nb1'.
                        nb1.sum()
                        ^
ubench/holder_comparison.py:103:28: B023 Function definition does not bind loop variable 'nb1'.
                    assert nb1.add(nb2) == data_size
                           ^
ubench/holder_comparison.py:103:36: B023 Function definition does not bind loop variable 'nb2'.
                    assert nb1.add(nb2) == data_size
                                   ^
ubench/holder_comparison.py:103:44: B023 Function definition does not bind loop variable 'data_size'.
                    assert nb1.add(nb2) == data_size
                                           ^
ubench/holder_comparison.py:106:25: B023 Function definition does not bind loop variable 'nb1'.
                        nb1.add(nb2)
                        ^
ubench/holder_comparison.py:106:33: B023 Function definition does not bind loop variable 'nb2'.
                        nb1.add(nb2)
                                ^
ubench/holder_comparison_extract_sheet_data.py:21:16: B023 Function definition does not bind loop variable 'header_row'.
            if header_row:
               ^
ubench/holder_comparison_extract_sheet_data.py:22:20: B023 Function definition does not bind loop variable 'header'.
                if header is None:  # type: ignore[unreachable]
                   ^
ubench/holder_comparison_extract_sheet_data.py:23:36: B023 Function definition does not bind loop variable 'header_row'.
                    print(",".join(header_row))
                                   ^
ubench/holder_comparison_extract_sheet_data.py:25:28: B023 Function definition does not bind loop variable 'header'.
                    assert header == header_row
                           ^
ubench/holder_comparison_extract_sheet_data.py:25:38: B023 Function definition does not bind loop variable 'header_row'.
                    assert header == header_row
                                     ^
ubench/holder_comparison_extract_sheet_data.py:26:16: B023 Function definition does not bind loop variable 'data_row'.
            if data_row is not None:
               ^
ubench/holder_comparison_extract_sheet_data.py:27:32: B023 Function definition does not bind loop variable 'data_row'.
                print(",".join(data_row))  # type: ignore[unreachable]
                               ^
ubench/holder_comparison_extract_sheet_data.py:28:17: B023 Function definition does not bind loop variable 'data_row_buffer'.
                data_row_buffer.append(data_row)
                ^
ubench/holder_comparison_extract_sheet_data.py:28:40: B023 Function definition does not bind loop variable 'data_row'.
                data_row_buffer.append(data_row)
                                       ^
ubench/holder_comparison_extract_sheet_data.py:29:20: B023 Function definition does not bind loop variable 'header_row'.
            return header_row
                   ^
```

* Properly visit self in >=3.9 traverse

* Add comment about 3.9 behavior

Very minor refactoring to ease development and debugging.

Having to declare a local `std::string` has bugged me many times. Nice to get this little nuisance out of the way.

Extracted from PR #4022, where it is used like this:

```
    std::fprintf(stdout,
                 "\nTYPE_CASTER_ODR_GUARD_IMPL %s %s\n",
                 clean_type_id(intrinsic_type_info.name()).c_str(),
                 source_file_line_from_sloc.c_str());
```

* Report `C++ Info:` from `pytest_configure()`

* Use pytest_report_header() as suggested by @skylion007

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Placeholder commit for 3.11 testing

* Does this fix it?

* Try suggestion

* Placeholder commit for 3.11 testing

* Does this fix it?

* Try suggestion

* fix: try using modern init for embedded interp

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix: error message changed in 3.11

* fix: apply logic in Python manually

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix autodetect dynamic attrs in 3.11

* fix: include error message if possible in error

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* ci: enable standard Python 3.11 testing

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Make dynamic attrs condtiion exclusive to ver.

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

* [pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/psf/black: 22.3.0 → 22.6.0](https://github.com/psf/black/compare/22.3.0...22.6.0)
- [github.com/Lucas-C/pre-commit-hooks: v1.2.0 → v1.3.0](https://github.com/Lucas-C/pre-commit-hooks/compare/v1.2.0...v1.3.0)
- [github.com/PyCQA/pylint: v2.14.3 → v2.14.4](https://github.com/PyCQA/pylint/compare/v2.14.3...v2.14.4)
- [github.com/pre-commit/mirrors-clang-format: v14.0.5 → v14.0.6](https://github.com/pre-commit/mirrors-clang-format/compare/v14.0.5...v14.0.6

)

* Update blacken-docs

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* chore(deps): bump actions/setup-python from 3 to 4

Bumps [actions/setup-python](https://github.com/actions/setup-python) from 3 to 4.
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](https://github.com/actions/setup-python/compare/v3...v4

)

---
updated-dependencies:
- dependency-name: actions/setup-python
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

* Apply suggestions from code review

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Ignores no longer needed after April 2022. Dependabot keeps the same style pinning now.

When converting an array to an Eigen matrix, ignore the strides if any
dimension size is 0. If the array is empty, the strides aren't relevant,
and especially numpy ≥ 1.23 explicitly sets the strides to 0 in this
case. (See numpy commit dd5ab7b11520.)

Update tests to verify that this works, and continues to work.

* Fix binding of `Pet::Attributes`

* omit `attributes` as it's not needed

Signed-off-by: Masaki Kozuki <mkozuki@nvidia.com>

* Support loading unique_ptr<derived> as unique_ptr<base>.

* Fix incorrect test

* pre commit fix

* Fix clang tidy

* Resolve comments

* Resolve comments

updates:
- [github.com/hadialqattan/pycln: v1.3.3 → v1.3.5](https://github.com/hadialqattan/pycln/compare/v1.3.3...v1.3.5

)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

updates:
- [github.com/PyCQA/pylint: v2.14.1 → v2.14.3](https://github.com/PyCQA/pylint/compare/v2.14.1...v2.14.3)
- [github.com/pre-commit/mirrors-clang-format: v14.0.4-1 → v14.0.5](https://github.com/pre-commit/mirrors-clang-format/compare/v14.0.4-1...v14.0.5

)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Bumps [pre-commit/action](https://github.com/pre-commit/action) from 2.0.3 to 3.0.0.
- [Release notes](https://github.com/pre-commit/action/releases)
- [Commits](https://github.com/pre-commit/action/compare/v2.0.3...v3.0.0

)

---
updated-dependencies:
- dependency-name: pre-commit/action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

* emplace field descriptors

* reserve sufficient capacity

* remove std::move

* properly iterate through dict

* make handle casting more explicit

* Revert to old dict api

updates:
- [github.com/asottile/pyupgrade: v2.32.1 → v2.33.0](https://github.com/asottile/pyupgrade/compare/v2.32.1...v2.33.0)
- [github.com/Lucas-C/pre-commit-hooks: v1.1.13 → v1.2.0](https://github.com/Lucas-C/pre-commit-hooks/compare/v1.1.13...v1.2.0)
- [github.com/hadialqattan/pycln: v1.3.2 → v1.3.3](https://github.com/hadialqattan/pycln/compare/v1.3.2...v1.3.3)
- [github.com/PyCQA/pylint: v2.13.8 → v2.14.1](https://github.com/PyCQA/pylint/compare/v2.13.8...v2.14.1)
- [github.com/pre-commit/mirrors-mypy: v0.950 → v0.960](https://github.com/pre-commit/mirrors-mypy/compare/v0.950...v0.960

)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

* enable two new clang-tidy checks

* Use better loop var for char

* error_already_set::what() is now constructed lazily

Prior to this commit throwing error_already_set was expensive due to the
eager construction of the error string (which required traversing the
Python stack). See #1853 for more context and an alternative take on the
issue.

Note that error_already_set no longer inherits from std::runtime_error
because the latter has no default constructor.

* Do not attempt to normalize if no exception occurred

This is not supported on PyPy-2.7 5.8.0.

* Extract exception name via tp_name

This is faster than dynamically looking up __name__ via GetAttrString.
Note though that the runtime of the code throwing an error_already_set
will be dominated by stack unwinding so the improvement will not be
noticeable.

Before:

396 ns ± 0.913 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

After:

277 ns ± 0.549 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

Benchmark:

const std::string foo() {
    PyErr_SetString(PyExc_KeyError, "");
    const std::string &s = py::detail::error_string();
    PyErr_Clear();
    return s;
}

PYBIND11_MODULE(foo, m) {
    m.def("foo", &::foo);
}

* Reverted error_already_set to subclass std::runtime_error

* Revert "Extract exception name via tp_name"

The implementation of __name__ is slightly more complex than that.
It handles the module name prefix, and heap-allocated types. We could
port it to pybind11 later on but for now it seems like an overkill.

This reverts commit f1435c7e6b068a1ed13ebd3db597ea3bb15aa398.

* Cosmit following @YannickJadoul's comments

Note that detail::error_string() no longer calls PyException_SetTraceback
as it is unncessary for pretty-printing the exception.

* Fixed PyPy build

* Moved normalization to error_already_set ctor

* Fix merge bugs

* Fix more merge errors

* Improve formatting

* Improve error message in rare case

* Revert back if statements

* Fix clang-tidy

* Try removing mutable

* Does build_mode release fix it

* Set to Debug to expose segfault

* Fix remove set error string

* Do not run error_string() more than once

* Trying setting the tracebackk to the value

* guard if m_type is null

* Try to debug PGI

* One last try for PGI

* Does reverting this fix PyPy

* Reviewer suggestions

* Remove unnecessary initialization

* Add noexcept move and explicit fail throw

* Optimize error_string creation

* Fix typo

* Revert noexcept

* Fix merge conflict error

* Abuse assignment operator

* Revert operator abuse

* See if we still need debug

* Remove unnecessary mutable

* Report "FATAL failure building pybind11::error_already_set error_string" and terminate process.

* Try specifying noexcept again

* Try explicit ctor

* default ctor is noexcept too

* Apply reviewer suggestions, simplify code, and make helper method private

* Remove unnecessary include

* Clang-Tidy fix

* detail::obj_class_name(), fprintf with [STDERR], [STDOUT] tags, polish comments

* consistently check m_lazy_what.empty() also in production builds

* Make a comment slightly less ambiguous.

* Bug fix: Remove `what();` from `restore()`.

It sure would need to be guarded by `if (m_type)`, otherwise `what()` fails and masks that no error was set (see update unit test). But since `error_already_set` is copyable, there is no point in releasing m_type, m_value, m_trace, therefore we can just as well avoid the runtime overhead of force-building `m_lazy_what`, it may never be used.

* Replace extremely opaque (unhelpful) error message with a truthful reflection of what we know.

* Fix clang-tidy error [performance-move-constructor-init].

* Make expected error message less specific.

* Various changes.

* bug fix: error_string(PyObject **, ...)

* Putting back the two eager PyErr_NormalizeException() calls.

* Change error_already_set() to call pybind11_fail() if the Python error indicator not set. The net result is that a std::runtime_error is thrown instead of error_already_set, but all tests pass as is.

* Remove mutable (fixes oversight in the previous commit).

* Normalize the exception only locally in error_string(). Python 3.6 & 3.7 test failures expected. This is meant for benchmarking, to determine if it is worth the trouble looking into the failures.

* clang-tidy: use auto

* Use `gil_scoped_acquire_local` in `error_already_set` destructor. See long comment.

* For Python < 3.8: `PyErr_NormalizeException` before `PyErr_WriteUnraisable`

* Go back to replacing the held Python exception with then normalized exception, if & when needed. Consistently document the side-effect.

* Slightly rewording comment. (There were also other failures.)

* Add 1-line comment for obj_class_name()

* Benchmark code, with results in this commit message.

          function                   #calls  test time [s]  μs / call
master    pure_unwind                729540      1.061      14.539876
          err_set_unwind_err_clear   681476      1.040      15.260282
          err_set_error_already_set  508038      1.049      20.640525
          error_already_set_restore  555578      1.052      18.933288
          pr1895_original_foo        244113      1.050      43.018168
                                                                       PR / master
PR #1895  pure_unwind                736981      1.054      14.295685       98.32%
          err_set_unwind_err_clear   685820      1.045      15.237399       99.85%
          err_set_error_already_set  661374      1.046      15.811879       76.61%
          error_already_set_restore  669881      1.048      15.645176       82.63%
          pr1895_original_foo        318243      1.059      33.290806       77.39%

master @ commit ad146b2a

Running tests in directory "/usr/local/google/home/rwgk/forked/pybind11/tests":
============================= test session starts ==============================
platform linux -- Python 3.9.10, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/google/home/rwgk/forked/pybind11/tests, configfile: pytest.ini
collecting ... collected 5 items

test_perf_error_already_set.py::test_perf[pure_unwind]
PERF pure_unwind,729540,1.061,14.539876
PASSED
test_perf_error_already_set.py::test_perf[err_set_unwind_err_clear]
PERF err_set_unwind_err_clear,681476,1.040,15.260282
PASSED
test_perf_error_already_set.py::test_perf[err_set_error_already_set]
PERF err_set_error_already_set,508038,1.049,20.640525
PASSED
test_perf_error_already_set.py::test_perf[error_already_set_restore]
PERF error_already_set_restore,555578,1.052,18.933288
PASSED
test_perf_error_already_set.py::test_perf[pr1895_original_foo]
PERF pr1895_original_foo,244113,1.050,43.018168
PASSED

============================== 5 passed in 12.38s ==============================

pr1895 @ commit 8dff51d12e4af11aff415ee966070368fe606664

Running tests in directory "/usr/local/google/home/rwgk/forked/pybind11/tests":
============================= test session starts ==============================
platform linux -- Python 3.9.10, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/google/home/rwgk/forked/pybind11/tests, configfile: pytest.ini
collecting ... collected 5 items

test_perf_error_already_set.py::test_perf[pure_unwind]
PERF pure_unwind,736981,1.054,14.295685
PASSED
test_perf_error_already_set.py::test_perf[err_set_unwind_err_clear]
PERF err_set_unwind_err_clear,685820,1.045,15.237399
PASSED
test_perf_error_already_set.py::test_perf[err_set_error_already_set]
PERF err_set_error_already_set,661374,1.046,15.811879
PASSED
test_perf_error_already_set.py::test_perf[error_already_set_restore]
PERF error_already_set_restore,669881,1.048,15.645176
PASSED
test_perf_error_already_set.py::test_perf[pr1895_original_foo]
PERF pr1895_original_foo,318243,1.059,33.290806
PASSED

============================== 5 passed in 12.40s ==============================

clang++ -o pybind11/tests/test_perf_error_already_set.os -c -std=c++17 -fPIC -fvisibility=hidden -Os -flto -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wnon-virtual-dtor -Wunused-result -isystem /usr/include/python3.9 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/local/google/home/rwgk/forked/pybind11/include -I/usr/local/google/home/rwgk/clone/pybind11/include /usr/local/google/home/rwgk/forked/pybind11/tests/test_perf_error_already_set.cpp

clang++ -o lib/pybind11_tests.so -shared -fPIC -Os -flto -shared ...

Debian clang version 13.0.1-3+build2
Target: x86_64-pc-linux-gnu
Thread model: posix

* Changing call_repetitions_target_elapsed_secs to 0.1 for regular unit testing.

* Adding in `recursion_depth`

* Optimized ctor

* Fix silly bug in recurse_first_then_call()

* Add tests that have equivalent PyErr_Fetch(), PyErr_Restore() but no try-catch.

* Add call_error_string to tests. Sample only recursion_depth 0, 100.

* Show lazy-what speed-up in percent.

* Include real_work in benchmarks.

* Replace all PyErr_SetString() with generate_python_exception_with_traceback()

* Better organization of test loops.

* Add test_error_already_set_copy_move

* Fix bug in newly added test (discovered by clang-tidy): actually use move ctor

* MSVC detects the unreachable return

* change test_perf_error_already_set.py back to quick mode

* Inherit from std::exception (instead of std::runtime_error, which does not make sense anymore with the lazy what)

* Special handling under Windows.

* print with leading newline

* Removing test_perf_error_already_set (copies are under https://github.com/rwgk/rwgk_tbx/commit/7765113fbb659e1ea004c5ba24fb578244bc6cfd).

* Avoid gil and scope overhead if there is nothing to release.

* Restore default move ctor. "member function" instead of "function" (note that "method" is Python terminology).

* Delete error_already_set copy ctor.

* Make restore() non-const again to resolve clang-tidy failure (still experimenting).

* Bring back error_already_set copy ctor, to see if that resolves the 4 MSVC test failures.

* Add noexcept to error_already_set copy & move ctors (as suggested by @skylion007 IIUC).

* Trying one-by-one noexcept copy ctor for old compilers.

* Add back test covering copy ctor. Add another simple test that exercises the copy ctor.

* Exclude more older compilers from using the noexcept = default ctors. (The tests in the previous commit exposed that those are broken.)

* Factor out & reuse gil_scoped_acquire_local as gil_scoped_acquire_simple

* Guard gil_scoped_acquire_simple by _Py_IsFinalizing() check.

* what() GIL safety

* clang-tidy & Python 3.6 fixes

* Use `gil_scoped_acquire` in dtor, copy ctor, `what()`. Remove `_Py_IsFinalizing()` checks (they are racy: https://github.com/python/cpython/pull/28525).

* Remove error_scope from copy ctor.

* Add `error_scope` to `get_internals()`, to cover the situation that `get_internals()` is called from the `error_already_set` dtor while a new Python error is in flight already. Also backing out `gil_scoped_acquire_simple` change.

* Add `FlakyException` tests with failure triggers in `__init__` and `__str__`

THIS IS STILL A WORK IN PROGRESS. This commit is only an important resting point.

This commit is a first attempt at addressing the observation that `PyErr_NormalizeException()` completely replaces the original exception if `__init__` fails. This can be very confusing even in small applications, and extremely confusing in large ones.

* Tweaks to resolve Py 3.6 and PyPy CI failures.

* Normalize Python exception immediately in error_already_set ctor.

For background see: https://github.com/pybind/pybind11/pull/1895#issuecomment-1135304081



* Fix oversights based on CI failures (copy & move ctor initialization).

* Move @pytest.mark.xfail("env.PYPY") after @pytest.mark.parametrize(...)

* Use @pytest.mark.skipif (xfail does not work for segfaults, of course).

* Remove unused obj_class_name_or() function (it was added only under this PR).

* Remove already obsolete C++ comments and code that were added only under this PR.

* Slightly better (newly added) comments.

* Factor out detail::error_fetch_and_normalize. Preparation for producing identical results from error_already_set::what() and detail::error_string(). Note that this is a very conservative refactoring. It would be much better to first move detail::error_string into detail/error_string.h

* Copy most of error_string() code to new error_fetch_and_normalize::complete_lazy_error_string()

* Remove all error_string() code from detail/type_caster_base.h. Note that this commit includes a subtle bug fix: previously error_string() restored the Python error, which will upset pybind11_fail(). This never was a problem in practice because the two PyType_Ready() calls in detail/class.h do not usually fail.

* Return const std::string& instead of const char * and move error_string() to pytypes.h

* Remove gil_scope_acquire from error_fetch_and_normalize, add back to error_already_set

* Better handling of FlakyException __str__ failure.

* Move error_fetch_and_normalize::complete_lazy_error_string() implementation from pybind11.h to pytypes.h

* Add error_fetch_and_normalize::release_py_object_references() and use from error_already_set dtor.

* Use shared_ptr for m_fetched_error => 1. non-racy, copy ctor that does not need the GIL; 2. enables guard against duplicate restore() calls.

* Add comments.

* Trivial renaming of a newly introduced member function.

* Workaround for PyPy

* Bug fix (oversight). Only valgrind got this one.

* Use shared_ptr custom deleter for m_fetched_error in error_already_set. This enables removing the dtor, copy ctor, move ctor completely.

* Further small simplification. With the GIL held, simply deleting the raw_ptr takes care of everything.

* IWYU cleanup

```
iwyu version: include-what-you-use 0.17 based on Debian clang version 13.0.1-3+build2
```

Command used:

```
iwyu -c -std=c++17 -DPYBIND11_TEST_BOOST -Iinclude/pybind11 -I/usr/include/python3.9 -I/usr/include/eigen3 include/pybind11/pytypes.cpp
```

pytypes.cpp is a temporary file: `#include "pytypes.h"`

The raw output is very long and noisy.

I decided to use `#include <cstddef>` instead of `#include <cstdio>` for `std::size_t` (iwyu sticks to the manual choice).

I ignored all iwyu suggestions that are indirectly covered by `#include <Python.h>`.

I manually verified that all added includes are actually needed.

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

* Add object rvalue overload for accessors. Enables reference stealing

* Fix comments

* Fix more comment typos

* Fix bug

* reorder declarations for clarity

* fix another perf bug

* should be static

* future proof operator overloads

* Fix perfect forwarding

* Add a couple of tests

* Remove errant include

* Improve test documentation

* Add dict test

* add object attr tests

* Optimize STL map caster and cleanup enum

* Reorder to match declarations

* adjust increfs

* Remove comment

* revert value change

* add missing move

* Add test_perf_accessors (to be merged into test_pytypes).

* Python < 3.8 f-string compatibility

* Use thread_local in inc_ref_counter()

* Intentional breakage, brute-force way to quickly find out how many platforms reach the PYBIND11_HANDLE_REF_DEBUG code, with and without threads.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Remove Intentional breakage

* Drop perf test, move inc_refs tests to test_pytypes

* Fold in PR #3970 with `#ifdef`s

* Complete test coverage for all newly added code.

* Condense new unit tests via a simple local helper macro.

* Remove PYBIND11_PR3970 define. See https://github.com/pybind/pybind11/pull/3977#issuecomment-1142526417



* Move static keyword first (fixes silly oversight).

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

* Add `error_scope` to `detail::get_internals()`

* Adjust test to tolerate macOS PyPy behavior.