* `container: silkeh/clang:18-bookworm` in .github/workflows/format.yml

* clang-tidy auto-fix (trivial, in test only)

* Disable `performance-enum-size` (noisy, low value)

* Temporarily turn off 3 diagnostics (to be tackled one-by-one).

* Add explicit `switch` `default` to resolve clang-tidy `bugprone-switch-missing-default-case`

Debian clang version 18.1.8 (++20240718080534+3b5b5c1ec4a3-1~exp1~20240718200641.143)
Target: x86_64-pc-linux-gnu

tests/test_numpy_dtypes.cpp:212:5: warning: switching on non-enum value without default case may not cover all cases [bugprone-switch-missing-default-case]

* Add clang-17 and clang-18 testing.

* Add `NOLINTNEXTLINE(clang-analyzer-optin.core.EnumCastOutOfRange)` in test_tagbased_polymorphic.cpp

Debian clang version 18.1.8 (++20240718080534+3b5b5c1ec4a3-1~exp1~20240718200641.143)
Target: x86_64-pc-linux-gnu

tests/test_tagbased_polymorphic.cpp:77:40: warning: The value '150' provided to the cast expression is not in the valid range of values for 'Kind' [clang-analyzer-optin.core.EnumCastOutOfRange]

* Fix inconsistent pybind11/eigen/tensor.h behavior:

This existing comment in pybind11/eigen/tensor.h

```
                // move, take_ownership don't make any sense for a ref/map:
```

is at odds with the `delete src;` three lines up.

In real-world client code `take_ownership` will not exist (unless the client code is untested and unused). I.e. the `delete` is essentially only useful to avoid leaks in the pybind11 unit tests.

While upgrading to clang-tidy 18, the warning below appeared. Apparently it is produced during LTO, and it appears difficult to suppress. Regardless, the best way to resolve this is to remove the `delete` and to simply make the test objects `static` in the unit test code.

________

Debian clang version 18.1.8 (++20240718080534+3b5b5c1ec4a3-1~exp1~20240718200641.143)
Target: x86_64-pc-linux-gnu
________

```
lto-wrapper: warning: using serial compilation of 3 LTRANS jobs
lto-wrapper: note: see the ‘-flto’ option documentation for more information
In function ‘cast_impl’,
    inlined from ‘cast’ at /mounted_pybind11/include/pybind11/eigen/tensor.h:414:25,
    inlined from ‘operator()’ at /mounted_pybind11/include/pybind11/eigen/../pybind11.h:296:40,
    inlined from ‘_FUN’ at /mounted_pybind11/include/pybind11/eigen/../pybind11.h:267:21:
/mounted_pybind11/include/pybind11/eigen/tensor.h:475:17: warning: ‘operator delete’ called on unallocated object ‘<anonymous>’ [-Wfree-nonheap-object]
  475 |                 delete src;
      |                 ^
/mounted_pybind11/include/pybind11/eigen/../pybind11.h: In function ‘_FUN’:
/mounted_pybind11/include/pybind11/eigen/../pybind11.h:297:75: note: declared here
  297 |                     std::move(args_converter).template call<Return, Guard>(cap->f),
      |                                                                           ^
```

* Disable `bugprone-chained-comparison`: this clang-tidy check is incompatible with the Catch2 `REQUIRE` macro (26 warnings like the one below).
________

Debian clang version 18.1.8 (++20240718080534+3b5b5c1ec4a3-1~exp1~20240718200641.143)
Target: x86_64-pc-linux-gnu
________

```
/mounted_pybind11/tests/test_embed/test_interpreter.cpp:127:9: warning: chained comparison 'v0 <= v1 == v2' may generate unintended results, use parentheses to specify order of evaluation or a logical operator to separate comparison expressions [bugprone-chained-comparison]
  127 |         REQUIRE(ret == 42);
      |         ^
/build/tests/catch/catch.hpp:17670:24: note: expanded from macro 'REQUIRE'
 17670 | #define REQUIRE( ... ) INTERNAL_CATCH_TEST( "REQUIRE", Catch::ResultDisposition::Normal, __VA_ARGS__  )
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/build/tests/catch/catch.hpp:2710:47: note: expanded from macro 'INTERNAL_CATCH_TEST'
 2710 |             catchAssertionHandler.handleExpr( Catch::Decomposer() <= __VA_ARGS__ ); \
      |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mounted_pybind11/tests/test_embed/test_interpreter.cpp:127:9: note: operand 'v0' is here
  127 |         REQUIRE(ret == 42);
      |         ^
/build/tests/catch/catch.hpp:17670:24: note: expanded from macro 'REQUIRE'
 17670 | #define REQUIRE( ... ) INTERNAL_CATCH_TEST( "REQUIRE", Catch::ResultDisposition::Normal, __VA_ARGS__  )
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/build/tests/catch/catch.hpp:2710:47: note: expanded from macro 'INTERNAL_CATCH_TEST'
 2710 |             catchAssertionHandler.handleExpr( Catch::Decomposer() <= __VA_ARGS__ ); \
      |                                               ^~~~~~~~~~~~~~~~~~~
/mounted_pybind11/tests/test_embed/test_interpreter.cpp:127:17: note: operand 'v1' is here
  127 |         REQUIRE(ret == 42);
      |                 ^
/build/tests/catch/catch.hpp:17670:90: note: expanded from macro 'REQUIRE'
 17670 | #define REQUIRE( ... ) INTERNAL_CATCH_TEST( "REQUIRE", Catch::ResultDisposition::Normal, __VA_ARGS__  )
       |                                                                                          ^~~~~~~~~~~
/build/tests/catch/catch.hpp:2710:70: note: expanded from macro 'INTERNAL_CATCH_TEST'
 2710 |             catchAssertionHandler.handleExpr( Catch::Decomposer() <= __VA_ARGS__ ); \
      |                                                                      ^~~~~~~~~~~
/mounted_pybind11/tests/test_embed/test_interpreter.cpp:127:24: note: operand 'v2' is here
  127 |         REQUIRE(ret == 42);
      |                        ^
/build/tests/catch/catch.hpp:17670:90: note: expanded from macro 'REQUIRE'
 17670 | #define REQUIRE( ... ) INTERNAL_CATCH_TEST( "REQUIRE", Catch::ResultDisposition::Normal, __VA_ARGS__  )
       |                                                                                          ^~~~~~~~~~~
/build/tests/catch/catch.hpp:2710:70: note: expanded from macro 'INTERNAL_CATCH_TEST'
 2710 |             catchAssertionHandler.handleExpr( Catch::Decomposer() <= __VA_ARGS__ ); \
      |                                                                      ^~~~~~~~~~~
```

* Add 8 `// NOLINT(bugprone-empty-catch)`

* Resolve clang-tidy `bugprone-multi-level-implicit-pointer-conversion` warnings.
________
Debian clang version 18.1.8 (++20240718080534+3b5b5c1ec4a3-1~exp1~20240718200641.143)
Target: x86_64-pc-linux-gnu
________
```
pybind11/detail/internals.h:556:53: warning: multilevel pointer conversion from 'internals **' to 'const void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]

pybind11/detail/type_caster_base.h:431:20: warning: multilevel pointer conversion from 'void **' to 'void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]

pybind11/numpy.h:904:81: warning: multilevel pointer conversion from '_object *const *' to 'const void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]
pybind11/numpy.h:1989:39: warning: multilevel pointer conversion from 'typename vectorize_arg<const double *>::type *' (aka 'const double **') to 'void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]
pybind11/numpy.h:1989:39: warning: multilevel pointer conversion from 'typename vectorize_arg<const VectorizeTestClass *>::type *' (aka 'const VectorizeTestClass **') to 'void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]

pybind11/stl/filesystem.h:75:44: warning: multilevel pointer conversion from 'PyObject **' (aka '_object **') to 'void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]
pybind11/stl/filesystem.h:83:42: warning: multilevel pointer conversion from 'PyObject **' (aka '_object **') to 'void *', please use explicit cast [bugprone-multi-level-implicit-pointer-conversion]
```

* Introduce `PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY` to resolve PyPy build errors:

```
In file included from /Users/runner/work/pybind11/pybind11/tests/test_stl.cpp:18:
/Users/runner/work/pybind11/pybind11/include/pybind11/stl/filesystem.h:75:17: error: no matching function for call to 'PyPyUnicode_FSConverter'
            if (PyUnicode_FSConverter(buf, reinterpret_cast<void *>(&native)) != 0) {
                ^~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:969:31: note: expanded from macro 'PyUnicode_FSConverter'
                              ^~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:970:17: note: candidate function not viable: cannot convert argument of incomplete type 'void *' to 'struct _object **' for 2nd argument
PyAPI_FUNC(int) PyUnicode_FSConverter(struct _object *arg0, struct _object **arg1);
                ^
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:969:31: note: expanded from macro 'PyUnicode_FSConverter'
                              ^
In file included from /Users/runner/work/pybind11/pybind11/tests/test_stl.cpp:18:
/Users/runner/work/pybind11/pybind11/include/pybind11/stl/filesystem.h:83:17: error: no matching function for call to 'PyPyUnicode_FSDecoder'
            if (PyUnicode_FSDecoder(buf, reinterpret_cast<void *>(&native)) != 0) {
                ^~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:971:29: note: expanded from macro 'PyUnicode_FSDecoder'
                            ^~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:972:17: note: candidate function not viable: cannot convert argument of incomplete type 'void *' to 'struct _object **' for 2nd argument
PyAPI_FUNC(int) PyUnicode_FSDecoder(struct _object *arg0, struct _object **arg1);
                ^
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:971:29: note: expanded from macro 'PyUnicode_FSDecoder'
                            ^
```

* clang-tidy auto-fix

* Fix silly oversight.