[BUG]: slow compile if very long std::array being used with clang-18

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

3.0.0

Problem description

I've found that pybind11 compile time very slow if very long std::array being used

clang-18 spends 90 seconds to finish, clang-20 11 seconds, g++-13 13 seconds.

tried 2.16.3/3.0.0 both compile slow with clang-18. clang ftime-trace shows that most of time spent in codegen

for function

std::array<int, 102400ul> pybind11::detail::vector_to_array_impl<std::array<int, 102400ul>, std::vector<int, std::allocator<int>>, 0ul, 1ul, 2ul, 3ul, 4ul, 5ul, 6ul, 7ul, 8ul, 9ul, 10ul, 11ul, ....

I've created a repo to show this issue https://github.com/comicfans/pybind11_slow_clang/tree/main

clang-18 compile trace (which can be loaded into chrome://tracing)
basic.cpp.json

Reproducible example code

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

struct SlowType {
  std::array<int, 102400> member{};
};

PYBIND11_MODULE(basic, module) {
  py::class_<SlowType>(module, "SlowType")
      .def(py::init<>())
      .def_readwrite("member", &SlowType::member);
}

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

Not a regression

Start with the minimal basic.cpp reproducer and the vector_to_array_impl instantiation reached through pybind11/stl.h; compare clang-18 with clang-20 and g++-13 while inspecting the supplied ftime trace. Done means the cause of the long std::array compile time is identified and a validated fix or documented limitation is agreed.