[BUG] recursive cast in type_caster of non-class type causes a segfault.

I have discussed this issue on gitter and it appears it might be a bug.

I've created a public github repos and a branch called feature/pybind11_cast_bugthat recreates the problem.
https://github.com/awr-paul/pybind11_helloworld/tree/feature/pybind11_cast_bug

Essentially if you use a non-class type with the pybind11::type_caster<> cast can get called recursively and segfault (at least when numpy is imported).

The problem, manifests itself here:
https://github.com/awr-paul/pybind11_helloworld/blob/feature/pybind11_cast_bug/src/pybind11_helloworld/submodule/main.cpp#L71-72

and can be recreated like this:

>>> from pybind11_helloworld.submodule import sometime
>>> sometime()
Segmentation fault (core dumped)

However, if a class type is used instead of a primitive then the code works fine (as demonstrated on the main branch).
https://github.com/awr-paul/pybind11_helloworld/blob/main/src/pybind11_helloworld/submodule/main.cpp#L75

If the pybind11::type_caster<> does not support non-class types then this should be documented and a helpful compile error message could be added to the type_caster rather than allowing it to compile and segfault:

static_assert(std::is_class<T>::value, "pybind11::type_caster only supports class types");

However, I suspect the issue is another bug, possibly with how pybind11 interacts with the numpy ABI?

Start by reproducing the crash with the linked feature/pybind11_cast_bug branch, importing numpy and calling sometime(), then inspect src/pybind11_helloworld/submodule/main.cpp around lines 71–72 and compare the class case around line 75. Determine whether non-class pybind11::type_caster<> inputs are supported; done means the example no longer segfaults and the supported behavior is documented or diagnosed at compile time.