[BUG]: std::move_constructible and pybind11::detail::is_move_constructible differ

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

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

3.0.1

Problem description

I'm not sure this is a bug.

Must std::move_constructible concept and pybind11::is_move_constructible evaluate the same?

When a struct has a value_type member typedef set to void the two predicates differ (cf. code below).
I stumbled into this problem using CGAL (cf. https://github.com/CGAL/cgal/issues/9319).

Reproducible example code

#include <concepts>
#include <pybind11/pybind11.h>

struct A {
    typedef int value_type;
};
static_assert(std::move_constructible<A>); // OK
static_assert(pybind11::detail::is_move_constructible<A>::value); // OK

struct B {
    typedef void value_type;
};
static_assert(std::move_constructible<B>); // OK
static_assert(pybind11::detail::is_move_constructible<B>::value); // fails

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

Not a regression

Start with the reproducible static assertions and trace the definition of pybind11::detail::is_move_constructible included through pybind11/pybind11.h. Compare its behavior with std::move_constructible for the A and B examples, then inspect existing trait tests if present. Done means the differing behavior is either aligned with the standard concept or clearly justified and covered by a regression test.