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

[Simon-Lopez]

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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. CGAL/cgal#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

[Skylion007]

"- The newly added pybind11::detail::is_move_constructible trait can be
specialized for cases in which std::is_move_constructible does not
work as needed. This is very similar to the long-established
pybind11::detail::is_copy_constructible.
#4631"

So no, they explicitly don't evaluate the same. The question is should they in the case you described