Am Do., 9. Sept. 2021 um 06:59 Uhr schrieb Jason McKesson via Std-Proposals <std-proposals@lists.isocpp.org>:
On Wed, Sep 8, 2021 at 2:29 PM D'Alessandro, Luke K via Std-Proposals
<std-proposals@lists.isocpp.org> wrote:
>
>
> I guess I have two questions:
>
> 1. Does std::construct_at change the active member of a union?

As stated in [class.union]/2, "a non-static data member is active if
its name refers to an object whose lifetime has begun and has not
ended." `construct_at` can begin the lifetime of objects (which can
also cause the lifetime of other overlapping objects to end).

Therefore, it can change the active union member.

While `std::construct_at` does indeed begin the lifetime of the element it constructs, the lifetime of the array has not begun. Therefore, forming a reference to an object outside of its lifetime is UB, so `&u.data[1]` is not possible. We had the exact same issue recently when implementing constexpr std::string for MSVC STL. There we have an indirection to `char_traits` and would need to call `char_traits::assign(&sso_storage[0], CharType())` which again is invalid due to forming of the reference to `sso_storage[0]` outside of its lifetime

If you have a non-assignable class then the best way around is an 
`
if constexpr(std::is_assignable_v) {
     // change the active member of the union to the array here.
}
std::construct_at(...)
`

Cheers
Michael