Re: [committed] libstdc++: Fix constexpr functions in <experimental/internet>

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 30 Mar 2023 at 00:40, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested powerpc64le-linux. Pushed to trunk.
>
> -- >8 --
>
> Change ip::basic_endpoint to work in constant expressions, but only for
> C++20 and later (due to the use of a union, which cannot change active
> member in constexpr evaluation until C++20).
>
> During constant evaluation we cannot inspect the common initial sequence
> of basic_endpoint's union members to check whether sin_family == AF_INET
> or AF_INET6.  This means we need to store an additional boolean member
> that remembers whether we have a v4 or v6 address. The address type can
> change behind our backs if a user copies an address to the data()
> pointer and then calls resize(n), so we need to inspect the sa_family_t
> member in the union after a resize and update the boolean. POSIX only
> guarantees that the sa_family_t member of each protocol-specific address
> structure is at the same offset and of the same type, not that there is
> a common initial sequence. The check in resize is done using memcmp, so
> that we avoid accessing an inactive member of the union if the
> sockaddr_in and sockaddr_in6 structures do not have a common initial
> sequence that includes the sa_family_t member.

If we had https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2641r2.html
then we wouldn't need the extra boolean. During constant evaluation we
could use the new magic function to check which union member is
active, and during runtime we could inspect the sa_family_t member
(still using memcmp, because POSIX doesn't guarantee a common initial
sequence, even though I think everybody does give it one in practice).