[Bug libstdc++/78830] std::prev accepts ForwardIterator-s

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78830

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There is an LWG issue requesting clarification in the standard:
https://wg21.link/lwg3197

Option B is consistent with the interpretation of libstdc++ (and recent
versions of libc++). If Option A or C is accepted, I will change libstdc++
accordingly. Until then, I maintain that we conform to the standard as
currently written.