[Bug libstdc++/68197] negative index to ios_base::iword lead to unpredictable result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68197

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I would argue that your program has undefined behaviour, there is no array
element at a negative index.