https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68737
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And the previous line an alloca call using __len:
_CharT* __ws = static_cast<_CharT*>(__builtin_alloca(sizeof(_CharT)
* __len));
__ctype.widen(__cs, __cs + __len, __ws);
The code assumes __convert_from_v always returns a valid length, but it seems
to be failing and returning -1.
