[Bug libstdc++/91788] std::variant index +1-1

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91788

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
Internally, it may also be possible to avoid calling index() so often and work
with the raw _M_index more often.