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

[glisse at gcc dot gnu.org]

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

            Bug ID: 91788
           Summary: std::variant index +1-1
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

(this is a detail, it probably has a negligible impact)

      constexpr size_t index() const noexcept
      { return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; }

IIUC, the whole +1-1 is here so that for a valueless variant, index_type(-1)
becomes size_t(-1). I think there are cases where we could do better. For
instance, for a never valueless type, we could just return _M_index. If there
are fewer than 128 alternatives, we could use a sign extension: "return (signed
char)_M_index;". Maybe some well-placed __builtin_unreachable to specify the
range of _M_index would work as well.