[Bug libstdc++/100290] New: join_view::_Iterator::operator++ copies _M_parent->_M_inner when _S_ref_is_glvalue is false

[rs2740 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100290

            Bug ID: 100290
           Summary: join_view::_Iterator::operator++ copies
                    _M_parent->_M_inner when _S_ref_is_glvalue is false
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

auto&& __inner_range = [this] () -> decltype(auto) {
              if constexpr (_S_ref_is_glvalue)
                return *_M_outer;
              else
                return _M_parent->_M_inner;
            }();

This is a copy in the else case due to decltype(auto) producing the declared
type of _M_inner. The lambda should return auto&&.