[Bug libstdc++/98677] std::regex constructor triggers valgrind under clang++ with undefined sanitizer; possible use-after-move

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98677

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think the use-after-move is the problem, because only the members of
the _State_base class are accessed after the move, and they are unaffected by
the move.

The move constructor does this:

      _State(_State&& __rhs) : _State_base(__rhs)
      {
        if (__rhs._M_opcode() == _S_opcode_match)
          new (this->_M_matcher_storage._M_addr())
            _MatcherT(std::move(__rhs._M_get_matcher()));
      }

So the _State_base::_M_opcode, _State_base::_M_alt and _State_base::_M_next
members of __rhs are not modified, and can be safely accessed after the move.