[Bug tree-optimization/101923] std::function's move ctor is slower than the copy one for empty source objects

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101923

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We can do better than just making the swap conditional:

      function(function&& __x) noexcept
      : _Function_base(), _M_invoker(__x._M_invoker)
      {
        if (static_cast<bool>(__x))
          {
            _M_functor = __x._M_functor;
            _M_manager = __x._M_manager;
            __x._M_manager = nullptr;
            __x._M_invoker = nullptr;
          }
      }