[Bug libstdc++/90105] std::forward_list::sort() is not "stable"

[stoyanovmk at ornl dot gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90105

--- Comment #2 from stoyanovmk at ornl dot gov ---
Tested the fix provided by Jonathan Wakely, I can confirm the fix.

Ran several tests with the included small example and the code where I found
the issue in the first place.(In reply to Jonathan Wakely from comment #1)
> I think this is the fix:
> 
> --- a/libstdc++-v3/include/bits/forward_list.tcc
> +++ b/libstdc++-v3/include/bits/forward_list.tcc
> @@ -469,9 +469,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>                         __p = static_cast<_Node*>(__p->_M_next);
>                         --__psize;
>                       }
> -                   else if (__comp(*__p->_M_valptr(), *__q->_M_valptr()))
> +                   else if (!__comp(*__q->_M_valptr(), *__p->_M_valptr()))
>                       {
> -                       // First node of p is lower; e must come from p.
> +                       // First node of q is not lower; e must come from p.
>                         __e = __p;
>                         __p = static_cast<_Node*>(__p->_M_next);
>                         --__psize;

Tested the fix, I can confirm it works.

Ran several tests with the included small example and the code where I found
the issue in the first place.