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

[redi at gcc dot gnu.org]

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Patch posted to https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00669.html