[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 #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Wed Apr 17 21:47:20 2019
New Revision: 270427

URL: https://gcc.gnu.org/viewcvs?rev=270427&root=gcc&view=rev
Log:
PR libstdc++/90105 make forward_list::sort stable

While testing the fix I also discovered that operator== assumes the
elements are comparable with operator!= which is not required.

        PR libstdc++/90105
        * include/bits/forward_list.h (operator==): Do not use operator!= to
        compare elements.
        (forward_list<T, A>::sort(Comp)): When elements are equal take the one
        earlier in the list, so that sort is stable.
        * testsuite/23_containers/forward_list/operations/90105.cc: New test.
        * testsuite/23_containers/forward_list/comparable.cc: Test with
        types that meet the minimum EqualityComparable and LessThanComparable
        requirements. Remove irrelevant comment.

Added:
    trunk/libstdc++-v3/testsuite/23_containers/forward_list/operations/90105.cc
      - copied, changed from r270425,
trunk/libstdc++-v3/testsuite/23_containers/forward_list/comparable.cc
Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/bits/forward_list.tcc
    trunk/libstdc++-v3/testsuite/23_containers/forward_list/comparable.cc