[PATCH] PR libstdc++/90105 make forward_list::sort stable

[Jonathan Wakely <jwakely at redhat dot com>]

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.

Tested powerpc64le-linux.

I'm surprised nobody has noticed either of these bugs before! I think
this is safe for stage 4, and for backporting to active branches. Any
objections?

Attachment: patch.txt
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