jdoerrie added inline comments.
================
Comment at: include/algorithm:4471
_RBi(__middle), _RBi(__first),
_RBi(__last), __negate<_Compare>(__comp));
}
----------------
As marshall pointed out in https://bugs.llvm.org//show_bug.cgi?id=31166 this is
where the problem lies. While reversing the input ranges is a very nifty trick
to be able to use the moved out space at the end of `[first, last)`, simply
negating the comparison functor is not enough to preserve stability.
Assuming a default `__comp` of `std::less`, in line 4431 a negated `__comp`
will evaluate to true for equivalent elements, resulting in a move from
`*__first2` and thus breaking stability. The fix for this to to still have
`__comp` evaluate to false for equivalent elements, what the change in 748
accomplishes.
Given that now `__negate` technically does not negate anymore, maybe it should
be renamed to a more suiting `__reverse` or `__swap` or similar. Doing a quick
grep over the code base line 4471 seems to be the only place where this is
used, so a rename should not have further consequences. Furthermore, the unary
`bool operator()` could be deleted.
https://reviews.llvm.org/D32788
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