Sadly the 25-line original code isn't presented; the code that is presented is the 5-line replacement using the STL's `find_if` and `rotate`. Bjarne sketches the idea that those five lines can be further condensed into two lines with the non-STL `gather` algorithm:
auto dest = std::find_if(v.begin(), v.end(), contains(p));
stdx::gather(v.begin(), dest, v.end(), [](const auto& elt) { return &elt == &*source; });
But this is overkill — replacing an O(distance(source,dest)) non-allocating rotate with an O(v.size()) potentially-allocating stable_partition — and more importantly it re-complicates the code.
Now, I think part of his point is that `stable_partition` is "simpler" than `gather` only because it's in the STL. If we add `gather` to the STL too and everyone learns what it means, then there's no objection to using `gather` for "simplification" like this: it would be a straightforward simplification in almost the same way that `std::equal_range(first, last, x)` is a straightforward simplification of `std::make_pair(std::lower_bound(first, last, x), std::upper_bound(first, last, x))`.
The "almost" is that actually there is an algorithmic advantage to `std::equal_range`: when you're looking for the upper bound, you don't have to consider any of the elements to the left of the lower bound you already found. You get a (very slight) performance boost by using the combined `equal_range` algorithm. `gather`, on the other hand, has no such advantage; and (as we've seen) has a (very slight) performance disadvantage when compared to the `rotate` that Bjarne's correspondent's code actually required.
We're not talking about replacing 25 lines of bespoke code with 1 line of Boost `gather`; we're talking about replacing 2 lines of STL `stable_partition` with 1 line of Boost `gather`. The former is probably worth it. The latter is not.
It may be just two stable partitions, but “just” is doing a lot of work there. The algorithm becomes obvious once someone has identified it.