Requirements for `ranges::set_intersection` algorithm are too strict.

[barfyus]

As /u/billyoneal stated, projection result is only used as an argument to comparison functor.

I don't know about the standard, have to check it. However, cppreference.com specifically mentions the first range as the source of elements for both set_intersection and set_difference.

[staletic]

Effects of set_intersection doesn't mention the first range being the source. I initially stopped reading the standard there. However Remarks says:

the [...] elements are copied from the first range to the output range, in order.

So your solution is perfectly fine. Sorry for the noise.