I don't think copy-on-write will prevent a copy here, since the copy is being written to inside of sorted. I don't think the compiler is smart enough to elide the copy, either.
It definitely can and does work for similar situations in other languages. It’s fragile though as aliasing accidentally somewhere or adding other kinds of uncertainty around which values are returned makes sinking the return allocation into the caller, a required prerequisite, much more likely to fail.
A good way to imagine this is having return value optimizations give you a copy which is placed on the original which allows the work to be skipped. But that can require a whole lot of other trades around calling conventions, optimization boundaries and so on. C++ has dealt with some of this complexity recently but it’s nuances too years to sort out between standard revisions and only became required in some cases rather than legal until after compilers had plenty of time to work on it.
Yeah, I don't doubt it if Clang can do this optimization for C++, but I don't think the Swift optimizer is quite there yet since it needs to peer through many more layers of complexity.
