Unfortunately, C++ uses ++ and -- for iterators, many of which cannot reasonably implement += or -=. This distinction is baked into the type system to tell whether or not an iterator supports efficient "multiple advance" (e.g. a linked list iterator doesn't have += but a pointer into a contiguous vector does).

There's no way to fix this in a reverse-compatible way for existing code (which is one of the constraints of cpp2- it must work with all existing C++ so that it is possible for existing projects to migrate regardless of size).