a lot of cool-looking stuff in my ECS is supported by Swift parameter packs; however, once you start using them a lot you find limits pretty soon.

One example: the following wouldn't compile in swift:

  func query<each T, each K>(
    _ body: ((repeat each T), (repeat each K)) -> Void
  ) { ... }

edit: the example is oversimplified just to show the point - in this example compiler can't really tell where T ends and K starts, so its logical; but I had more complex cases where a human would've been able to infer types correctly