{
  modularFlake,
  ...
}:
let
  ov = final: prev: { __canary = "meowmeow"; };
in
{
  outputs = with modularFlake.defaultOutputs; [ overlays ];
  modules = [
    # fine to expect the system argument
    ({ system, ... }: {
      config = {
        # but not use it (note it is not used here)
        overlays.a = builtins.seq (
          assert 2 + 2 == 4;
          null
        ) ov;
      };
    })
  ];
  # should pass
  fails = false;
  check =
    produced:
    let
      isFunction = val: builtins.isFunction val || (builtins.isAttrs val && val ? __functor);
      ov1 = produced.overlays.a;
    in
    isFunction ov1 && isFunction (ov1 { }) && (ov1 { } { } == ov { } { });
}