clarification on recursive application

[cueckoo]

Originally opened by @proppy in cuelang/cue#510

Starting with https://cue-review.googlesource.com/c/cue/+/6522 the following is resulting rightfully in a structural cycle error:

#sum: {
  A=arg: [...int]
  if len(A) > 1 {
    result: A[0] + (#sum & {arg: A[1:]}).result
  }
  if len(A) == 1 {
    result: A[0]
  }
}

sum1: (#sum & {arg: [1,2,3,4]}).result

But this doesn't:

#box: {
  V=value: int
  sum: {
    A=arg: [...#box]
    if len(A) > 1 {
      result: V + (A[0].sum & {arg: A[1:]}).result
    }
    if len(A) == 1 {
      result: V + A[0].value
    }
  }
}

box1: [...#box] & [for a in [1,2,3,4] {value: a}]
sum1: (box1[0].sum & {arg: box1[1:]}).result

And I'm not exactly clear if that's the expected behavior.

Is that fine to evaluate something recursively as long as we keep referring to different sub-struct unification, or should a bug should be filed against structural cycle detection.