1. You cannot return anything (say an immutable result that has consumed the input)

Okay, so don't return anything, just mutate the original. Except:

2. You cannot mutate the original, return nothing, but the mutated original isn't a subset of the original. For example: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABB...