Issue Description:

With a struct, one can override the default constructor to verify the data meets certain preconditions:

struct Content<T> {
  let list: [T]
  
  init(list: [T]) {
    precondition(!list.isEmpty)

    self.list = list
  }
}

However, the equivalent is not currently possible with enums:

enum Content<T> {
  case loading
  case loaded([T])
}

I'm not sure what the syntax for this would be, but I'd like to be able to detect an invalid definition of this enum at "creation time", rather than having these preconditions in a method with Content as a parameter.