1

Have a helper method:

extension MutableCollection {
  fileprivate mutating func overwrite<I: IteratorProtocol>(
    prefixUsing source: inout I,
    _ transform: (I.Element) -> Element
  ) -> Index {
    var current = startIndex
    let end = endIndex
    while current < end, let seed = source.next() {
      self[current] = transform(seed)
      formIndex(after: &current)
    }
    return current
  }
}

Which is used by:

extension MutableCollection {
  @discardableResult
  public mutating func overwrite<I: IteratorProtocol>(
    prefixUsing source: inout I
  ) -> Index where I.Element == Element {
    return overwrite(prefixUsing: &source, { $0 })
  }
}

Instead of "{ $0 }," I wanted to use "\Element.self," but the compiler said there was no conversion between the (writable) key path and an (Element) -> Element function. Using the latest Xcode. Isn't this supposed to work? (I added an "as KeyPath" once, and it made no difference, so the problem isn't the mutability.)

2

It's probably this bug:

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