swift - 为什么我必须执行 append(contentsOf newElements: S) in my custom RangeReplaceableCollection

Question

我目前正在尝试将自定义集合类型更新为 Swift 4.1。但是，当我遵守文档并实现 Collection 的所有要求时和 RangeReplaceableCollection , Xcode 还在提示我的类型不符合 RangeReplaceableCollection .

Here's一个mcve对于这个问题(由 Hamish 慷慨提供，谢谢你:)

class Foo<Element : AnyObject> {
  required init() {}
  private var base: [Element] = []
}

extension Foo : Collection {
  typealias Index = Int

  var startIndex: Index {
    return base.startIndex
  }

  var endIndex: Index {
    return base.endIndex
  }

  func index(after i: Index) -> Index {
    return base.index(after: i)
  }

  subscript(index: Index) -> Element {
    return base[index]
  }
}

extension Foo : RangeReplaceableCollection {
  func replaceSubrange<C : Collection>(
    _ subrange: Range<Index>, with newElements: C
  ) where Element == C.Element {}
}

According to the documentation ，代码应该编译:

To add RangeReplaceableCollection conformance to your custom collection, add an empty initializer and the replaceSubrange(_:with:) method to your custom type. RangeReplaceableCollection provides default implementations of all its other methods using this initializer and method.

不幸的是，事实并非如此。相反，Xcode 发出以下错误消息:

// error: type 'Foo<Element>' does not conform to protocol 'RangeReplaceableCollection'
// extension Foo : RangeReplaceableCollection {
// ^
//   Swift.RangeReplaceableCollection:5:26: note: candidate has non-matching type '<Self, S> (contentsOf: S) -> ()' [with SubSequence = Foo<Element>.SubSequence]
//   public mutating func append<S>(contentsOf newElements: S) where S : Sequence, Self.Element == S.Element
//                        ^
//   Swift.RangeReplaceableCollection:9:26: note: protocol requires function 'append(contentsOf:)' with type '<S> (contentsOf: S) -> ()'; do you want to add a stub?
//   public mutating func append<S>(contentsOf newElements: S) where S : Sequence, Self.Element == S.Element
//   

为了确保这不是文档中的错误，我检查了 the source code of Swift 4.1并找到了 func append<S>(contentsOf newElements: S) where S: Sequence, Element == S.Element 的默认实现在 RangeReplaceableCollection.swift 中，第 442-452 行:

@_inlineable
public mutating func append<S : Sequence>(contentsOf newElements: S) where S.Element == Element {
  let approximateCapacity = self.count + numericCast(newElements.underestimatedCount)
  self.reserveCapacity(approximateCapacity)
  for element in newElements {
    append(element)
  }
}

---

问题:

• 为什么 Xcode 要求实现此功能，尽管提供了默认实现？
• 如何编译我的代码？

Answer 1

尽管提供了默认实现，但为什么 Xcode 要求实现此功能？

这是由于 Swift 4.1 中引入的 TypeChecker 中的一个错误:
[SR-7429]: Cannot conform a non-final class to a protocol with a defaulted requirement with a generic placeholder constrained to an associated type

如何编译我的代码？

在错误修复之前，目前有三种方法可以编译它。

• 实现这两个功能:

  • required init<S : Sequence>(_ elements: S) where Element == S.Element {}
  • func append<S : Sequence>(contentsOf newElements: S) where Element == S.Element

• 将类标记为 final .

• 将集合类型实现为 struct .为此，required init() {}必须删除。