为什么私有(set)在Swift中不起作用？

来自Apple文档:

"上面的每个访问级别修饰符都可以选择接受一个参数,该参数由括在括号中的关键字集合组成(例如,private(set)).如果要为变量或下标的setter指定一个小于或等于变量或下标本身的访问级别的访问级别,请使用此形式的访问级别修饰符,如Getters和Setters中所述."

摘录自:Apple Inc."The Swift Programming Language."iBooks. https://itun.es/ru/jEUH0.l

我尝试在Playground中测试的一个例子:

import UIKit


class A {
  private(set) var name: String {
  get { return "Hello, \(self.name)" }
  set { self.name = "Unknown" }
  }

  init(_ name:String) {
    self.name = name
  }
}

let a = A("Andrew")
a.name = "Hello"

我在控制台输出中得到的错误:

Playground execution failed: error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=2, address=0x7fff5056eff8).
The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.
* thread #1: tid = 0xea721, 0x00000001104f308c libsystem_pthread.dylib`__mtx_droplock + 222, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x7fff5056eff8)
  * frame #0: 0x00000001104f308c libsystem_pthread.dylib`__mtx_droplock + 222
    frame #1: 0x00000001104f2f07 libsystem_pthread.dylib`pthread_mutex_unlock + 68
    frame #2: 0x000000010ffbd2b5 libc++.1.dylib`std::__1::mutex::unlock() + 9
    frame #3: 0x000000010f040b94 libswift_stdlib_core.dylib`swift_getGenericMetadata + 260
    frame #4: 0x000000010ef28a24 libswift_stdlib_core.dylib`Swift.IndexingGenerator.init (Swift.IndexingGenerator.Type)(A) -> Swift.IndexingGenerator + 164
    frame #5: 0x000000010ef55f1a libswift_stdlib_core.dylib`protocol witness for Swift._Sequence_.generate (@inout Swift._Sequence_.Self)() -> Swift._Sequence_.Self.GeneratorType in conformance Swift._ContiguousArrayBuffer : Swift._Sequence_ + 154
    frame #6: 0x000000010ef284d5 libswift_stdlib_core.dylib`Swift._copyCollectionToNativeArrayBuffer >(A) -> Swift._ContiguousArrayBuffer + 1061
    frame #7: 0x000000010ef40281 libswift_stdlib_core.dylib`Swift.Array.convertFromArrayLiteral (Swift.Array.Type)(Swift.Array...) -> Swift.Array + 641
    frame #8: 0x000000010f1eaae4 PlaygroundLogger`Swift.UInt64.toBytes (Swift.UInt64)() -> Swift.Array + 292
    frame #9: 0x000000010f1eb6a4 PlaygroundLogger`protocol witness for PlaygroundLogger.ToBytes.toBytes (@inout PlaygroundLogger.ToBytes.Self)() -> Swift.Array in conformance Swift.UInt64 : PlaygroundLogger.ToBytes + 20
    frame #10: 0x000000010f1dbe3d PlaygroundLogger`PlaygroundLogger.BytesStream.write (PlaygroundLogger.BytesStream)(PlaygroundLogger.ToBytes) -> PlaygroundLogger.BytesStream + 77
    frame #11: 0x000000010f1dbd74 PlaygroundLogger`PlaygroundLogger.BytesStream.write (PlaygroundLogger.BytesStream)(Swift.String) -> PlaygroundLogger.BytesStream + 164
    frame #12: 0x000000010f20f04b PlaygroundLogger`PlaygroundLogger.PlaygroundWriter.encode_config_info (PlaygroundLogger.PlaygroundWriter)() -> () + 203
    frame #13: 0x000000010f20f2bf PlaygroundLogger`PlaygroundLogger.PlaygroundWriter.encode_header (PlaygroundLogger.PlaygroundWriter)() -> () + 127
    frame #14: 0x000000010f20ecda PlaygroundLogger`PlaygroundLogger.PlaygroundScopeWriter.encode_scope_event (PlaygroundLogger.PlaygroundScopeWriter)(PlaygroundLogger.ScopeEvent) -> () + 58
    frame #15: 0x000000010f1eb997 PlaygroundLogger`playground_log_scope_entry + 87
    frame #16: 0x000000011ae20771

struct TrackedString {
  private(set) var numberOfEdits = 0
  var value: String = "" {
  didSet {
    numberOfEdits++
  }
  }
}

var stringToEdit = TrackedString()
stringToEdit.value = "Hello"
stringToEdit
stringToEdit.numberOfEdits += 10
stringToEdit