经过一些测试和调试（通过子类）后，我发现对于 CKRecord ， objectForKey：确实调用了 objectForKeyedSubscript：。另外，在隐式标记为 @objc 的Swift类中实现下标（通过从降级） NSObject ）或明确表示下标被实现为 objectForKeyedSubscript：。

After some testing and debugging (via a subclass), I discovered that, for CKRecord, objectForKey: does indeed call objectForKeyedSubscript:. Also, implementing subscript in a Swift class that is marked @objc implicitly (by descending from NSObject) or explicitly means that subscript is implemented as objectForKeyedSubscript:.

这意味着在扩展中的 CKRecord 上实现下标实现，这会导致堆栈溢出。

This means that implementing subscript on CKRecord in an extension hides the default implementation, which causes the stack overflow.