1

I'm trying to compile this example in a Playground page:

import Foundation


public struct HeartRate: Hashable {
  public let bpm: Double
  public let interval: DateInterval
}

@dynamicMemberLookup public struct HeartRateContext: Equatable {
  public enum Activity {
    case sleep
    case regular
    case workout
  }
  
  public let age: Int
  public let activity: Activity
  private let heartRate: HeartRate
  
  public init(age: Int, activity: Activity, heartRate: HeartRate) {
    self.age = age
    self.activity = activity
    self.heartRate = heartRate
  }
  
  public subscript<Value> (dynamicMember keyPath: KeyPath<HeartRate, Value>) -> Value {
    heartRate[keyPath: keyPath]
  }
}

let heartRate = HeartRate(bpm: 100, interval: DateInterval())
let heartRateContext = HeartRateContext(age: 30, activity: .workout, heartRate: heartRate)
print (heartRateContext.bpm)
print (heartRateContext.activity)

get this compile error:

error: DynamicMemberLookup.xcplaygroundpage:8:24: 'DateInterval' is only available in iOS 10.0 or newer; clients of '__lldb_expr_16' may have a lower deployment target
public let interval: DateInterval
^

DynamicMemberLookup.xcplaygroundpage:6:15: note: add @available attribute to enclosing struct
public struct HeartRate: Hashable {

There is no problem with using DateInterval in another page. So this is a problem with this code.

What's wrong?