编译器无法在合理的时间内对这个表达式进行类型检查（XCode 12 Beta 1，swift）

Question

尝试使用xcode 12 beta 1构建以下快速代码，但收到错误消息“编译器无法在合理的时间内对该表达式进行类型检查；请尝试将该表达式分解为不同的子表达式”。

知道发生了什么吗？

class UICommandRouter {
  private typealias CommandHandler = (_: Data?) -> Void
  private var builtInHandlers: [String: CommandHandler] = [:]

  init() {
    builtInHandlers = [
        "a": { [weak self] in self?.handleCommandA(data: $0);},
        "b": { [weak self] in self?.handleCommandB(data: $0);},

        "c": { [weak self] in self?.handleCommandC(data: $0);},
        "d": { [weak self] in self?.handleCommandD(data: $0);},
        "e": { [weak self] in self?.handleCommandE(data: $0);},

        "f": { [weak self] in self?.handleCommandF(data: $0);},
        "g": { [weak self] in self?.handleCommandG(data: $0);},
        "h": { [weak self] in self?.handleCommandH(data: $0);},
        "i": { [weak self] in self?.handleCommandI(data: $0);},
        "j": { [weak self] in self?.handleCommandJ(data: $0);},

        "k": { [weak self] in self?.handleCommandK(data: $0);},
        "l": { [weak self] in self?.handleCommandL(data: $0);},
    ]
  }
 
  private func handleCommandA(data _: Data?) {}

  private func handleCommandB(data _: Data?) {}

  private func handleCommandC(data _: Data?) {}

  private func handleCommandD(data _: Data?) {}

  private func handleCommandE(data _: Data?) {}

  private func handleCommandF(data _: Data?) {}

  private func handleCommandG(data _: Data?) {}

  private func handleCommandH(data _: Data?) {}

  private func handleCommandI(data _: Data?) {}

  private func handleCommandJ(data _: Data?) {}

  private func handleCommandK(data _: Data?) {}

  private func handleCommandL(data _: Data?) {}
}