Swift 协议(protocol)扩展 `var { get }` 覆盖实现的 `let`

Question

当将实现结构分配给类型为 protocol 的变量时，使用 var 和 let 实现属性的行为不同。

protocol Req {
  var path: String? { get }
}
extension Req {
  var path: String? { return "Req" }
}

struct LetReq: Req {
  let path = "LetReq"
}
struct VarReq: Req {
  var path: String? { return "VarReq" }
}

var req: Req!

req = VarReq()
req.path // prints "VarReq"

req = LetReq()
req.path // prints "Req" not "LetReq" which seems very awkward.

这是 Swift 设计的行为吗？

Answer 1

我认为这是一个编译器错误。如果您将 LetReq 设为可选字符串，它会按预期工作:

struct LetReq: Req {
    let path: String? = "LetReq"
}

File a bug与 Apple。