泛型辅助函数应输出 `invoice`，实际却输出 `default`。找出契约错误。
swift
protocol Labeled {}
extension Labeled {
func label() -> String { "default" }
}
struct Invoice: Labeled {
func label() -> String { "invoice" }
}
func show<Value: Labeled>(_ value: Value) {
print(value.label())
}
show(Invoice())