泛型辅助函数应输出`invoice`，实际却输出`default`。找出契约错误。
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())