自定义类型的Golang切片在另一个类型中作为参考

我的Go测试代码出现此错误：

$ go run test.go 
# command-line-arguments
./test.go:43: cannot use &ol1 (type *Orderline) as type Orderline in array element
./test.go:43: cannot use &ol2 (type *Orderline) as type Orderline in array element

码

package main

import (
    "fmt"
)

type Customer struct {
    Id int64
    Name string
}

type Order struct {
    Id int64
    Customer *Customer
    Orderlines *[]Orderline
}

type Orderline struct {
    Id int64
    Product *Product
    Amount int64
}

type Product struct {
    Id int64
    Modelnr string
    Price float64
}

func (o *Order) total_amount() float64 {
    return 0.0 // Total amount collector for each Orderline goes here
}

func main() {
    c := Customer{1, "Customername"}

    p1 := Product{30, "Z97", 9.95}
    p2 := Product{31, "Z98", 25.00}

    ol1 := Orderline{10, &p1, 2}
    ol2 := Orderline{11, &p2, 6}

    ols := []Orderline{&ol1, &ol2}

    o := Order{1, &c, &ols}

    fmt.Println(o)
}

我也尝试直接在Order中追加到Slice，但是它也失败了：

o := new(Order)
o.Id = 1
o.Customer = &c
append(o.Orderlines, &ol1, &ol2)

抛出：

$ go run test.go 
# command-line-arguments
./test.go:48: append(o.Orderlines, &ol1, &ol2) evaluated but not used