I have a module named “abc”.

go.mod

module abc

go 1.22.2

Then I have a struct that this module exports.

struct.go

package abc

import "fmt"

type A struct {
    Left  int
    Right int
}

func (a A) String() string {
    return fmt.Sprintf("%d %d", a.Left, a.Right)
}

And a test for that struct.

abc_test.go

package abc_test

import (
    "testing"

    abc "abc"
)

func TestOne(t *testing.T) {
    a := abc.A{Left: 1, Right: 2}
    s := a.String()

    if s != Expected() {
        t.Errorf("Failed, s is %s", s)
    }
}

The function Expected() is defined in a separate file.

helper.go

package abc_test

func Expected() string {
    return "1 2"
}

The issue is file abc_test.go sees the function Expected() as undefined. I would like to create a function that is only usable inside the test package so that when such a module would be exposed on the internet, the user could not use the helper function for the test directly.

How can I do that?