KDOC 82: errors.Unwrapを読む

https://github.com/kd-collective/go/blob/b8ac61e6e64c92f23d8cf868a92a70d13e20a124/src/errors/wrap.go#L11-L25

// Unwrap returns the result of calling the Unwrap method on err, if err's
// type contains an Unwrap method returning error.
// Otherwise, Unwrap returns nil.
//
// Unwrap only calls a method of the form "Unwrap() error".
// In particular Unwrap does not unwrap errors returned by [Join].
func Unwrap(err error) error {
	u, ok := err.(interface {
		Unwrap() error
	})
	if !ok {
		return nil
	}
	return u.Unwrap()
}

import (
	"errors"
	"testing"

	"github.com/stretchr/testify/assert"
)

type wrapped struct {
	msg string
	err error
}

func (e wrapped) Error() string { return e.msg }
func (e wrapped) Unwrap() error { return e.err }

func TestUnwrap(t *testing.T) {
	err1 := errors.New("1")
	err2 := wrapped{"wrap 2", err1}

	assert.Equal(t, nil, errors.Unwrap(wrapped{"wrapped", nil}))
	assert.Equal(t, err1, errors.Unwrap(wrapped{"wrapped", err1}))
	assert.Equal(t, err2, errors.Unwrap(wrapped{"wrapped", err2}))

	assert.Equal(t, "wrap 2", err2.Error())
	assert.Equal(t, "wrap 2", errors.Unwrap(wrapped{"wrapped", err2}).Error())
}