Nil Slice Becomes Empty Slice After Copying

[helloqiu]

Reproducible Example

Check https://go.dev/play/p/wIwXSvhuj4j

package main

import (
	"fmt"
	"reflect"

	"github.com/jinzhu/copier"
)

type A struct {
	SomeField []string
}

type B struct {
	SomeField []uint
}

func main() {
	a := A{}
	b := B{}
	if err := copier.Copy(&b, &a); err != nil {
		panic(err)
	}
	anotherB := B{}
	if !reflect.DeepEqual(b, anotherB) {
		fmt.Println("b and anotherB should be equal")
	}
	fmt.Printf("a.SomeField : len(%d), is-nil:(%t)\n", len(a.SomeField), a.SomeField == nil)
	fmt.Printf("b.SomeField : len(%d), is-nil:(%t)\n", len(b.SomeField), b.SomeField == nil)
	fmt.Printf("anotherB.SomeField : len(%d), is-nil:(%t)\n", len(anotherB.SomeField), anotherB.SomeField == nil)
}

Result:

b and anotherB should be equal
a.SomeField : len(0), is-nil:(true)
b.SomeField : len(0), is-nil:(false)
anotherB.SomeField : len(0), is-nil:(true)

Description

If two struts have two fields with the same name but they are slice of different types, the value on the target struct would be assigned to an empty slice rather than a nil slice.