It changes the set 0 to the default value.

[GoHippo]

conf.yaml :
num: 0

type Config struct {
	Num int `yaml:"num" env-default:"1"`
}

func main() {
	var conf Config
	if err := cleanenv.ReadConfig("conf.yaml", &conf); err != nil {
		log.Fatalf("Error reading config file: %v", err)
	}
	fmt.Printf("num:%v\n", conf)
}

result: "num:{1}"