Embedded struct of pointer types will overwrite the whole destination struct

[randallmlough]

If you create a struct that contains multiple pointer types and then attempt to merge two of them together where only one of those types has a value, then the whole destination struct will be overwritten.

Simple example

type Config struct {
	Foo string `json:"foo"`
	Bar string `json:"bar"`
	Params *Params `json:"params"`

}
type Params struct{
	Name string `json:"name"`
	Multi *MultiPtr `json:"multi"`
	Final *Final `json:"Final"`
}
type MultiPtr struct {
	Text string `json:"text"`
	Number int `json:"number"`
}
type Final struct {
	Msg1 string `json:"msg"`
	Msg2 string `json:"msg_2"`
}
func main() {
	a := &Config{
		Foo: "something",
		Params: &Params{
			Name: "should remain untouched", // will be removed/destroyed if `mergo.WithOverride` is enabled
			Multi: &MultiPtr{ // will be removed/destroyed if `mergo.WithOverride` is enabled
				Text: "should stay since b MultiPtr is nil",
				Number: 5,
			},
			Final: &Final{
				Msg1: "will only be overwritten when `mergo.WithOverride` is enabled",
				Msg2: "", // will always overwritten
			},
		},
	}
	b := &Config{
		Bar: "from b",
		Params: &Params{
			Final: &Final{
				Msg1: "I overwrote something",
				Msg2: "thanks for looking into this",
			},
		},
	}
	// with mergo.WithOverride disabled
	//if err := mergo.Merge(a, b); err != nil {
	//	log.Println(err)
	//}
	// outputs: {"foo":"something","bar":"from b","params":{"name":"should remain untouched","multi":{"text":"should stay since b MultiPtr is nil"},"Final":{"msg":"will only be overwritten when `mergo.WithOverride` is enabled","msg_2":"thanks for looking into this"}}}

	// with mergo.WithOverride enabled
	if err := mergo.Merge(a, b, mergo.WithOverride); err != nil {
		log.Println(err)
	}
        // Outputs: {"foo":"something","bar":"from b","params":{"name":null,"multi":null,"Final":{"msg":"I overwrote something","msg_2":"thanks for looking into this"}}}

	j, _ := json.Marshal(a)
	fmt.Println(string(j))
}

I saw issue #47 and the PR that followed it, but that doesn't seem to solve this issue

[thaJeztah]

@vdemeester looks like this can be closed (fixed by #120)

[vdemeester]

Indeed, thanks 👍