func mergeAny(out, in reflect.Value, viaPtr bool, prop *Properties) {
	if in.Type() == protoMessageType {
		if !in.IsNil() {
			if out.IsNil() {
				out.Set(reflect.ValueOf(Clone(in.Interface().(Message))))
			} else {
				Merge(out.Interface().(Message), in.Interface().(Message))
			}
		}
		return
	}
	switch in.Kind() {
	case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Int32, reflect.Int64,
		reflect.String, reflect.Uint32, reflect.Uint64:
		if !viaPtr && isProto3Zero(in) {
			return
		}
		out.Set(in)
	case reflect.Map:
		if in.Len() == 0 {
			return
		}
		if out.IsNil() {
			out.Set(reflect.MakeMap(in.Type()))
		}
		elemKind := in.Type().Elem().Kind()
		for _, key := range in.MapKeys() {
			var val reflect.Value
			switch elemKind {
			case reflect.Ptr:
				val = reflect.New(in.Type().Elem().Elem())
				mergeAny(val, in.MapIndex(key), false, nil)
			case reflect.Slice:
				val = in.MapIndex(key)
				val = reflect.ValueOf(append([]byte{}, val.Bytes()...))
			default:
				val = in.MapIndex(key)
			}
			out.SetMapIndex(key, val)
		}
	case reflect.Ptr:
		if in.IsNil() {
			return
		}
		if out.IsNil() {
			out.Set(reflect.New(in.Elem().Type()))
		}
		mergeAny(out.Elem(), in.Elem(), true, nil)
	case reflect.Slice:
		if in.IsNil() {
			return
		}
		if in.Type().Elem().Kind() == reflect.Uint8 {
			if prop != nil && prop.proto3 && in.Len() == 0 {
				return
			}
			out.SetBytes(append([]byte{}, in.Bytes()...))
			return
		}
		n := in.Len()
		if out.IsNil() {
			out.Set(reflect.MakeSlice(in.Type(), 0, n))
		}
		switch in.Type().Elem().Kind() {
		case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Int32, reflect.Int64,
			reflect.String, reflect.Uint32, reflect.Uint64:
			out.Set(reflect.AppendSlice(out, in))
		default:
			for i := 0; i < n; i++ {
				x := reflect.Indirect(reflect.New(in.Type().Elem()))
				mergeAny(x, in.Index(i), false, nil)
				out.Set(reflect.Append(out, x))
			}
		}
	case reflect.Struct:
		mergeStruct(out, in)
	default:
		log.Printf("proto: don't know how to copy %v", in)
	}
}
mergeAny performs a merge between two values of the same type.
viaPtr indicates whether the values were indirected through a pointer (implying proto2).
prop is set if this is a struct field (it may be nil).
package proto
import (
	"log"
	"reflect"
	"strings"
)
func Clone(pb Message) Message {
	in := reflect.ValueOf(pb)
	if in.IsNil() {
		return pb
	}
	out := reflect.New(in.Type().Elem())
	mergeStruct(out.Elem(), in.Elem())
	return out.Interface().(Message)
}
func Merge(dst, src Message) {
	in := reflect.ValueOf(src)
	out := reflect.ValueOf(dst)
	if out.IsNil() {
		panic("proto: nil destination")
	}
	if in.Type() != out.Type() {
		panic("proto: type mismatch")
	}
	if in.IsNil() {
		return
	}
	mergeStruct(out.Elem(), in.Elem())
}
func mergeStruct(out, in reflect.Value) {
	sprop := GetProperties(in.Type())
	for i := 0; i < in.NumField(); i++ {
		f := in.Type().Field(i)
		if strings.HasPrefix(f.Name, "XXX_") {
			continue
		}
		mergeAny(out.Field(i), in.Field(i), false, sprop.Prop[i])
	}
	if emIn, ok := in.Addr().Interface().(extensionsMap); ok {
		emOut := out.Addr().Interface().(extensionsMap)
		mergeExtension(emOut.ExtensionMap(), emIn.ExtensionMap())
	} else if emIn, ok := in.Addr().Interface().(extensionsBytes); ok {
		emOut := out.Addr().Interface().(extensionsBytes)
		bIn := emIn.GetExtensions()
		bOut := emOut.GetExtensions()
		*bOut = append(*bOut, *bIn...)
	}
	uf := in.FieldByName("XXX_unrecognized")
	if !uf.IsValid() {
		return
	}
	uin := uf.Bytes()
	if len(uin) > 0 {
		out.FieldByName("XXX_unrecognized").SetBytes(append([]byte(nil), uin...))
	}
}