fix empty slice is converted to nil in un ConvertibleTo case

copier.go

@@ -176,26 +176,27 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 		return
 	}
 
-	if from.Kind() == reflect.Slice && to.Kind() == reflect.Slice && fromType.ConvertibleTo(toType) {
+	if from.Kind() == reflect.Slice && to.Kind() == reflect.Slice {
 		if to.IsNil() {
 			slice := reflect.MakeSlice(reflect.SliceOf(to.Type().Elem()), from.Len(), from.Cap())
 			to.Set(slice)
 		}
+		if fromType.ConvertibleTo(toType) {
+			for i := 0; i < from.Len(); i++ {
+				if to.Len() < i+1 {
+					to.Set(reflect.Append(to, reflect.New(to.Type().Elem()).Elem()))
+				}
 
-		for i := 0; i < from.Len(); i++ {
-			if to.Len() < i+1 {
-				to.Set(reflect.Append(to, reflect.New(to.Type().Elem()).Elem()))
-			}
-
-			if !set(to.Index(i), from.Index(i), opt.DeepCopy, converters) {
-				// ignore error while copy slice element
-				err = copier(to.Index(i).Addr().Interface(), from.Index(i).Interface(), opt)
-				if err != nil {
-					continue
+				if !set(to.Index(i), from.Index(i), opt.DeepCopy, converters) {
+					// ignore error while copy slice element
+					err = copier(to.Index(i).Addr().Interface(), from.Index(i).Interface(), opt)
+					if err != nil {
+						continue
+					}
 				}
 			}
+			return
 		}
-		return
 	}
 
 	if fromType.Kind() != reflect.Struct || toType.Kind() != reflect.Struct {

copier_test.go

@@ -1666,3 +1666,33 @@ func TestSqlNullFiled(t *testing.T) {
 		t.Errorf("to (%v) value should equal from (%v) value", to.MkExpiryDateType, from.MkExpiryDateType.Int32)
 	}
 }
+
+func TestEmptySlice(t *testing.T) {
+	type Str1 string
+	type Str2 string
+	type Input1 struct {
+		Val Str1
+	}
+	type Input2 struct {
+		Val Str2
+	}
+	to := []*Input1(nil)
+	from := []*Input2{}
+	err := copier.Copy(&to, &from)
+	if err != nil {
+		t.Error("should not error")
+	}
+	if from == nil {
+		t.Error("from should be empty slice not nil")
+	}
+
+	to = []*Input1(nil)
+	from = []*Input2(nil)
+	err = copier.Copy(&to, &from)
+	if err != nil {
+		t.Error("should not error")
+	}
+	if from != nil {
+		t.Error("from should be empty slice nil")
+	}
+}