feat: if err occur, dst will recover to init

copier.go

@@ -136,6 +136,15 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 		return ErrInvalidCopyFrom
 	}
 
+	cacheToValue := indirect(reflect.New(to.Type()))
+	cacheToValue.Set(to)
+	defer func() {
+		//  if err occur, toValue needs to recover to init state.
+		if err != nil {
+			to.Set(cacheToValue)
+		}
+	}()
+
 	fromType, isPtrFrom := indirectType(from.Type())
 	toType, _ := indirectType(to.Type())
 

copier_test.go

@@ -1665,7 +1665,6 @@ func TestDeepCopyAnonymousFieldTime(t *testing.T) {
 }
 
 func TestSqlNullFiled(t *testing.T) {
-
 	type sqlStruct struct {
 		MkId              sql.NullInt64
 		MkExpiryDateType  sql.NullInt32
@@ -1762,3 +1761,70 @@ func TestNestedNilPointerStruct(t *testing.T) {
 		t.Errorf("to (%v) value should equal from (%v) value", to.Title, from.Title)
 	}
 }
+
+func TestOccurErr(t *testing.T) {
+	t.Run("CopyWithOption err occur", func(t *testing.T) {
+		type srcTags struct {
+			Field string
+			Index int
+		}
+		type destTags struct {
+			Field string
+			Index string
+		}
+
+		dst := &destTags{
+			Field: "init",
+			Index: "0",
+		}
+		src := &srcTags{
+			Field: "copied",
+			Index: 1,
+		}
+		err := copier.CopyWithOption(dst, src, copier.Option{
+			Converters: []copier.TypeConverter{
+				{
+					SrcType: 1,
+					DstType: "",
+					Fn: func(src interface{}) (dst interface{}, err error) {
+						return nil, fmt.Errorf("return err")
+					},
+				},
+			},
+		})
+		if err == nil {
+			t.Errorf("should return err")
+		}
+		if dst.Field != "init" || dst.Index != "0" {
+			t.Error("when err occur, the dst should be init")
+		}
+
+	})
+	t.Run("copy err occur", func(t *testing.T) {
+		type srcTags struct {
+			field  string
+			Field2 string
+		}
+
+		type destTags struct {
+			Field  string `copier:"field"`
+			Field2 string `copier:"Field2"`
+		}
+
+		dst := &destTags{
+			Field:  "init",
+			Field2: "init2",
+		}
+		src := &srcTags{
+			field:  "Field1->Field1",
+			Field2: "Field2->Field2",
+		}
+		err := copier.Copy(dst, src)
+		if err == nil {
+			t.Errorf("should return err")
+		}
+		if dst.Field != "init" || dst.Field2 != "init2" {
+			t.Error("when err occur, the dst should be init")
+		}
+	})
+}