allow overriding old, non-map, map element with map

issue202_test.go

@@ -0,0 +1,138 @@
+package mergo_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+func TestIssue202(t *testing.T) {
+	tests := []struct {
+		name           string
+		dst, src, want map[string]interface{}
+	}{
+		{
+			name: "slice override string",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": "foo",
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": []int{1, 2, 3},
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": []int{1, 2, 3},
+			},
+		},
+		{
+			name: "string override slice",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": []int{1, 2, 3},
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": "foo",
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": "foo",
+			},
+		},
+		{
+			name: "map override string",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": "foo",
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+		},
+		{
+			name: "string override map",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": "foo",
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": "foo",
+			},
+		},
+		{
+			name: "map override map",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": map[string]interface{}{
+					"a": 10,
+				},
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+		},
+		{
+			name: "map override map with merge",
+			dst: map[string]interface{}{
+				"x": 456,
+				"y": map[string]interface{}{
+					"a": 10,
+					"b": 100,
+				},
+			},
+			src: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+				},
+			},
+			want: map[string]interface{}{
+				"x": "123",
+				"y": map[string]interface{}{
+					"a": true,
+					"b": 100,
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := mergo.Merge(&tt.dst, tt.src, mergo.WithOverride); err != nil {
+				t.Error(err)
+			}
+
+			if !reflect.DeepEqual(tt.dst, tt.want) {
+				t.Errorf("maps not equal.\nwant:\n%v\ngot:\n%v\n", tt.want, tt.dst)
+			}
+		})
+	}
+}

merge.go

@@ -194,8 +194,13 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 					dst.SetMapIndex(key, dstSlice)
 				}
 			}
-			if dstElement.IsValid() && !isEmptyValue(dstElement) && (reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Map || reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Slice) {
-				continue
+			if dstElement.IsValid() && !isEmptyValue(dstElement) {
+				if reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Slice {
+					continue
+				}
+				if reflect.TypeOf(srcElement.Interface()).Kind() == reflect.Map && reflect.TypeOf(dstElement.Interface()).Kind() == reflect.Map {
+					continue
+				}
 			}
 
 			if srcElement.IsValid() && ((srcElement.Kind() != reflect.Ptr && overwrite) || !dstElement.IsValid() || isEmptyValue(dstElement)) {