fixes issue darccio#187. adds test to verify the fix.

issue187_test.go

@@ -0,0 +1,71 @@
+package mergo_test
+
+import (
+	"dario.cat/mergo"
+	"testing"
+)
+
+func TestIssue187MergeStructToMap(t *testing.T) {
+	dst := map[string]interface{}{
+		"empty": "data",
+	}
+
+	src := struct {
+		Foo   string
+		Bar   int
+		Empty string
+	}{
+		Foo: "hello",
+		Bar: 42,
+	}
+	if err := mergo.Map(&dst, src); err != nil {
+		t.Error(err)
+	}
+	if dst["foo"] != "hello" || dst["bar"] != 42 || dst["empty"] != "data" {
+		t.Errorf("expected dst to be {foo: hello, bar: 42, empty: data}, got {foo: %v, bar: %v, empty: %v}", dst["foo"], dst["bar"], dst["empty"])
+	}
+}
+
+func TestIssue187MergeStructToMapWithOverwrite(t *testing.T) {
+	dst := map[string]interface{}{
+		"foo":   "initial",
+		"bar":   1,
+		"empty": "data",
+	}
+	src := struct {
+		Foo   string
+		Bar   int
+		Empty string
+	}{
+		Foo: "hello",
+		Bar: 42,
+	}
+	if err := mergo.Map(&dst, src, mergo.WithOverride); err != nil {
+		t.Error(err)
+	}
+	if dst["foo"] != "hello" || dst["bar"] != 42 || dst["empty"] != "data" {
+		t.Errorf("expected dst to be {foo: hello, bar: 42, empty: data}, got {foo: %v, bar: %v, empty: %v}", dst["foo"], dst["bar"], dst["empty"])
+	}
+}
+
+func TestIssue187MergeStructToMapWithOverwriteWithEmptyValue(t *testing.T) {
+	dst := map[string]interface{}{
+		"foo":   "initial",
+		"bar":   1,
+		"empty": "data",
+	}
+	src := struct {
+		Foo   string
+		Bar   int
+		Empty string
+	}{
+		Foo: "hello",
+		Bar: 42,
+	}
+	if err := mergo.Map(&dst, src, mergo.WithOverwriteWithEmptyValue); err != nil {
+		t.Error(err)
+	}
+	if dst["foo"] != "hello" || dst["bar"] != 42 || dst["empty"] != "" {
+		t.Errorf("expected dst to be {foo: hello, bar: 42, empty: }, got {foo: %v, bar: %v, empty: %v}", dst["foo"], dst["bar"], dst["empty"])
+	}
+}

map.go

@@ -58,7 +58,7 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 			}
 			fieldName := field.Name
 			fieldName = changeInitialCase(fieldName, unicode.ToLower)
-			if _, ok := dstMap[fieldName]; !ok || (!isEmptyValue(reflect.ValueOf(src.Field(i).Interface()), !config.ShouldNotDereference) && overwrite) {
+			if _, ok := dstMap[fieldName]; !ok || (!isEmptyValue(reflect.ValueOf(src.Field(i).Interface()), !config.ShouldNotDereference) && overwrite) || config.overwriteWithEmptyValue {
 				dstMap[fieldName] = src.Field(i).Interface()
 			}
 		}