From 2f1a6156ce0c8b8a6291926b75bc27b9b8fc2bfe Mon Sep 17 00:00:00 2001
From: vsemichev <semichev@yahoo.com>
Date: Thu, 8 Aug 2024 17:01:49 -0400
Subject: [PATCH] fixes issue #187. adds test to verify the fix.

---
 issue187_test.go | 71 ++++++++++++++++++++++++++++++++++++++++++++++++
 map.go           |  2 +-
 2 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 issue187_test.go

diff --git a/issue187_test.go b/issue187_test.go
new file mode 100644
index 0000000..49aabfc
--- /dev/null
+++ b/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"])
+	}
+}
diff --git a/map.go b/map.go
index 95ccd12..759b4f7 100644
--- a/map.go
+++ b/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()
 			}
 		}