From 24feece1215559a844cc6fccede5c8e284adcdc5 Mon Sep 17 00:00:00 2001
From: magiccwarss <matem.mayreni@gmail.com>
Date: Sun, 8 Nov 2026 11:17:23 +0400
Subject: [PATCH] struct: preserve embedded pointer defaults when mapping

---
 struct.go      |  2 +-
 struct_test.go | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/struct.go b/struct.go
index 819a6e5..602a514 100644
--- a/struct.go
+++ b/struct.go
@@ -301,7 +301,7 @@ func (s *Section) mapToField(val reflect.Value, isStrict bool, sectionIndex int,
 		isStruct := tpField.Type.Kind() == reflect.Struct
 		isStructPtr := tpField.Type.Kind() == reflect.Pointer && tpField.Type.Elem().Kind() == reflect.Struct
 		isAnonymousPtr := tpField.Type.Kind() == reflect.Pointer && tpField.Anonymous
-		if isAnonymousPtr {
+		if isAnonymousPtr && field.IsNil() {
 			field.Set(reflect.New(tpField.Type.Elem()))
 		}
 
diff --git a/struct_test.go b/struct_test.go
index ad6ad04..7904c12 100644
--- a/struct_test.go
+++ b/struct_test.go
@@ -910,3 +910,28 @@ Title = Consultant Engineer
 		)
 	})
 }
+
+func TestMapToWithMapperPreservesEmbeddedPointerDefaults(t *testing.T) {
+	type Options struct {
+		Name string
+		Port int
+	}
+	type Config struct {
+		*Options
+	}
+	for _, source := range []string{"", "[options]\nname = configured\n"} {
+		t.Run(source, func(t *testing.T) {
+			defaults := &Options{Name: "default", Port: 1234}
+			got := Config{Options: defaults}
+			err := MapToWithMapper(&got, TitleUnderscore, []byte(source))
+			require.NoError(t, err)
+			require.Same(t, defaults, got.Options)
+			assert.Equal(t, 1234, got.Port)
+			if source == "" {
+				assert.Equal(t, "default", got.Name)
+			} else {
+				assert.Equal(t, "configured", got.Name)
+			}
+		})
+	}
+}