fix #1106 UnmarshalKey looses some data if a subitem is overridden

overrides_test.go

@@ -45,7 +45,7 @@ func TestNestedOverrides(t *testing.T) {
 	deepCheckValue(assert, v, overrideLayer, []string{"tom", "size"}, 4)
 
 	// Case 4: key:value overridden by a map
-	v = overrideDefault(assert, "tom.size", 4, "tom", map[string]any{"age": 10}) // "tom.size" is first given "4" as default value, then "tom" is overridden by map{"age":10}
+	v = overrideDefault(assert, "tom.size", 4, "tom", map[string]any{"age": 10, "size": 4}) // "tom.size" is first given "4" as default value, then "tom" is overridden by map{"age":10}
 	assert.Equal(4, v.Get("tom.size"))                                                      // "tom.size" should still be reachable
 	assert.Equal(10, v.Get("tom.age"))                                                      // new value should be there
 	deepCheckValue(assert, v, overrideLayer, []string{"tom", "age"}, 10)                    // new value should be there

viper.go

@@ -899,6 +899,13 @@ func GetViper() *Viper {
 // Get returns an interface. For a specific value use one of the Get____ methods.
 func Get(key string) any { return v.Get(key) }
 
+func isStringMapInterface(val any) bool {
+	vt := reflect.TypeOf(val)
+	return vt.Kind() == reflect.Map &&
+		vt.Key().Kind() == reflect.String &&
+		vt.Elem().Kind() == reflect.Interface
+}
+
 func (v *Viper) Get(key string) any {
 	lcaseKey := strings.ToLower(key)
 	val := v.find(lcaseKey, true)
@@ -906,6 +913,29 @@ func (v *Viper) Get(key string) any {
 		return nil
 	}
 
+	// when section is partially overridden,
+	// make sure to return the complete map.
+	if isStringMapInterface(val) {
+		val := val.(map[string]interface{})
+		prefix := lcaseKey + v.keyDelim
+		keys := v.AllKeys()
+		for _, key := range keys {
+			if !strings.HasPrefix(key, prefix) {
+				continue
+			}
+			mk := strings.TrimPrefix(key, prefix)
+			mk = strings.Split(mk, v.keyDelim)[0]
+			if _, exists := val[mk]; exists {
+				continue
+			}
+			mv := v.Get(lcaseKey + v.keyDelim + mk)
+			if mv == nil {
+				continue
+			}
+			val[mk] = mv
+		}
+	}
+
 	if v.typeByDefValue {
 		// TODO(bep) this branch isn't covered by a single test.
 		valType := val