fix: Unmarshal struct decoding when passing in a pointer to a pointer

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

viper.go

@@ -1127,6 +1127,13 @@ func (v *Viper) Unmarshal(rawVal any, opts ...DecoderConfigOption) error {
 func (v *Viper) decodeStructKeys(input any, opts ...DecoderConfigOption) ([]string, error) {
 	var structKeyMap map[string]any
 
+	rv := reflect.ValueOf(input)
+	for rv.Kind() == reflect.Ptr {
+		rv = reflect.Indirect(rv)
+	}
+
+	input = rv.Interface()
+
 	err := decode(input, defaultDecoderConfig(&structKeyMap, opts...))
 	if err != nil {
 		return nil, err

viper_test.go

@@ -1080,6 +1080,48 @@ func TestUnmarshalWithAutomaticEnv(t *testing.T) {
 	})
 }
 
+func TestUnmarshalIndirection(t *testing.T) {
+	v := New()
+
+	v.Set("foo", "bar")
+
+	type config struct {
+		Foo string
+	}
+
+	var C config
+
+	CC := &C
+	CCC := &CC
+
+	err := v.Unmarshal(&CCC)
+	require.NoError(t, err)
+
+	assert.Equal(t, config{"bar"}, C)
+}
+
+func TestUnmarshalInterface(t *testing.T) {
+	v := New()
+
+	v.Set("foo", "bar")
+
+	type someInterface interface {
+		Foo() string
+	}
+
+	type config struct {
+		Foo   string
+		Fooer someInterface
+	}
+
+	var C config
+
+	err := v.Unmarshal(&C)
+	require.NoError(t, err)
+
+	assert.Equal(t, config{Foo: "bar"}, C)
+}
+
 func TestBindPFlags(t *testing.T) {
 	v := New() // create independent Viper object
 	flagSet := pflag.NewFlagSet("test", pflag.ContinueOnError)