fix: merge missing struct keys inside UnmarshalExact

Author: krakowski

viper.go

@@ -1179,7 +1179,14 @@ func (v *Viper) UnmarshalExact(rawVal any, opts ...DecoderConfigOption) error {
 	config := defaultDecoderConfig(rawVal, opts...)
 	config.ErrorUnused = true
 
-	return decode(v.AllSettings(), config)
+	// TODO: make this optional?
+	structKeys, err := v.decodeStructKeys(rawVal, opts...)
+	if err != nil {
+		return err
+	}
+
+	// TODO: struct keys should be enough?
+	return decode(v.getSettings(append(v.AllKeys(), structKeys...)), config)
 }
 
 // BindPFlags binds a full flag set to the configuration, using each flag's long

viper_test.go

@@ -1052,6 +1052,32 @@ func TestUnmarshalWithAutomaticEnv(t *testing.T) {
 
 		assert.Error(t, err, "expected viper.Unmarshal to return error due to unset field 'FLAG'")
 	})
+
+	t.Run("Exact", func(t *testing.T) {
+		var config Configuration
+
+		v.Set("port", 1234)
+		if err := v.UnmarshalExact(&config); err != nil {
+			t.Fatalf("unable to decode into struct, %v", err)
+		}
+
+		assert.Equal(
+			t,
+			Configuration{
+				Name:     "Steve",
+				Port:     1234,
+				Duration: time.Second + time.Millisecond,
+				Modes:    []int{1, 2, 3},
+				Authentication: AuthConfig{
+					Secret: "42",
+				},
+				Storage: StorageConfig{
+					Size: 4096,
+				},
+			},
+			config,
+		)
+	})
 }
 
 func TestBindPFlags(t *testing.T) {