fix the special case for error / empty string

logger.go

@@ -636,10 +636,12 @@ func toJSON[T ValueTypes](v T) string {
 
 func (v ValueType[T]) String() string {
 	// if the type is numeric, use Sprint(v.val) otherwise use Sprintf("%q", v.Val) to quote it.
-	switch /*s :=*/ any(v.Val).(type) {
+	switch s := any(v.Val).(type) {
 	case bool, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64,
 		float32, float64:
 		return fmt.Sprint(v.Val)
+	case error: // struct with no public fields so we need to call Error() to get a string. otherwise we get {}
+		return fmt.Sprintf("%q", s.Error())
 	/* It's all handled by json fallback now - todo test if this is/was cheaper?
 	case []interface{}:
 		return arrayToString(s)

logger_test.go

@@ -752,6 +752,18 @@ func TestNoLevel(t *testing.T) {
 	}
 }
 
+func TestSerializationOfError(t *testing.T) {
+	err := fmt.Errorf("test error")
+	Errf("Error on purpose: %v", err)
+	S(Error, "Error on purpose", Any("err", err))
+	kv := Any("err", err)
+	kvStr := kv.StringValue()
+	expected := `"test error"`
+	if kvStr != expected {
+		t.Errorf("unexpected:\n%s\nvs:\n%s\n", kvStr, expected)
+	}
+}
+
 // io.Discard but specially known to by logger optimizations for instance.
 type discard struct{}