Add #530 as test to ensure it remains fixed (#531)

tests/530_untagged_union.rs

@@ -0,0 +1,21 @@
+use std::ops::Bound;
+
+#[derive(serde::Deserialize, serde::Serialize, PartialEq, Eq, Debug)]
+#[serde(untagged)]
+enum EitherInterval<V> {
+    B(Interval<V>),
+    D(V, Option<V>),
+}
+
+type Interval<V> = (Bound<V>, Bound<V>);
+
+#[test]
+fn serde_roundtrip() {
+    let interval = EitherInterval::B((Bound::Excluded(0u8), Bound::Unbounded));
+
+    let ron: String = ron::ser::to_string(&interval).unwrap();
+    assert_eq!(ron, "(Excluded(0),Unbounded)");
+
+    let de = ron::de::from_str(&ron).unwrap();
+    assert_eq!(interval, de);
+}