fix: append decimal with different scale

crates/polars-core/src/datatypes/dtype.rs

@@ -505,6 +505,8 @@ pub(crate) fn can_extend_dtype(left: &DataType, right: &DataType) -> PolarsResul
             Ok(must_cast)
         },
         (DataType::Null, DataType::Null) => Ok(false),
+        #[cfg(feature = "dtype-decimal")]
+        (DataType::Decimal(_, s1), DataType::Decimal(_, s2)) => Ok(s1 != s2),
         // Other way around we don't allow because we keep left dtype as is.
         // We don't go to supertype, and we certainly don't want to cast self to null type.
         (_, DataType::Null) => Ok(true),

crates/polars-core/src/series/mod.rs

@@ -993,6 +993,26 @@ mod test {
         assert!(s1.append(&s2).is_err())
     }
 
+    #[test]
+    #[cfg(feature = "dtype-decimal")]
+    fn series_append_decimal() {
+        let s1 = Series::new("a", &[1.1, 2.3]).cast(&DataType::Decimal(None, Some(2))).unwrap();
+        let s2 = Series::new("b", &[3]).cast(&DataType::Decimal(None, Some(0))).unwrap();
+
+        {
+            let mut s1 = s1.clone();
+            s1.append(&s2).unwrap();
+            assert_eq!(s1.len(), 3);
+            assert_eq!(s1.get(2).unwrap(), AnyValue::Decimal(300, 2));
+        }
+
+        {
+            let mut s2 = s2.clone();
+            s2.append(&s1).unwrap();
+            assert_eq!(s2.get(2).unwrap(), AnyValue::Decimal(2, 0))
+        }
+    }
+
     #[test]
     fn series_slice_works() {
         let series = Series::new("a", &[1i64, 2, 3, 4, 5]);