Test setting existing component with entry.

src/world/mod.rs

@@ -1708,6 +1708,26 @@ mod tests {
         assert_eq!(result, vec![1, 2, 3]);
     }
 
+    #[test]
+    fn entry_set_existing_component() {
+        let mut world = World::<Registry>::new();
+
+        let entity_identifier = world.insert(entity!(A(1), B('a')));
+        world.insert(entity!(A(2)));
+        world.insert(entity!(B('b')));
+        world.insert(entity!());
+
+        let mut entry = assert_some!(world.entry(entity_identifier));
+        entry.add(A(3));
+
+        let mut result = world
+            .query::<views!(&A), filter::None>()
+            .map(|result!(a)| a.0)
+            .collect::<Vec<_>>();
+        result.sort();
+        assert_eq!(result, vec![2, 3]);
+    }
+
     #[test]
     fn entry_remove_component() {
         let mut world = World::<Registry>::new();