diff --git a/intercom-attributes/tests/data/run/generic-coclass.rs b/intercom-attributes/tests/data/run/generic-coclass.rs
new file mode 100644
index 0000000..752e909
--- /dev/null
+++ b/intercom-attributes/tests/data/run/generic-coclass.rs
@@ -0,0 +1,181 @@
+extern crate intercom;
+use intercom::attributes::HasInterface;
+use intercom::prelude::*;
+use std::cell::RefCell;
+
+#[com_interface]
+trait IAnimal
+{
+    fn get_name(&self) -> ComResult<String>;
+    fn describe(&self) -> ComResult<String>;
+}
+
+// Crab
+
+#[com_interface]
+trait ICrab
+{
+    fn pick_up(&self, item: &str) -> ComResult<()>;
+}
+
+#[com_class(IAnimal, ICrab)]
+#[derive(Default)]
+struct Crab
+{
+    name: &'static str,
+    item: RefCell<Option<String>>,
+}
+
+impl Crab
+{
+    fn new(name: &'static str) -> Crab
+    {
+        Crab {
+            name,
+            item: RefCell::new(None),
+        }
+    }
+}
+
+impl ICrab for Crab
+{
+    fn pick_up(&self, item: &str) -> ComResult<()>
+    {
+        self.item.replace(Some(item.to_string()));
+        Ok(())
+    }
+}
+
+impl IAnimal for Crab
+{
+    fn get_name(&self) -> ComResult<String>
+    {
+        Ok(self.name.to_string())
+    }
+
+    fn describe(&self) -> ComResult<String>
+    {
+        match *self.item.borrow() {
+            Some(ref item) => Ok(format!("A crab called {} carrying {}", self.name, item)),
+            None => Ok(format!("A crab called {} who is empty pincered", self.name)),
+        }
+    }
+}
+
+// Dog
+
+#[com_interface]
+trait IDog
+{
+}
+
+#[com_class(IAnimal, IDog)]
+#[derive(Default)]
+struct Dog
+{
+    name: &'static str,
+}
+
+impl IDog for Dog {}
+impl IAnimal for Dog
+{
+    fn get_name(&self) -> ComResult<String>
+    {
+        Ok(self.name.to_string())
+    }
+
+    fn describe(&self) -> ComResult<String>
+    {
+        Ok(format!("A dog called {}", self.name))
+    }
+}
+
+// Generic collection
+
+#[com_interface]
+trait ICollection
+{
+    fn get_count(&self) -> usize;
+    fn get_item(&self, idx: usize) -> ComResult<ComRc<dyn IAnimal>>;
+}
+
+#[com_class(ICollection)]
+struct Collection<T: HasInterface<dyn IAnimal> + Default>
+{
+    items: Vec<ComBox<T>>,
+}
+
+impl<T: HasInterface<dyn IAnimal> + Default> ICollection for Collection<T>
+{
+    fn get_count(&self) -> usize
+    {
+        self.items.len()
+    }
+
+    fn get_item(&self, idx: usize) -> ComResult<ComRc<dyn IAnimal>>
+    {
+        Ok(self
+            .items
+            .get(idx)
+            .map(|item| item.into())
+            .unwrap_or_else(|| ComBox::new(T::default()).into()))
+    }
+}
+
+// A factory that returns generic collections.
+
+#[com_class(Self)]
+struct Factory;
+
+#[com_interface]
+impl Factory
+{
+    fn get_crabs(&self) -> ComResult<ComRc<dyn ICollection>>
+    {
+        let collection = ComBox::new(Collection {
+            items: vec![
+                ComBox::new(Crab::new("Ferris")),
+                ComBox::new(Crab::new("Pincers")),
+                ComBox::new(Crab::new("Shelly")),
+            ],
+        });
+
+        Ok(collection.into())
+    }
+
+    fn get_dogs(&self) -> ComResult<ComRc<dyn ICollection>>
+    {
+        let collection = ComBox::new(Collection {
+            items: vec![
+                ComBox::new(Dog { name: "Spot" }),
+                ComBox::new(Dog { name: "Stripe" }),
+                ComBox::new(Dog { name: "Checkers" }),
+            ],
+        });
+
+        Ok(collection.into())
+    }
+}
+
+fn main()
+{
+    // Instantiate Factory as a COM interface.
+    let factory = ComBox::new(Factory);
+    let factory: ComRc<Factory> = factory.into();
+
+    // Make crabs pick up stuff.
+    let crab_collection = factory.get_crabs().expect("Failed to get crabs");
+    let items = ["borrowck", "fish", "gear"];
+    for i in 0..crab_collection.get_count() {
+        let animal = crab_collection.get_item(i).expect("Failed to get item");
+        let crab: ComRc<dyn ICrab> = intercom::ComItf::query_interface(&animal)
+            .expect("Crab collection had a non-crab item");
+        crab.pick_up(items[i]);
+    }
+
+    // Describe each crab.
+    for i in 0..crab_collection.get_count() {
+        let animal = crab_collection.get_item(i).expect("Failed to get item");
+        println!("{}", animal.describe().expect("Failed to describe"));
+    }
+}
diff --git a/intercom-attributes/tests/data/run/generic-coclass.rs.stdout b/intercom-attributes/tests/data/run/generic-coclass.rs.stdout
new file mode 100644
index 0000000..77eaa68
--- /dev/null
+++ b/intercom-attributes/tests/data/run/generic-coclass.rs.stdout
@@ -0,0 +1,3 @@
+A crab called Ferris carrying borrowck
+A crab called Pincers carrying fish
+A crab called Shelly carrying gear