Add a test case for covariant asyncio.Task (#8833)

test_cases/stdlib/asyncio/check_task.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import asyncio
+
+
+class Waiter:
+    def __init__(self) -> None:
+        self.tasks: list[asyncio.Task[object]] = []
+
+    def add(self, t: asyncio.Task[object]) -> None:
+        self.tasks.append(t)
+
+    async def join(self) -> None:
+        await asyncio.wait(self.tasks)
+
+
+async def foo() -> int:
+    ...
+
+
+async def main() -> None:
+    # asyncio.Task is covariant in its type argument, which is unusual since its parent class
+    # asyncio.Future is invariant in its type argument. This is only sound because asyncio.Task
+    # is not actually Liskov substitutable for asyncio.Future: it does not implement set_result.
+    w = Waiter()
+    t: asyncio.Task[int] = asyncio.create_task(foo())
+    w.add(t)
+    await w.join()