concurrent.futures: allow as_completed to act covariantly (#11291)

test_cases/stdlib/check_concurrent_futures.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from collections.abc import Callable, Iterator
+from concurrent.futures import Future, ThreadPoolExecutor, as_completed
+from typing_extensions import assert_type
+
+
+class Parent:
+    ...
+
+
+class Child(Parent):
+    ...
+
+
+def check_as_completed_covariance() -> None:
+    with ThreadPoolExecutor() as executor:
+        f1 = executor.submit(lambda: Parent())
+        f2 = executor.submit(lambda: Child())
+        fs: list[Future[Parent] | Future[Child]] = [f1, f2]
+        assert_type(as_completed(fs), Iterator[Future[Parent]])
+        for future in as_completed(fs):
+            assert_type(future.result(), Parent)
+
+
+def check_future_invariance() -> None:
+    def execute_callback(callback: Callable[[], Parent], future: Future[Parent]) -> None:
+        future.set_result(callback())
+
+    fut: Future[Child] = Future()
+    execute_callback(lambda: Parent(), fut)  # type: ignore
+    assert isinstance(fut.result(), Child)