Disallow async callable class instances as callable for `sync.SyncToA…

…sync` (#328)

i.e. passing an instance defining `async def __call__(...)` to `sync.SyncToAsync` will raise a `TypeError`

asgiref/sync.py

@@ -356,7 +356,11 @@ def __init__(
         thread_sensitive: bool = True,
         executor: Optional["ThreadPoolExecutor"] = None,
     ) -> None:
-        if not callable(func) or _iscoroutinefunction_or_partial(func):
+        if (
+            not callable(func)
+            or _iscoroutinefunction_or_partial(func)
+            or _iscoroutinefunction_or_partial(getattr(func, "__call__", func))
+        ):
             raise TypeError("sync_to_async can only be applied to sync functions.")
         self.func = func
         functools.update_wrapper(self, func)

tests/test_sync.py

@@ -99,6 +99,18 @@ async def test_function(*args):
     )
 
 
+@pytest.mark.asyncio
+async def test_sync_to_async_raises_typeerror_for_async_callable_instance():
+    class CallableClass:
+        async def __call__(self):
+            return None
+
+    with pytest.raises(
+        TypeError, match="sync_to_async can only be applied to sync functions."
+    ):
+        sync_to_async(CallableClass())
+
+
 @pytest.mark.asyncio
 async def test_sync_to_async_decorator():
     """