Switch async_to_sync type error to a warning

Detecting if a function is async in Python has a lot of false
negatives, so this is a lot safer while async code we call all makes
sure it has the coroutinefunction flag set.

asgiref/sync.py

@@ -4,6 +4,7 @@
 import os
 import sys
 import threading
+import warnings
 import weakref
 from concurrent.futures import Future, ThreadPoolExecutor
 from typing import Any, Callable, Dict, Optional, Union
@@ -118,7 +119,9 @@ class AsyncToSync:
 
     def __init__(self, awaitable, force_new_loop=False):
         if not callable(awaitable) or not _iscoroutinefunction_or_partial(awaitable):
-            raise TypeError("async_to_sync can only be applied to async functions.")
+            # Python does not have very reliable detection of async functions
+            # (lots of false negatives) so this is just a warning.
+            warnings.warn("async_to_sync was passed a non-async-marked callable")
         self.awaitable = awaitable
         try:
             self.__self__ = self.awaitable.__self__

tests/test_sync.py

@@ -203,26 +203,26 @@ def test_async_to_sync_fail_non_function():
     """
     async_to_sync raises a TypeError when applied to a non-function.
     """
-    with pytest.raises(TypeError) as excinfo:
+    with pytest.warns(UserWarning) as warnings:
         async_to_sync(1)
 
-    assert excinfo.value.args == (
-        "async_to_sync can only be applied to async functions.",
+    assert warnings[0].message.args == (
+        "async_to_sync was passed a non-async-marked callable",
     )
 
 
 def test_async_to_sync_fail_sync():
     """
     async_to_sync raises a TypeError when applied to a sync function.
     """
-    with pytest.raises(TypeError) as excinfo:
+    with pytest.warns(UserWarning) as warnings:
 
         @async_to_sync
         def test_function(self):
             pass
 
-    assert excinfo.value.args == (
-        "async_to_sync can only be applied to async functions.",
+    assert warnings[0].message.args == (
+        "async_to_sync was passed a non-async-marked callable",
     )