(Merge 3.4) Issue #21596: asyncio.wait(): mention that the sequence of futures

must not be empty.

Doc/library/asyncio-task.rst

@@ -521,6 +521,8 @@ Task functions
    to complete.  Coroutines will be wrapped in Tasks. Returns two sets of
    :class:`Future`: (done, pending).
 
+   The sequence *futures* must not be empty.
+
    *timeout* can be used to control the maximum number of seconds to wait before
    returning.  *timeout* can be an int or float.  If *timeout* is not specified
    or ``None``, there is no limit to the wait time.

Lib/asyncio/tasks.py

@@ -387,6 +387,8 @@ ALL_COMPLETED = concurrent.futures.ALL_COMPLETED
 def wait(fs, *, loop=None, timeout=None, return_when=ALL_COMPLETED):
     """Wait for the Futures and coroutines given by fs to complete.
 
+    The sequence futures must not be empty.
+
     Coroutines will be wrapped in Tasks.
 
     Returns two sets of Future: (done, pending).