message_send: Avoid unchecked cast.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/semgrep.yml

@@ -8,17 +8,6 @@ rules:
     languages: [python]
     severity: ERROR
 
-  - id: useless-if-body
-    patterns:
-      - pattern: |
-          if $X:
-              $S
-          else:
-              $S
-    message: "Useless if statment; both blocks have the same body"
-    languages: [python]
-    severity: ERROR
-
   - id: dont-use-stream-objects-filter
     pattern: Stream.objects.filter(...)
     message: "Please use access_stream_by_*() to fetch Stream objects"

zerver/views/message_send.py

@@ -195,10 +195,10 @@ def send_message_backend(request: HttpRequest, user_profile: UserProfile,
             # Also, mypy can't detect that a single-item
             # list populated from a Union[int, str] is actually
             # a Union[Sequence[int], Sequence[str]].
-            message_to = cast(
+            if isinstance(stream_indicator, int):
-                Union[Sequence[int], Sequence[str]],
+                message_to = [stream_indicator]
-                [stream_indicator],
+            else:
-            )
+                message_to = [stream_indicator]
         else:
             message_to = extract_private_recipients(req_to)