Correctly emulate old to_id behaviour

telethon/client/updates.py

@@ -616,15 +616,8 @@ class EventBuilderDict:
         try:
             return self.__dict__[builder]
         except KeyError:
-            # Updates may arrive before login (like updateLoginToken) and we
-            # won't have our self ID yet (anyway only new messages need it).
-            self_id = (
-                self.client._self_input_peer.user_id
-                if self.client._self_input_peer
-                else None
-            )
             event = self.__dict__[builder] = builder.build(
-                self.update, self.others, self_id)
+                self.update, self.others, self.client._self_id)
 
             if isinstance(event, EventCommon):
                 event.original_update = self.update

telethon/client/users.py

@@ -164,6 +164,16 @@ class UserMethods:
         except errors.UnauthorizedError:
             return None
 
+    @property
+    def _self_id(self: 'TelegramClient') -> typing.Optional[int]:
+        """
+        Returns the ID of the logged-in user, if known.
+
+        This property is used in every update, and some like `updateLoginToken`
+        occur prior to login, so it gracefully handles when no ID is known yet.
+        """
+        return self._self_input_peer.user_id if self._self_input_peer else None
+
     async def is_bot(self: 'TelegramClient') -> bool:
         """
         Return `True` if the signed-in user is a bot, `False` otherwise.

telethon/tl/custom/message.py

@@ -608,9 +608,14 @@ class Message(ChatGetter, SenderGetter, TLObject, abc.ABC):
     @property
     def to_id(self):
         """
-        Alias for ``.peer_id``. Telegram renamed the field, and this property
-        exists to avoid breaking code.
+        Returns the peer to which this message was sent to. This used to exist
+        to infer the ``.peer_id``.
         """
+        # If the client wasn't set we can't emulate the behaviour correctly,
+        # so as a best-effort simply return the chat peer.
+        if self._client and not self.out and self.is_private:
+            return types.PeerUser(self._client._self_id)
+
         return self.peer_id
 
     # endregion Public Properties