Create temporary DMChannels from message create events

This allows for DMChannels to work without falling back to the
Object error case since there is enough information to build a pseudo
DMChannel object.

This is a breaking change since it changes the type of
DMChannel.recipient to Optional[User] for when this faux object is
created.

discord/channel.py

@@ -1195,8 +1195,10 @@ class DMChannel(discord.abc.Messageable, Hashable):
 
     Attributes
     ----------
-    recipient: :class:`User`
+    recipient: Optional[:class:`User`]
         The user you are participating with in the direct message channel.
+        If this channel is received through the gateway, the recipient information
+        may not be always available.
     me: :class:`ClientUser`
         The user presenting yourself.
     id: :class:`int`
@@ -1215,11 +1217,26 @@ class DMChannel(discord.abc.Messageable, Hashable):
         return self
 
     def __str__(self):
-        return f'Direct Message with {self.recipient}'
+        if self.recipient:
+            return f'Direct Message with {self.recipient}'
+        return 'Direct Message with Unknown User'
 
     def __repr__(self):
         return f'<DMChannel id={self.id} recipient={self.recipient!r}>'
 
+    @classmethod
+    def _from_message(cls, state, channel_id, payload):
+        # The MESSAGE_CREATE payload no longer gives bots
+        # an appropriate CHANNEL_CREATE.
+        # However, it has enough data for us to pretend since
+        # bots can no longer be in a group DM.
+        self = cls.__new__(cls)
+        self._state = state
+        self.id = channel_id
+        self.recipient = None
+        self.me = state.user
+        return self
+
     @property
     def type(self):
         """:class:`ChannelType`: The channel's Discord type."""

discord/state.py

@@ -338,10 +338,10 @@ class ConnectionState:
 
         if len(self._private_channels) > 128:
             _, to_remove = self._private_channels.popitem(last=False)
-            if isinstance(to_remove, DMChannel):
+            if isinstance(to_remove, DMChannel) and to_remove.recipient:
                 self._private_channels_by_user.pop(to_remove.recipient.id, None)
 
-        if isinstance(channel, DMChannel):
+        if isinstance(channel, DMChannel) and channel.recipient:
             self._private_channels_by_user[channel.recipient.id] = channel
 
     def add_dm_channel(self, data):
@@ -371,7 +371,7 @@ class ConnectionState:
         try:
             guild = self._get_guild(int(data['guild_id']))
         except KeyError:
-            channel = self.get_channel(channel_id)
+            channel = DMChannel._from_message(self, channel_id, data)
             guild = None
         else:
             channel = guild and guild.get_channel(channel_id)