Add upgraded Member to Message.mentions in case of no cache.

discord/member.py

@@ -192,6 +192,17 @@ class Member(discord.abc.Messageable, _BaseUser):
         data['user'] = author._to_minimal_user_json()
         return cls(data=data, guild=message.guild, state=message._state)
 
+    @classmethod
+    def _try_upgrade(cls, *,  data, guild, state):
+        # A User object with a 'member' key
+        try:
+            member_data = data.pop('member')
+        except KeyError:
+            return state.store_user(member_data)
+        else:
+            member_data['user'] = data
+            return cls(data=member_data, guild=guild, state=state)
+
     @classmethod
     def _from_presence_update(cls, *, data, guild, state):
         clone = cls(data=data, guild=guild, state=state)

discord/message.py

@@ -412,16 +412,20 @@ class Message:
             self.author = Member._from_message(message=self, data=member)
 
     def _handle_mentions(self, mentions):
-        self.mentions = []
-        if self.guild is None:
-            self.mentions = [self._state.store_user(m) for m in mentions]
+        self.mentions = r = []
+        guild = self.guild
+        state = self._state
+        if guild is None:
+            self.mentions = [state.store_user(m) for m in mentions]
             return
 
         for mention in filter(None, mentions):
             id_search = int(mention['id'])
-            member = self.guild.get_member(id_search)
+            member = guild.get_member(id_search)
             if member is not None:
-                self.mentions.append(member)
+                r.append(member)
+            else:
+                r.append(Member._try_upgrade(data=mention, guild=guild, state=state))
 
     def _handle_mention_roles(self, role_mentions):
         self.role_mentions = []