[commands] Fetch user if an ID is passed and cache lookup fails.

discord/ext/commands/converter.py

@@ -206,6 +206,10 @@ class UserConverter(IDConverter):
 
     .. versionchanged:: 1.5
          Raise :exc:`.UserNotFound` instead of generic :exc:`.BadArgument`
+
+    .. versionchanged:: 1.6
+        This converter now lazily fetches users from the HTTP APIs if an ID is passed
+        and it's not available in cache.
     """
     async def convert(self, ctx, argument):
         match = self._get_id_match(argument) or re.match(r'<@!?([0-9]+)>$', argument)
@@ -215,7 +219,14 @@ class UserConverter(IDConverter):
         if match is not None:
             user_id = int(match.group(1))
             result = ctx.bot.get_user(user_id) or _utils_get(ctx.message.mentions, id=user_id)
-        else:
+            if result is None:
+                try:
+                    result = await ctx.bot.fetch_user(user_id)
+                except discord.HTTPException:
+                    raise UserNotFound(argument) from None
+
+            return result
+
         arg = argument
 
         # Remove the '@' character if this is the first character from the argument