[p]warn ask to ban when user not in server

redbot/cogs/warnings/warnings.py

@@ -15,9 +15,10 @@
 )
 from redbot.core import Config, commands, modlog
 from redbot.core.bot import Red
-from redbot.core.commands import UserInputOptional
+from redbot.core.commands import UserInputOptional, RawUserIdConverter
 from redbot.core.i18n import Translator, cog_i18n
 from redbot.core.utils import AsyncIter
+from redbot.core.utils.views import ConfirmView
 from redbot.core.utils.chat_formatting import warning, pagify
 from redbot.core.utils.menus import menu
 
@@ -373,7 +374,7 @@ async def actionlist(self, ctx: commands.Context):
     async def warn(
         self,
         ctx: commands.Context,
-        member: discord.Member,
+        user: Union[discord.Member, RawUserIdConverter],
         points: UserInputOptional[int] = 1,
         *,
         reason: str,
@@ -386,6 +387,49 @@ async def warn(
         or a custom reason if ``[p]warningset allowcustomreasons`` is set.
         """
         guild = ctx.guild
+        member = None
+        if isinstance(user, discord.Member):
+            member = user
+        elif isinstance(user, int):
+            if not ctx.channel.permissions_for(ctx.guild.me).ban_members:
+                await ctx.send(_("User `{user}` is not in the server.").format(user=user))
+                return
+            user_obj = self.bot.get_user(user) or discord.Object(id=user)
+
+            confirm = ConfirmView(ctx.author, timeout=30)
+            confirm.message = await ctx.send(
+                _(
+                    "User `{user}` is not in the server. Would you like to ban them instead?"
+                ).format(user=user),
+                view=confirm,
+            )
+            await confirm.wait()
+            if confirm.result:
+                try:
+                    await ctx.guild.ban(user_obj, reason=reason)
+                    await modlog.create_case(
+                        self.bot,
+                        guild,
+                        ctx.message.created_at,
+                        "hackban",
+                        user,
+                        ctx.author,
+                        reason,
+                        until=None,
+                        channel=None,
+                    )
+                except discord.HTTPException as error:
+                    await ctx.send(
+                        _("An error occurred while trying to ban the user. Error: {error}").format(
+                            error=error
+                        )
+                    )
+            else:
+                confirm.message = await ctx.send(_("No action taken."))
+
+            await ctx.tick()
+            return
+
         if member == ctx.author:
             return await ctx.send(_("You cannot warn yourself."))
         if member.bot: