types(Message): remove & this from #inGuild typeguard (#8704)

Author: suneettipirneni

packages/discord.js/typings/index.d.ts

@@ -1750,7 +1750,7 @@ export class Message<InGuild extends boolean = boolean> extends Base {
   public toJSON(): unknown;
   public toString(): string;
   public unpin(reason?: string): Promise<Message<InGuild>>;
-  public inGuild(): this is Message<true> & this;
+  public inGuild(): this is Message<true>;
 }
 
 export class AttachmentBuilder {

packages/discord.js/typings/index.test-d.ts

@@ -309,6 +309,11 @@ declare const assertIsMessage: (m: Promise<Message>) => void;
 
 client.on('messageCreate', async message => {
   const { client, channel } = message;
+
+  if (!message.inGuild() && message.partial) {
+    expectNotType<never>(message);
+  }
+
   expectType<Client<true>>(client);
   assertIsMessage(channel.send('string'));
   assertIsMessage(channel.send({}));