Here’s a minimal snippet to reproduce this

class Stuff<InGuild extends boolean = boolean> {
    public inGuild(): this is Stuff<true> {
        return true;
    }
}

const x = new Stuff();

if (x.inGuild()) {

    
    x
}

Hover over x on VSCode and I see this:

x is still a Stuff<boolean> and not a Stuff<true>, why is that?

I got this idea from discord.js here and I wonder why they can do that and I’m not able to reproduce this here.