Didn't know `Expect` existed. Can't find it docs?

It's actually not in the standard library, but you can write it yourself:

type Expect = T;

`T extends true` puts a type constraint on the parameter, which then needs to be assignable to the literal type `true` to type-check.

OK cool, now what about `Equal` :).

type Equal = A extends B ? (B extends A ? true : false) : false;

type Equal = (() => T extends X ? 1 : 2) extends < T >() => T extends Y ? 1 : 2 ? true : false;

Understanding this requires a bit more context, but I'll explain why we need something so complicated in the Advanced Union Types chapter :)

I picked it from https://github.com/type-challenges/type-challenges which is an awesome resource too