Cons

Constructs a new tuple with the specified type at the head. If you declare Cons<A, [B, C]> you will get back [A, B, C].

type Cons<X, Y extends readonly any[]> = ((arg: X, ...rest: Y) => any) extends (...args: infer U) => any ? U : never;

§Type Parameters

Y extends readonly any[]

((arg: X, ...rest: Y) => any) extends (...args: infer U) => any ? U : never