Typescript: How to add an item to a tuple?

Question

Why this produces an error "A rest element type must be an array type."?

type QWE<T extends [number, string]> = [boolean, ...T]
                                                 ^^^^ the error is here

Answer 1

If someone can find it useful, this is a version of Titian's type, with an improvement covering some more cases:

type R = TupleUnshift<boolean, [number, string]> // [boolean, number, string]


type TupleUnshift<A, B extends [...any[]]> = ((a: A, ...r: ForcedTuple<B>) => void) extends (...a: infer R) => any ? R : never

type ForcedTuple<T> =
    T extends [
        infer A,
        infer B,
        infer C,
        infer D,
        infer E,
        infer F,
        infer G,
        infer H,
        infer I,
        infer J,
        infer K,
        infer L,
        infer M,
        infer N,
        infer O,
        infer P,
        infer Q,
        infer R,
        infer S,
        infer T,
        infer U,
        infer V,
        infer W,
        infer X,
        infer Y,
        infer Z
    ]
    ?
    [A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z]
    : T