/**
 * Unshift T onto the arguments list of S.
 */
type Unshift<S extends (...args: any[]) => any, T> =
    S extends (...args: infer U) => any ? (arg: T, ...args: U) => any : never;

/**
 * Generate all the prefixes of T, not including
 * the nullary function nor T itself (unless T variadic).
 * Return types are not preserved.
 */
type Prefixes<T extends (...args: any[]) => any> = {
    0: () => any,
    1: T extends (_: infer U, ...args: infer V) => any
        ? Unshift<Prefixes<(...args: V) => any>, U> | ((arg: U) => any)
        : never,
    2: T
}[
    T extends (arg: infer U, ...args: infer V) => any
        ? U[] extends V
            ? 2
            : V extends []
                ? 0 : 1
        : 0
];