Purpose of `[]` in TypeScript type signatures

declare const tap: {
  <A, X>(
    f: (a: NoInfer<A>) => X
  ): <E, R>(
    self: Effect<A, E, R>
  ) => [X] extends [Effect<infer _A1, infer E1, infer R1>] ? Effect<A, E | E1, R | R1>
    : [X] extends [PromiseLike<infer _A1>] ? Effect<A, E | Cause.UnknownException, R>
    : Effect<A, E, R>
  <X>(
    f: NotFunction<X>
  ): <A, E, R>(
    self: Effect<A, E, R>
  ) => [X] extends [Effect<infer _A1, infer E1, infer R1>] ? Effect<A, E | E1, R | R1>
    : [X] extends [PromiseLike<infer _A1>] ? Effect<A, E | Cause.UnknownException, R>
    : Effect<A, E, R>
  <A, E, R, X>(
    self: Effect<A, E, R>,
    f: (a: NoInfer<A>) => X
  ): [X] extends [Effect<infer _A1, infer E1, infer R1>] ? Effect<A, E | E1, R | R1>
    : [X] extends [PromiseLike<infer _A1>] ? Effect<A, E | Cause.UnknownException, R>
    : Effect<A, E, R>
  <A, E, R, X>(
    self: Effect<A, E, R>,
    f: NotFunction<X>
  ): [X] extends [Effect<infer _A1, infer E1, infer R1>] ? Effect<A, E | E1, R | R1>
    : [X] extends [PromiseLike<infer _A1>] ? Effect<A, E | Cause.UnknownException, R>
    : Effect<A, E, R>
}