Encoding HKTs in typescript without declaration merging

HKT.ts

export interface HKT {
  param: unknown;
  result: unknown;
}

interface NotHKT extends HKT {
  result: this['param'] extends true ? false : true;
}

interface FstHKT extends HKT {
  result: this['param'] extends [infer A, infer _] ? A : never;
}

interface ArrayHKT extends HKT {
  result: Array<this['param']>;
}

export type Apply<f extends HKT, x>
  = (f & { param: x })['result'];

type Test1 = Apply<NotHKT, true> // be deducedd to `false`
type Test2 = Apply<FstHKT, [string, boolean]> // be deduced to `string`
type Test3 = Apply<ArrayHKT, number> // be deduced to `number[]`