type-challengesにリベンジした

type MyPick<T, K extends keyof T> = {
  [k in K]: T[k]
}

type MyReadonly<T> = {
  readonly [k in keyof T]: T[k]
}

type TupleToObject<T extends readonly any[]> = {
  [k in T[number]]: k
}

type First<T extends any[]> = T extends [] ? never : T[0]

type MyExclude<T, U> = T extends U ? never : T

type MyAwaited<T> = T extends Promise<infer P> ? P extends Promise<any> ? MyAwaited<P> : P : never

type If<C, T, F> = C extends true ? T : F

type Concat<T extends any[], U extends any[]> = [...T, ...U]

type Push<T extends any[], U> = [...T, U]

type Unshift<T extends any[], U> = [U, ...T]

type MyParameters<T extends (...args: any[]) => any> = T extends (...args: infer P) => any ? P : never

type MyReturnType<T> = T extends (...args: any[]) => infer P ? P : never

type MyReadonly2<T, K extends keyof T = keyof T> = {
  readonly[k in K]: T[k]
} & {
  [k in keyof T as k extends K ? never : k]: T[k]
}

type DeepReadonly<T> = {
  readonly [k in keyof T]: T[k] extends {}|any[] ? DeepReadonly<T[k]> : T[k]
}

type TupleToUnion<T extends any[]|any> = T extends [infer Left, ...infer Right] ? Left|TupleToUnion<Right> : never

type Chainable<X = {}> = {
  option<T, U extends string>(key: U, value: T):Chainable<{[k in U]: T} & Omit<X, U>>
  get(): X
}