test-cases.ts

import type { Alike, Expect } from '@type-challenges/utils'

declare const a: Chainable

const result1 = a
  .option('foo', 123)
  .option('bar', { value: 'Hello World' })
  .option('name', 'type-challenges')
  .get()

const result2 = a
  .option('name', 'another name')
  // @ts-expect-error
  .option('name', 'last name')
  .get()

const result3 = a
  .option('name', 'another name')
  // @ts-expect-error
  .option('name', 123)
  .get()

type cases = [
  Expect<Alike<typeof result1, Expected1>>,
  Expect<Alike<typeof result2, Expected2>>,
  Expect<Alike<typeof result3, Expected3>>,
]

type Expected1 = {
  foo: number
  bar: {
    value: string
  }
  name: string
}

type Expected2 = {
  name: string
}

type Expected3 = {
  name: number
}

// *** Tests ***
type T1 = typeof result1
//    ^?

type T3 = typeof result3
//   ^?


// *** Solution ***

type Chainable<T = {}> = {
  option<K extends string, V>(
    key: K extends keyof T ? never : K,
    value: V
  ): Chainable<
    Omit<T, K> & { [X in K]: V }
  >
  get(): T
}