phantom.ts

class Money<T extends Money<T>> {
  constructor(
    private readonly _value: number
  ) {
  }

  equals(other: T) {
    return this._value === other._value
  }

  add(other: T): T{
    return Money.create<T>(this._value + other._value) as T
  }
  
  static create<U extends Money<U>>(value: number) {
    return new Money<U>(value) as U
  }
}

type Dollar = Money<Dollar> & { _dollar: never }
type Yen = Money<Yen> & { _yen: never }

// Value Object
const dollar = Money.create<Dollar>(123)
const yen = Money.create<Yen>(123)

// ok
console.log(dollar.equals(dollar))
console.log(dollar.add(dollar))

// 型エラー
dollar.equals(yen)
dollar.add(yen)