ts勉強初日目、なぜなのか

why.ts

export class Id<A> {
  constructor(public value: Number) { }
}

export interface Iso<A, B> {
  to(a: A): B;
  from(b: B): A;
}

export class IdIso<A> implements Iso<Number, Id<A>> {
  public to(a: Number): Id<A> {
    return new Id<A>(a);
  }
  public from(b: Id<A>): Number {
    return b.value;
  }
}

export function idIso<A>() {
  return new IdIso<A>();
}

class User {
  static getJson(id: Id<User>): User {
    // dummy
    return new User();
  }

}
class Member {
  static getJson(id: Id<Member>): Member {
    // dummy
    return new Member();
  }
}

User.getJson(idIso<User>().to(1));
Member.getJson(idIso<Member>().to(1));
// required: Id<User>
// found: Id<Member>
// but not compile error
User.getJson(idIso<Member>().to(1));
// required: Id<Member>
// found: Id<User>
// but not compile error
Member.getJson(idIso<User>().to(1));