type State = "locked" | "unlocked";

class CredentialManager<S extends State> {
  private readonly state?: S;

  unlock(this: CredentialManager<"locked">, master_key: string) {
    return this as unknown as CredentialManager<"unlocked">;
  }

  set(this: CredentialManager<"unlocked">, key: string, value: any) {
    // ...
  }

  get(this: CredentialManager<"unlocked">, key: string): string | undefined {
    // ...
    return undefined;
  }

  lock(this: CredentialManager<"unlocked">): CredentialManager<"locked"> {
    return this as unknown as CredentialManager<"locked">;
  }
}

function main2() {
  let pw_locked = new CredentialManager<"locked">();
  let pw_unlocked = pw_locked.unlock("psst");
  pw_unlocked.set("foo", "bar");
  let val = pw_unlocked.get("foo");
  pw_locked = pw_unlocked.lock();
}