Need help with dynamically typed rest parameters

type UnionToIntersection<U> = 
  (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

type LastOf<T> = 
  UnionToIntersection<T extends any ? () => T : never> extends () => (infer R) ? R : never;

type Merge<A, B> = 
  { [K in keyof A]: K extends keyof B ? B[K] : A[K] } & B extends infer O ? { [K in Exclude<keyof O, keyof A>]: O[K] } : never;

type ValueOf<T> = T[keyof T];

type StateOf<T> = T extends StateSlice<any, infer S> ? S : never;

class StateSlice<T, K extends { [key: string]: T }> {
  constructor(public state: K) {}

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  mergeStates<T extends StateSlice<any, any>>(
    ...slices: T[]
  ): StateSlice<ValueOf<Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>>, Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>> {
    const newState = Object.assign(
      {},
      // eslint-disable-next-line @typescript-eslint/no-unsafe-argument, @typescript-eslint/no-unsafe-return
      ...slices.map((slice) => slice.state)
    ) as Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>;
    return new StateSlice(newState);
  }


  trim(keys: (keyof K)[]): StateSlice<T, Pick<K, (typeof keys)[number]>> {
    const newState = keys.reduce(
      (state, key) => ({ ...state, [key]: this.state[key] }),
      {} as Pick<K, (typeof keys)[number]>
    );
    return new StateSlice(newState);
  }

  append<J extends string, V>(
    key: J,
    value: V
  ): StateSlice<T | V, K & { [P in J]: V }> {
    return new StateSlice({ ...this.state, [key]: value });
  }

  split(
    keys: (keyof K)[]
  ): [
    StateSlice<T, Pick<K, (typeof keys)[number]>>,
    StateSlice<T, Omit<K, (typeof keys)[number]>>
  ] {
    const includedState = this.trim(keys).state;
    const excludedState = Object.keys(this.state)
      .filter((key) => !keys.includes(key as keyof K))
      .reduce(
        (state, key) => ({ ...state, [key]: this.state[key as keyof K] }),
        {} as Omit<K, (typeof keys)[number]>
      );
    return [new StateSlice(includedState), new StateSlice(excludedState)];
  }
}

function createStateSlice<T, K extends { [key: string]: T }>(
  initialState: K
): StateSlice<T, K> {
  return new StateSlice(initialState);
}

const UserSignIn = createStateSlice({
  username: "",
  password: "",
});

const UserPreferences = createStateSlice({
  displayName: "",
  email: "",
});

const UserPreferences2 = createStateSlice({
  firstName: "",
  lastName: "",
  confirmPassword: "",
});

const UserSignUp = UserSignIn.mergeStates(UserPreferences, UserPreferences2);

type UnionToIntersection<U> = 
  (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

type LastOf<T> = 
  UnionToIntersection<T extends any ? () => T : never> extends () => (infer R) ? R : never;

type Merge<A, B> = 
  { [K in keyof A]: K extends keyof B ? B[K] : A[K] } & B extends infer O ? { [K in Exclude<keyof O, keyof A>]: O[K] } : never;

type ValueOf<T> = T[keyof T];

type StateOf<T> = T extends StateSlice<any, infer S> ? S : never;

class StateSlice<T, K extends { [key: string]: T }> {
  constructor(public state: K) {}

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  mergeStates<T extends StateSlice<any, any>>(
    ...slices: T[]
  ): StateSlice<ValueOf<Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>>, Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>> {
    const newState = Object.assign(
      {},
      // eslint-disable-next-line @typescript-eslint/no-unsafe-argument, @typescript-eslint/no-unsafe-return
      ...slices.map((slice) => slice.state)
    ) as Merge<StateOf<LastOf<T>>, UnionToIntersection<StateOf<T>>>;
    return new StateSlice(newState);
  }


  trim(keys: (keyof K)[]): StateSlice<T, Pick<K, (typeof keys)[number]>> {
    const newState = keys.reduce(
      (state, key) => ({ ...state, [key]: this.state[key] }),
      {} as Pick<K, (typeof keys)[number]>
    );
    return new StateSlice(newState);
  }

  append<J extends string, V>(
    key: J,
    value: V
  ): StateSlice<T | V, K & { [P in J]: V }> {
    return new StateSlice({ ...this.state, [key]: value });
  }

  split(
    keys: (keyof K)[]
  ): [
    StateSlice<T, Pick<K, (typeof keys)[number]>>,
    StateSlice<T, Omit<K, (typeof keys)[number]>>
  ] {
    const includedState = this.trim(keys).state;
    const excludedState = Object.keys(this.state)
      .filter((key) => !keys.includes(key as keyof K))
      .reduce(
        (state, key) => ({ ...state, [key]: this.state[key as keyof K] }),
        {} as Omit<K, (typeof keys)[number]>
      );
    return [new StateSlice(includedState), new StateSlice(excludedState)];
  }
}

function createStateSlice<T, K extends { [key: string]: T }>(
  initialState: K
): StateSlice<T, K> {
  return new StateSlice(initialState);
}

const UserSignIn = createStateSlice({
  username: "",
  password: "",
});

const UserPreferences = createStateSlice({
  displayName: "",
  email: "",
});

const UserPreferences2 = createStateSlice({
  firstName: "",
  lastName: "",
  confirmPassword: "",
});

const UserSignUp = UserSignIn.mergeStates(UserPreferences, UserPreferences2);

Argument of type 'StateSlice<unknown, { firstName: string; lastName: string; confirmPassword: string; }>' is not assignable to parameter of type 'StateSlice<unknown, { displayName: string; email: string; }>'.
  Type '{ firstName: string; lastName: string; confirmPassword: string; }' is missing the following properties from type '{ displayName: string; email: string; }': displayName, emailts(2345)
const UserPreferences2: StateSlice<unknown, {
    firstName: string;
    lastName: string;
    confirmPassword: string;
}>

Argument of type 'StateSlice<unknown, { firstName: string; lastName: string; confirmPassword: string; }>' is not assignable to parameter of type 'StateSlice<unknown, { displayName: string; email: string; }>'.
  Type '{ firstName: string; lastName: string; confirmPassword: string; }' is missing the following properties from type '{ displayName: string; email: string; }': displayName, emailts(2345)
const UserPreferences2: StateSlice<unknown, {
    firstName: string;
    lastName: string;
    confirmPassword: string;
}>