TS method shorthand syntax - bivariance pitfall

demo.ts

interface Client {
  pay(): void;
}

interface VIP extends Client {
  fastPay(): void;
}

interface Cashier {
  handleAnyClient(client: Client): void; // wider and narrower Clients allowed
  handleOnlyGeneric: (client: Client) => void; // only Clients allowed
}

const vipCashier: Cashier = {
  handleAnyClient: (vip: VIP) => null,
  handleOnlyGeneric: (vip: VIP) => null,
  // ^^^^^^
  // Types of parameters 'client' and 'client' are incompatible.
  // Property 'fastPay' is missing in type 'Client' but required in type 'VIP'
}