In this question I will ask you, why TS fails here. And I can say there is a valid reason why such construct is wrong, its not a language bug. Can you spot why, and what is example type which proves TypeScript rightly prevents such code to compile?

type User = {
  id: number;
  kind: string;
};

function makeCustomer<T extends User>(u: T): T {
  // Below error, why?
  return {
    id: u.id,
    kind: 'customer'
  }
}

You can start playing with this code here - Playground link.
Post your answers in comments. Have fun! Answer will be published soon!

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