Introduction to infer

// "If T has a payload property, call that payload P"
type PayloadType<T> = T extends { payload: infer P } ? P : never

type A = PayloadType<{ payload: 'ping' }> // 'ping'
type B = PayloadType<{ payload: 42 }> // 42
type C = PayloadType<string> // never - no payload to infer

T extends SomePattern<infer X> ? X : Fallback

1. Create ArrayElement<T> — element type for arrays, otherwise never
2. Create PromiseResult<T> — resolved type for Promises, otherwise T
3. Create FunctionReturn<T> — return type for functions, otherwise never
4. Add a runtime fetchUser that resolves to { id: '1', name: 'Alice' } and is typed as Promise<{ id: string; name: string }>
5. Create FetchUserResult by deriving it from fetchUser's result type with PromiseResult (so it resolves to the user object shape, not a Promise)
6. Export fetchUser