How to use generic inside the function when typing with a type alias?

Question

Let say I have a function like this(I know it is a nonsense):

const fn = <T>(a: number) => {
  return a as unknown as Promise<T>
}

When I separate the type and the function with type alias like this:

type Fn = <T>(a: number) => Promise<T>
const fn: Fn = <T>(a) => {
  return a as unknown as Promise<T>
}

I will get the following error:

Parameter 'a' implicitly has an 'any' type.

Why is that? How can I use generic when I type with type alias?