Typescript claims it is a CustomType | Undefined, but it should be only CustomType

The problem I have is different, but the core of the problem is listed bellow.

Here is the working code.

type User = {
   name: string;
}

let user: User | undefined
user = await UserService.getUserById(1)

if(!user) {
   throw new Error('No user found');
}

await passUserToAnotherFunction(user) // <-- user is defined here and of type User

If I want to do user validation logic in different function and throw an error there, the user in passUserToAnotherFunction is of type User | undefined. Here is an example:

type User = {
   name: string;
}

let user: User | undefined
user = await UserService.getUserById(1)

validateExistingUser(user)

await passUserToAnotherFunction(user) // <-- Here user is of type User | undefined and this line is red

const validateExistingUser(user: User | undefined) {
  if(!user) {
    throw new Error('No user found');
  }
} 

Why is it behaving like this when this function executes the same lines of code and how to avoid it?

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