How to enforce generic parameter to allow undefined

Question

I would like to use a wrapper for different API requests where I don't always know what the response will be and I don't know whether the request will be successful. I would like to be able to specify the expected return type of my function in a few cases without making it mandatory so my first attempt was

function fetch<T=any>(){
let body:T = undefined
// fetch the request
return body
}

Obviously the compiler will mark body here with

'T' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.

so next I tried to do

function fetch<T extends any|undefined = any>(){
let body:T = undefined
// fetch the request
return body
}

but this gives me the error:

'undefined' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.

Now I am out of ideas and I am not sure what the right language is to describe the problem and google it. If somebody could point me in the right direction, that would be highly appreciated!