[Tuples.Reduce] How do you constrain the callback's return type to be the same as the accumulator type?

[haivuw]

I want to constrain the reduce callback to only return the same type as the accumulator:

// Hotscript
interface Callback extends Fn {
  return: 'not a number' // expect to show error
}

type T0 = Call<T.Reduce<Callback,0>,[1,2,3]>;

Normally when I use recursive type, I can constrain the accumulator type like this. You will get an error if you try to pass a non-number to Acc on recursive calls:

// Recursive type
type Reduce<T extends number[], Acc extends number = 0> = ...

Back to Hotscript, below is what I came up with:

// Hotscript
type Accumulator = number

interface CallbackConstraint extends Fn {
  return: Accumulator
}

interface Callback extends CallbackConstraint { // actually showing error
  return: 'not a number'
}

but even then there is no connection between my Accumulator type and the Init argument of T.Reduce.