Generic function returning type of component type

[pd-giz-dave]

I have a generic function that is given the type of a struct that always has a component 'x' that is of another type, I want the function to return a value of the type of 'x'. How do I do that?
E.g.

struct IPoint {
  x int
  y int
}
struct FPoint {
  x f32
  y f32
}
fn add[T](a T,b T) typeof(T.x) {
  return a.x + b.x
}

The clunky way to do it is:

fn add[T,F](a T, b T) F {
  return a.x + b.x
}

But that is redundant and the call site becomes messy:

add[IPoint,int](a, b)
add[FPoint,f32](c, d)

[pd-giz-dave]

allow generic types to be compile time expressions