why doesn't this work?

num.fs

type Num<'a> = {
        add : 'a -> 'a -> 'a;
        subtract : 'a -> 'a -> 'a;
        multiply : 'a -> 'a -> 'a;
        divide : 'a -> 'a -> 'a;
    }

type System.Int32 with
    static member NumInstance () = 
        {
            add = fun x y -> x + y;
            subtract = fun x y -> x - y;
            multiply = fun x y -> x * y;
            divide = fun x y -> x / y;
        }

let inline getNum () : Num<'a> = (^a : (static member NumInstance : unit -> Num<'a>) ())

let inline func x y =
    let num = getNum()
    num.divide (num.add x y) x

[<EntryPoint>]
let main argv = 

    printfn "%A" (func 3 2) // the type 'int' does not support the operator 'NumInstance'
    0

See https://github.com/gmpl/FSharpPlus/blob/master/FSharpPlus/Operators.fs#L184-L267 and https://github.com/gmpl/FsControl/blob/master/FsControl.Core/Numeric.fs