Efficiently constrained isNull

isNull.fsx

type A = class end
type B = class inherit A end
type C = class inherit B end
type D = class inherit C end

type IsNull = 
    inherit D
    static member inline ($) (_:A  , x  :'a when 'a : not struct) = System.Object.ReferenceEquals(x, null)
    static member inline ($) (_:B  , x  :'a when 'a : null      ) = match x with null -> true | _ -> false
    static member inline ($) (_:C  , _:D) = false
    static member inline ($) (_:D  , _:C) = false

let inline isNull x = Unchecked.defaultof<IsNull> $ x