[<AbstractClass>]
type A<'This when 'This :> A<'This>> (x) =
    member a.X = x
    abstract member SetX : float -> 'This

module A =

    let WithX x (a: 'T when 'T :> A<'T>) =
        a.SetX x

type B (x, y) =
    inherit A<B>(x)

    member b.Y = y
    override b.SetX x =
        B (x, b.Y)