为什么我在FSharp中得到一个无效的运算符定义(~!)

type Parameter =
    | Fixed of double
    | Free of double ref
    with
    override m.ToString() = 
        match m with
        | Fixed v -> sprintf "%f" v
        | Free  v -> sprintf "$%f" v.Value
    static member (~!) m =
        match m with
        | Fixed v -> v
        | Free v -> !v
type Parameter =
    | Fixed of double
    | Fr