类型推断在具有静态成员约束的泛型类型上失败

我已经定义了以下类型(从代码中简化):

type Polynomial<'a when 'a :(static member public Zero : 'a) 
                and 'a: (static member (+): 'a*'a -> 'a) 
                and 'a : (static member (*): 'a*'a -> 'a) >  =
    | Polynomial of 'a list
    with 
    static member inline (+) (x: Polynomial<'a> , y : Polynomial<'a>) : Polynomial<'a>= 
        match x,y with
        |Polynomial xlist, Polynomial ylist ->
            let longer, shorter = 
                if xlist.Length> ylist.Length then xlist, ylist
                else ylist, xlist
            let shorterExtended = List.append shorter (List.init (longer.Length - shorter.Length) (fun _ -> LanguagePrimitives.GenericZero<'a>))
            List.map2 (+) longer shorterExtended |> Polynomial

当我建立时,我收到警告:

警告FS0193:类型参数缺少约束'when(^ a或^？23604):( static> member(+):^ a*^？23604 - > ^？23605)'

在最后一行的"更长"一词.据我所知,它应该能够推断它总是添加'a'的两个成员.我怎么能摆脱这个？