f# - optional parameters in abstract class type signature -

the class:

type notabstract () =      member this.withoptionalparameters (x, ?y) =          let y = defaultarg y 10         x + y 

has following type signature:

type notabstract =   class     new : unit -> notabstract     member withoptionalparameters : x:int * ?y:int -> int   end 

however, not work:

[<abstractclass>] type abstractexample () =      abstract withoptionalparameters: int * ?int -> int /// ouch...  type notabstract () =      inherit abstractexample ()     override this.withoptionalparameters (x, ?y) =          let y = defaultarg y 10         x + y 

how write proper type signature in abstract definition of function optional parameters? did not find hint here.

ps: aware (similar) result achieved polymorphism

declaring argument option type doesn't make argument optional.

notabstract().withoptionalparameters(2) // expression expected have type //     int * option<int>     // here has type //     int     

the spec §8.13.6 has it:

in signature, optional arguments appear follows: static member onenormaltwooptional : arg1:int * ?arg2:int * ?arg3:int -> int

naming optional argument in abstract member signature thus

[<abstractclass>] type abstractexample () =      abstract withoptionalparameters: int * ?y:int -> int        type notabstract () =      inherit abstractexample ()     override this.withoptionalparameters (x, ?y) =          let y = defaultarg y 10         x + y  notabstract().withoptionalparameters(42)  // val : int = 52