`[<System.ParamArray>]` parameter in member function causes type check error.

[muqiuhan]

When I use the parameter of [<ParamArray>] attribute in member function, I cannot pass the parameter using pipeline operator:

> type X () = member _.ID ([<System.ParamArray>] arr) = arr;;
type X =
  new: unit -> X
  member ID: [<System.ParamArray>] arr: 'a -> 'a

> let x = X();;                                              
val x: X

> x.ID([| 1; 2; 3 |]);;                                      
val it: int array = [|1; 2; 3|]

> [| 1; 2; 3 |] |> x.ID;;

  [| 1; 2; 3 |] |> x.ID;;
  -----------------^^^^
/home/muqiu/stdin(10,18): error FS0001: This expression was expected to have type
    'int array'    
but here has type
    'unit'

And no type check error occurs when the [<ParamArray>] attribute is not used or using [<ParamArray>] at the top level:

> type X () = member _.ID (arr) = arr;;
type X =
  new: unit -> X
  member ID: arr: 'a -> 'a

> let x = X();;                        
val x: X

> [| 1; 2; 3 |] |> x.ID;;
val it: int array = [|1; 2; 3|]

> let ID ([<System.ParamArray>] arr) = arr;;     
val ID: [<System.ParamArray>] arr: 'a -> 'a

> [| 1; 2; 3 |] |> ID;;                     
val it: int array = [|1; 2; 3|]

Why does [<ParamArray>] behave differently in member functions? Is this a compiler bug?

[brianrourkeboll]

Probably a duplicate of #11918.

I think it's a type inference + overload resolution thing: a method with a single parameter annotated with [<ParamArray>] can also be treated as a nullary method (or a method taking a single parameter of type unit).

But there is not perfect symmetry in how overload resolution works for directly-invoked methods and methods used in a first-class way (e.g., with piping). See #11918 (comment).

[brianrourkeboll]

The error message is confusing, though:

> open System
-
- type T =
-     static member M ([<ParamArray>] xs : int array) = xs
-
- let xs = [|1..10|]
-
- xs |> T.M;;

  xs |> T.M;;
  ------^^^

stdin(8,7): error FS0001: This expression was expected to have type
    'int array'
but here has type
    'unit'

It sounds like the opposite of what the source code looks like.

Especially since this works:

() |> T.M