OptionalAttribute and DefaultParameterValueAttribute is not output to signature file by --sig option

[hafuu]

Repro steps

1. Compile following code with --sig option and generate signature file.

namespace Library1
open System.Runtime.InteropServices
type A() =
  member this.M([<Optional; DefaultParameterValue("")>]x: string) = ()

Expected behavior

OptionalAttribute and DefaultParameterValueAttribute is output to signature file.

Actual behavior

OptionalAttribute and DefaultParameterValueAttribute is not output.

namespace Library1
  type A =
    class
      new : unit -> A
      member M : x:string -> unit
    end

Known workarounds

none

Related information

Windows 10, F# 4.1, Visual Studio 2017

[dsyme]

Covering problems with the --sig option by the single bug #393