Unable to implement abstract class

[saul]

namespace Microsoft.VisualStudio.FSharp.Editor.Tests.Roslyn

open Microsoft.CodeAnalysis.Options
open System.Collections.Immutable

type TestOptionSet(?values: ImmutableDictionary<OptionKey, obj>) =
 inherit OptionSet()

 let values = values |> Option.defaultValue ImmutableDictionary.Empty

 override __.GetOption(optionKey: OptionKey) =
 values.[optionKey]

 override __.WithChangedOption(optionAndLanguage: OptionKey, value: obj) =
 upcast TestOptionSet(values.SetItem(optionAndLanguage, value))

 // The member that I cannot override (FS0855)
 override __.GetChangedOptions(optionSet: OptionSet) : seq<OptionKey> =
 values
 |> Seq.filter (fun (KeyValue(k,v)) ->
 optionSet.GetOption(k) <> v
 )
 |> Seq.map (fun (KeyValue(k,_)) -> k) 

I get the error message:

FS0855	No abstract or interface member was found that corresponds to this override

If I remove the override keyword from the GetChangedOptions member, I then get the following error:

FS0365	No implementation was given for 'OptionSet.GetChangedOptions(optionSet: OptionSet) : System.Collections.Generic.IEnumerable<OptionKey>'

I believe this may have something to do with the fact that GetChangedOptions is internal, but I can't specify that with the override keyword, otherwise I get:

FS0941	Accessibility modifiers are not permitted on overrides or interface implementations

This is currently blocking me completing #3313

[majocha]

This looks not F# specific 😞 but rather a Roslyn issue.

[majocha]

My guess is you'd need an additional InternalsVisibleToFSharp in Roslyn to implement this class.

[dsyme]

cc @Pilchie

[Pilchie]

Tags @jasonmalinowski and @sharwell on this one.

(Sorry for the delay, was on vacation when this hit my inbox).