Add extension versions of UpCast and DownCast

[FreeApophis]

Just had an example where I replaced Wheres and Selects with a DownCast, but it is awkward, because the solution before was fluent, and now it is mixed.

        CannotReadRemoteService = DownCast<ServerIniUpdateSection.Exists>.From(_source.LeftOrNone())
            .Select(s => s.IsServiceOnOtherMachine)
            .GetOrElse(false);

An extension version probably would look like this:

        CannotReadRemoteService = _source
            .LeftOrNone()
            .DownCast<ServerIniUpdateSection.Exists>()
            .Select(s => s.IsServiceOnOtherMachine)
            .GetOrElse(false);

[bash]

I thought the main issue was that such an extension method requires two generic type parameters, see also my comment on the discussion about OfType. Is this maybe fixed with the upcoming extension members?

[FreeApophis]

I will test if it works with the new extension syntax, it looks possible if you have a generic type on the extension and the other type on the method. However I have not read that to be a feature, but it might have been overlooked that this is a new possibility.

Well the other solution would be, to not implement it as an Extension, it is certainly possible to implement it directly on Option<T>

OfType uses the non-generic IEnumerable interface to be able to implement it on IEnumerable we could also do that on IOption.

[FreeApophis]

The implemetation in C# was possible, but the Method is not found on the Option. I am a bit confused. I'd like to report that issue.

public static class OptionExtensions
{
    extension<TItem>(Option<TItem> option)
        where TItem : class
    {
        public Option<TResult> DownCast<TResult>()
            where TResult : class
            => DownCast<TResult>.From(option);
    }
}

Update: oh boy, this works:

Option<string> stringOption = option.DownCast<object, string>();

I started a discussion here: dotnet/csharplang#9714