Return "partial" OneOf types

[juantxorena]

I'm liking this library, but I've found a situation which IMHO is a bit annoying and doesn't really make sense. Consider this code:

public async Task<OneOf<None, Unknown, False>> DoThings(Object input)
{
    if (something)
    {
        return new False();
    }
    OneOf<None, Unknown> result = await OtherMethod(input);
    return result;
}

It fails to compile, because it cannot convert from OneOf<None, Unknown> to OneOf<None, Unknown, False>. It makes sense, but considering that I'm returning a False, or a None or Unknown, I think it should work. Instead, I have to manually match the output and return the types, like this, which is pretty redundant:

public async Task<OneOf<None, Unknown, False>> DoThings(Object input)
{
    if (something)
    {
        return new False();
    }
    OneOf<None, Unknown> result = await OtherMethod(input);
    return result.Match<OneOf<None, Unknown, False>>(
	none => new None(),
	unknown => new Unknown()
    );
}

Maybe it's difficult to implement, but IMHO it makes sense that it should work.