Attribute decoration to generate cast from one OneOfBase to another

[BreadTh]

Considder the following code:

public class Scenario
{
    private readonly Random random = new();

    public RealOutcome DoSomething()
    {
        if (random.Next() > int.MaxValue / 2)
            return new ErrorA();

        //Error: Cannot implicitly convert 'PartialOutcome' to 'RealOutcome'
        return DoSomethingElse();
    }

    private PartialOutcome DoSomethingElse()
    {
        if (random.Next() > int.MaxValue / 2)
            return new ErrorB();

        if (random.Next() > int.MaxValue / 2)
            return new ErrorC();

        return new TheResult();
    }
}


[GenerateOneOf]
public partial class RealOutcome : OneOfBase<
    TheResult,
    ErrorA,
    ErrorB,
    ErrorC>
{ }

[GenerateOneOf]
public partial class PartialOutcome : OneOfBase<
    TheResult,
    ErrorB,
    ErrorC>
{ }

public class TheResult { }
public class ErrorA { }
public class ErrorB { }
public class ErrorC { }

To remedy the error, one can add to RealOutcome the following boilerplate method:

public static implicit operator RealOutcome(PartialOutcome other)
{
    if (other.TryPickT0(out TheResult pick0, out OneOf<ErrorB, ErrorC> rest0))
        return pick0;
    if (rest0.TryPickT0(out ErrorB pick1, out var pick2))
        return pick1;
    return pick2;
}

I've done this more than a few times now (love this library!), so it would be really nice to have a shorthand that just generates the cast. Something like [GenerateOneOfImplicitCastAttribute<PartialOutcome>] (Now that we have generic attributes!)

This should of course only work if the type cast from's generic types are all contained in the cast to's generic types

I'd write it myself, but source generators still bend my mind for the time being.

[GuyWhoKnowsTheGuy]

This is particularly useful when when using option types, for example

[GenerateOneOf]
public partial class StringOrNumber : OneOfBase<string, int> { }

StringOrNumber myString = "My String"; // this compiles
StringOrNumber myNumber = 2; // this compiles

OneOf<StringOrNumber, None> foo = 3; // this does NOT compile
OneOf<StringOrNumber, None> bar = "hello world"; // this does NOT compile