Combine of covarince/contravariance delegates

[gafter]

The delegate combine operator does not properly account for delegate variance.

See dotnet/roslyn#3711 for details.

[gafter]

The managed language design team requests that this be addressed for the next VS product release.

[gafter]

@LLITCHEV: Can you please suggest who I should tag to get some commitment on this?

[LLITCHEV]

@jkotas Jan, could you please help Neal with the right people to talk to. I assume that this will require changes in the VM. Not so much in the Jit, but maybe yes?

[jkotas]

cc @gkhanna79 This is type system issue.

I do not think that it is a simple bug fix. We would need a new overload of Combine method that takes the type of the combined delegate as an argument, and change C# compiler to use it to combine delegates.

Consider this case - there is no way for the existing Combine method to figure out that the result type should be Func<IFace, object>:

internal class Program
{
    interface IBaseA { }
    interface IBaseB { }

    interface IFace1 : IBaseA, IBaseB { }
    interface IFace2 : IBaseA, IBaseB { }

    interface IFace : IFace1, IFace2 { }

    static void Main(string[] args)
    {
        Func<IFace1, object> d1 = SomeFunction1;
        Func<IFace2, object> d2 = SomeFunction2;

        Func<IFace, object> e1 = d1;
        Func<IFace, object> e2 = d2;

        var d = e1 + e2;
    }

    static object SomeFunction1(IFace1 input)
    {
        return null;
    }

    static object SomeFunction2(IFace2 input)
    {
        return null;
    }
}

[gafter]

The C# (and VB and F#) compiler team will be happy to use whatever API you need to add.

[gkhanna79]

@JohnChen0 Can you please look into this for RC2?

[gafter]

@fadimounir @JohnChen0 @gkhanna79 Do you anticipate having this fixed in time to ship with C#7? The C# and VB compiler teams are happy to do our end of the work, but we need you to tell us which API we should be using.

[gkhanna79]

@gafter What is the timeframe for C# 7? We are heads down with .NET Core RTM and it will be good to understand the timeline before commenting on the next steps.

[gafter]

@gkhanna79 The time frame for getting this fixed in C# 7 is the same as the time frame for the ValueTuple libraries. Having this in place in the next month would enable us to fix the compiler for Dev15.

[gkhanna79]

@gafter I think we can look into this for .NET Core 1.2 as part of the overall API edition effort (it will be a Dev15 Update and not RTM).