Why has a lambda with no capture changed from a static in C# 5 to an instance method in C# 6?

Question

This code throws an exception on the marked line:

using System;
using System.Linq.Expressions;

namespace ConsoleApplication2
{
    class Program
    {
              


        

Answer 1

Why is it so?

I don't know why, and honestly was unaware of that change, but taking a quick look at the decompiled code showed that for all similar lambdas inside the class Roslyn generates instance methods in a singleton nested class called <>c like this

internal class Program
{
    [CompilerGenerated]
    [Serializable]
    private sealed class <>c
    {
        public static readonly Program.<>c <>9;
        public static Action <>9__0_0;

        static <>c()
        {
            Program.<>c.<>9 = new Program.<>c();
        }

        internal void 
b__0_0(int x, int y)
        {
            Console.WriteLine(x + y);
        }
    }
}

To me this is a breaking change, but I didn't find any information about that.

What about how to make your code working, I think @Rob answer covers that part.