using System;

class Program
{
    delegate T F<T>() where T : class;
    static void Main()
    {
        new Program().Foo(() => 1);
    }
    void Foo<T>(F<T> x) where T : class { }
}

static class A
{
    public static void Foo<T>(this T x, Func<int> f) { }
}