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Program.cs

using System.Text;

namespace ConsoleApplication
{
    class Program
    {
        interface IHoge<out T>
        {
            T Foo();
        }

        class Hoge<T> : IHoge<T> where T : new()
        {
            public T Foo() => new T();
        }
        static void Main(string[] args)
        {
            var hoge = new Hoge<StringBuilder>();
            IHoge<object> ihoge = hoge;
        }
    }
}