public struct Some<T>
    {
        public T Value { get; }

        public Some(T t) => Value = t;

        public static implicit operator Some<T>(T value)
        {
            return new Some<T>(value);
        }
    }

    public interface ITest { }

    public class Test : ITest { }

    class Program
    {
        static void Main(string[] args)
        {
            Test test1 = new Test();
            ITest test2 = new Test();

            Some<int> some0 = 123; // OK
            Some<ITest> some1 = test1; // OK
            Some<ITest> some2 = test2; // ошибка.. почему???
        }
    }