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/Program.cs

using System;

namespace Program
{
	public class Program
	{
		public static void Main(string[] args)
		{
			Nullable<int> i = 5;
			Console.WriteLine(i.GetType());
			OwnNullable<int> j = 6;
			Console.WriteLine(j.GetType());
		}
	}
}

struct OwnNullable<T>
	where T : struct
{
	private T field;
	public OwnNullable(T value)
	{
		field = value;
	}

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