class Program
{
    static void Main(string[] args)
    {
        Get();
        GetGeneric<string>();
    }

    static void Check(object value) { }

    static string Get()
    {
        var possibleNullValue = JsonConvert.DeserializeObject<string>("null");

        if (possibleNullValue == null)
        {
            Console.WriteLine("null value");
        }

        Check(possibleNullValue);

        return possibleNullValue;
    }

    static T GetGeneric<T>()
    {
        var possibleNullValue = JsonConvert.DeserializeObject<T>("null");

        if (possibleNullValue == null)
        {
            Console.WriteLine("null value");
        }

        Check(possibleNullValue); // "Possible null reference" warning here

        return possibleNullValue;
    }
}

public static class JsonConvert
{
    public static T DeserializeObject<T>() => default;
}

#nullable safeonly

class NullTestWithoutContraint
{
    static T GetGeneric<T>()
    {
        var possibleNullValue = JsonConvert.DeserializeObject<T>();

       if (possibleNullValue is null) {} 
  
       return possibleNullValue;  // expecting a warning here, if T=string for example
    }
}