title: How do I enforce readonlyness of ref readonly in C# 7.2?
body: <p>I want to write a method returning ref a ref readonly <a href="https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/readonly-ref.md" rel="nofollow noreferrer">as described</a>: "The feature allows a member to return variables by reference <strong>without exposing them to mutations.</strong>"</p>
  
  <p>Unfortunately, my code compiles and does such a mutation. How do I make sure it cannot be modified? It is readonly for good reasons. I expect a compiler error. Should I doe something in addition? 200 is modified to 201. I do not want that.</p>
  
  <pre><code>internal class TryClass
  {
      private static int _result = 0;
  
      public static ref readonly int Multiply(int a, int b)
      {
          _result = a * b;
          return ref _result;
      }
  }
  
  internal class Program
  {
      private static void Main(string[] args)
      {
          int x = 10;
          int y = 20;
          var rez = TryClass.Multiply(x, y);
          rez++;
          Console.WriteLine(rez);
          Console.ReadLine();
      }
  }
  </code></pre>