---- Reported by nazgul@omega.pl 2006-07-27 14:48:36 MST ----

Please fill in this template when reporting a bug, unless you know what you
are doing.
Description of Problem:
In .NET IL (but unfortunately not in C#) it is possible to mark generic
parameters as covariant (+) or contravariant (-)
It makes possible to convert generic classes to other versions with respect
to their generic arguments (like IList<string> to IList<object>).
According to .NET 2.0 spec it is allowed only for interfaces and delegates.

Unfortunately this aspect of .NET specification is not implemented in Mono
now. I present an example

Steps to reproduce the problem:
1. Run attached out.exe binary (you can obtain it also by compiling
attached IL example)

Actual Results:
aa

Unhandled Exception: System.NullReferenceException: Object reference not
set to an instance of an object
  at Test.processObjects (ICovariantEnumerator`1 x) [0x00000] 
  at Test.Main () [0x00000] 

Expected Results:
aa
bb

How often does this happen? 
Always

Additional Information:
The example is created by Nemerle compiler and works fine on MS.NET. The
original code is:

interface ICovariantEnumerator [+T] 
{
  MoveNext () : bool;
  Current : T { get; }
}

[Record]
class Enumerator [T] : ICovariantEnumerator [T]
{
  enu : System.Collections.Generic.IEnumerator [T];
  
  public MoveNext () : bool
  {
    enu.MoveNext ();
  }
  
  public Current : T {
    get { enu.Current }
  }
}

module Test {
  public processObjects (x : ICovariantEnumerator [object]) : void { 
    _ = x.MoveNext ();
    System.Console.WriteLine (x.Current);
  }

  public processStrings (x : ICovariantEnumerator [string]) : void { 
    _ = x.MoveNext ();    
    System.Console.WriteLine (x.Current);    
  }
  
  public Main () : void 
  {
    def x = System.Collections.Generic.List ();
    x.Add ("aa");
    x.Add ("bb");
    def y = Enumerator (x.GetEnumerator ());
    Test.processStrings (y);
    Test.processObjects (y);
  }
}



---- Additional Comments From nazgul@omega.pl 2006-07-27 14:50:19 MST ----

Created an attachment (id=170170)
test.zip


Imported an attachment (id=170170)