[Mono-bugs] [Bug 78961][Wis] New - [GMCS] runtime should support Covariant / Contravariant generic parameters

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Changed by nazgul at omega.pl.

http://bugzilla.ximian.com/show_bug.cgi?id=78961

--- shadow/78961	2006-07-27 14:48:36.000000000 -0400
+++ shadow/78961.tmp.14040	2006-07-27 14:48:36.000000000 -0400
@@ -0,0 +1,95 @@
+Bug#: 78961
+Product: Mono: Runtime
+Version: 1.0
+OS: All
+OS Details: 
+Status: NEW   
+Resolution: 
+Severity: 
+Priority: Wishlist
+Component: misc
+AssignedTo: mono-bugs at ximian.com                            
+ReportedBy: nazgul at omega.pl               
+QAContact: mono-bugs at ximian.com
+TargetMilestone: ---
+URL: 
+Cc: 
+Summary: [GMCS] runtime should support Covariant / Contravariant generic parameters
+
+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);
+  }
+}