Add support of HasValue/Value idiom to the ?? operator.

Now you can write the following code:
WriteLine(VSome(42) ?? -1);
WriteLine(VNone() ?? -1);

macros/operators.n

@@ -306,6 +306,16 @@ namespace Nemerle.Core
       toption.Equals (t)
       || (t.BaseType != null && toption.Equals (t.BaseType))
     }
+    def isHasValuePattern(ti : TypeInfo) : bool
+    {
+      def isBoolPropertyOrField(members : list[IMember]) : bool
+      {
+        | [member is IField] | [member is IProperty] => true
+        | _ => false
+      }
+      isBoolPropertyOrField(ti.LookupMember("Value",    false)) &&
+      isBoolPropertyOrField(ti.LookupMember("HasValue", false))
+    }
 
     def tExprA = typer.TypeExpr(exprA);
     def tExprB = typer.TypeExpr(exprB);
@@ -335,12 +345,13 @@ namespace Nemerle.Core
             Message.Error(exprA.Location, $"Can't infer type of left operand ($exprA) of the `??' operator");
             None()
 
-        | Some (Class (tiA, _)) when tiA.IsValueType && !isNullable(tiA) =>
+        | Some (Class(tiA, _)) when isHasValuePattern(tiA) => resolve(isHasValuePattern, lastTry)
+        | Some (Class(tiA, _)) when tiA.IsValueType && !isNullable(tiA) =>
             Message.Error(exprA.Location, $"Left operand ($exprA) of the `??' operator should be reference or nullable type ");
             None()
 
-        | Some (Class (tiA, _))   when isNullable(tiA) => resolve(isNullable, lastTry)
-        | Some (Class (tiA, [_])) when isOption(tiA)   => resolve(isOption,   lastTry)
+        | Some (Class(tiA, _))   when isNullable(tiA) => resolve(isNullable, lastTry)
+        | Some (Class(tiA, [_])) when isOption(tiA)   => resolve(isOption,   lastTry)
         | _ => Some(<[ if ($exprA != null) $exprA else $exprB ]>)
       }
     }