c# - Mono + named/optional parameters = compiler bug? -

i have encountered unforeseen consequences of porting mono 2.8.1. problem can boiled down sample program (i have been unable reduce further, after cutting several classes , ~1000 lines of code file quoted below)

public class person {     public person(int age, string name = null){}     public person(double income, string name = null){}     public person(double income, int age, string name = null){} }  class program {     static void main()     {         person p = new person(1.0, name: "john doe");     } } 

compilation of above code mcs gives output:

test.cs(22,24): error cs0584: internal compiler error: internal error test.cs(22,20): error cs0266: cannot implicitly convert type `object' `namedparams.person'.  explicit conversion exists (are missing cast?) compilation failed: 2 error(s), 0 warnings 

removing use of optional/named parameter (i.e. calling new person(1.0, null, "john doe") or new person(1.0, null, name:"john doe"), or new person(1.0, "john doe") ) leads flawless compilation. also, under vs2010 file (and whole solution started with) compiles fine. casting removes error cs0266, not cs0584 -- no surprise there.

my question: me doing wrong, or mcs (i.e. bug in mcs obvious me -- else ,,internal error'' mean, perhaps it's ok such program won't compile), or maybe microsoft compiler in vs2010 should not let such code compile?

i bet it's mcs who's wrong (unable guess right constructor), perhaps it's otherwise , shouldn't know better?

ps. tried searching known bug in both google , novell's bugzilla, unable find relevant. again, may blind ;)

okay, here goes. crash indeed due third overload, person(double income, int age, string name = null). compiler sees trying pass less arguments listed in signature goes looking optional arguments. happily notices name optional , assumes not passing argument. appending placeholder @ end of supplied arguments. next, goes reorder named arguments in list end in right position. means john doe correctly @ last position name, placeholder gets age position. compiler tries fill in default values, shocked find placeholder @ location has no default value. thinks can not happen, since placeholder added optional argument , not optional anymore. not knowing do, throws exception.

the following patch seems fix issue (however may break else, no warranty):

--- mono-2.6.7.orig/mcs/mcs/ecore.cs    2009-10-02 12:51:12.000000000 +0200 +++ mono-2.6.7/mcs/mcs/ecore.cs 2010-12-21 02:26:44.000000000 +0100 @@ -3803,6 +3803,15 @@                                  int args_gap = math.abs (arg_count - param_count);                                 if (optional_count != 0) { +                                       // readjust optional arguments passed named arguments +                                       (int = 0; < arguments.count; i++) { +                                               namedargument na = arguments[i] namedargument; +                                               if (na == null) +                                                       continue; +                                               int index = pd.getparameterindexbyname (na.name.value); +                                               if (pd.fixedparameters[index].hasdefaultvalue) +                                                       optional_count--; +                                       }                                         if (args_gap > optional_count)                                                 return int.maxvalue - 10000 + args_gap - optional_count;