Re: Overloading for T and T'Class

[comar@cs.nyu.edu (Cyrille Comar)]

jgv@swl.msd.ray.com (John Volan) writes:

: However, doing this might cause overload resolution problems when you
: make an invocation:
: 
: 	with P;
: 	procedure P_Client (X: in out P.T) is
: 	begin
: 	  ...
: 	  P.Op (X); -- Error in overload resolution -- ambiguous.
: 	  ...
: 	  P.Op (P.T'(X));  -- Although maybe these would work?
: 	  P.Op (P.T'Class'(X));
: 	  ...
: 	end P_Client;
: 
: On the other hand, the following doesn't appear to be ambiguous:
: 
: 	with P;
: 	procedure Another_P_Client (X: in out P.T'Class) is
: 	begin
: 	  ...
: 	  P.Op (X); -- No problem, only one way to interpret this
: 	  ...
: 	end Another_P_Client;
: 
: (FYI: GNAT-1.83 seems to like the package spec P, but dies with an
: assertion failure on an attempt to invoke.  (Bug report has been
: filed.)  Anyway, it would be nice to get some human confirmation on
: this.  Tucker, you have the last word: Is this legal?)

Jon, 
your analysis is not quite correct, all the calls in your example are ambiguous
even the last one: it can be interpreted either as a dispatching call to the
primitive Op or a simple call to the classwide Op.  The latest version of gnat
(to be released shortly :-) does a better job than 1.83 and give the right
diagnostics:

gcc -c -gnatl ess.adb

NYU GNAT Compiler Version 2.0 (C) Copyright NYU, 1992,1993,1994

Compiling: ess.adb  94-11-19 04:45.08

     1. procedure ess is
     2.    package P is
     3.       type T is tagged null record;
     4.       procedure Op (X : in out T);
     5.       procedure Op (X : in out T'Class);
     6.    end P;
     7. 
     8.    package body P is
     9.       procedure Op (X : in out T) is begin null; end;
    10.       procedure Op (X : in out T'Class) is begin null; end;
    11.    end P;
    12. 
    13.    procedure P_Client (X: in out P.T) is
    14.    begin
    15.       P.Op (X); -- Error in overload resolution -- ambiguous.
               |
        >>> ambiguous expression (cannot resolve "Op")
        >>> possible interpretation at line 5
        >>> possible interpretation at line 4

    16.       P.Op (P.T'(X));  -- Although maybe these would work?
               |
        >>> ambiguous expression (cannot resolve "Op")
        >>> possible interpretation at line 5
        >>> possible interpretation at line 4

    17.       P.Op (P.T'Class'(X));
               |
        >>> ambiguous expression (cannot resolve "Op")
        >>> possible interpretation at line 5
        >>> possible interpretation at line 4

    18.    end P_Client;
    19. 
    20.    procedure Another_P_Client (X: in out P.T'Class) is
    21.    begin
    22.       P.Op (X);
               |
        >>> ambiguous expression (cannot resolve "Op")
        >>> possible interpretation at line 5
        >>> possible interpretation at line 4

    23.    end Another_P_Client;
    24. 
    25. begin
    26.    null;
    27. end;
-- 
------------------------------------------------------------------------
Cyrille Comar,                                  E-mail: comar@cs.nyu.edu
Gnat Project                                    US phone: (212) 998-3489