Re: Dispatching Example

["Ed Colbert" <colbert@abssw.com>]

Hi Tucker,

"Tucker Taft" <stt@avercom.net> wrote in message
news:3B9547D9.BE2CC282@avercom.net...
> Ed Colbert wrote:
> >
> > Hi All,
> >
> > Below is a simplified example from Richard Riehle's & my course on Ada.
> > GNAT v3.13p says that the call Is_Item is an mbiguous expression, and
> > ObjectAda v 7.2 says the statement is illegal based on LRM 5.2(6).  Both
> > Richard and I thought that the expression is tag indeterminate, LRM
> > 3.9.2(6), and would static resolved, LRM 3.9.2(19).  Are Richard and I
> > missing something or are both compilers wrong?
>
> The call on Is_Item is ambiguous, even though one of the
> possible interpretations (the one returning type T1) violates
> 5.2(6).  This is one of those cases where the overload
> resolution rules are less picky than the legality rules,
> which can be a bit confusing.
>
> ObjectAda is not being very helpful by telling you that one
> of the interpretations is illegal, without first telling
> you that the expression is considered ambiguous by the
> overloading rules.
>

Why is the expression ambiguous?  Isn't the tag of C, which in this case is
compile time determinable, used to resolve the call to Is_Item per 5.2(9)?

I suppose it could be ambiguous if the expression is evaluated first as
allowed by 5.2(7); but then when would 5.2(8-10) apply?

> In any case, if only the Is_Item that returned type T1 were
> directly visible, the expression would not be ambiguous, and
> the call would be legal.  It would involve a "dispatching
> on result" where the tag of "C" would determine which body
> of Is_Item were called.
>
> In general the overload resolution and legality rules relating
> to classwide assignment are confusing at best.  We tried other
> rules, but they had worse problems, so I suppose the current
> rules are the best of a set of unpleasant alternatives.
> ObjectAda's error message was unfortunately only adding to
> the confusion.
>

Clearly, there's something that's confusing Richard & I.

> >
> > package Dispatching_Examples
> > is
> >   type T1 is tagged private;
> >   function Is_Item return T1;
> >
> >   type T2 is new T1 with private;
> >   function Is_Item return T2;
> >
> > private
> >    -- full definition of T1 and T2
> >   type T1 is tagged null record;
> >   type T2 is new T1 with null record;
> > end Dispatching_Examples;
> >
> > with Dispatching_Examples;
> > use Dispatching_Examples;
> > procedure Dispatching_Examples_Driver is
> >     A, B : T1;                    -- simple variables
> >     Q, R : T2;                    -- simple variables
> >     C    : T1'Class := A;     -- class-wide variable
> >     D    : T2'Class := R ;    -- class-wide variable
> > begin
> >     C := Is_Item ;              -- static resolution?
> > end Dispatching_Examples_Driver;
>
> --
> -Tucker Taft   stt@avercom.net   http://www.avercom.net
> Chief Technology Officer, AverCom Corporation (A Titan Company)
> Bedford, MA  USA (AverCom was formerly the Commercial Division of
AverStar:
> http://www.averstar.com/~stt)
>