Re: Operator visibility question

[Robert A Duff <bobduff@world.std.com>]

rod@praxis-cs.co.uk (Rod Chapman) writes:

> Can anyone please tell me if the following code is legal?

In what language -- Ada or SPARK?  ;-) ;-)

> package MT
> is
>    type T is range 0 .. 4;
> end MT;
> 
> with MT;
> --# inherit MT;
> package VT
> is
>    function "+" (Left, Right : in MT.T) return MT.T renames MT."+";
>    A : constant := 2;
>    B : constant MT.T := 3;
> 
>    C : constant Boolean := (A + 2) < B; -- Is "<" visible here?

There are several "<" operators directly visible here, including the one
for root_integer.  But none of them apply -- the "<" for MT.T is not
directly visible here.  So I believe this is illegal in Ada, unless I'm
missing something.

A and 2 are both of type universal_integer, and there are implicit
conversions from u_i to any integer type, but there is no implicit
conversion from MT.T to root_real.

Sounds like a GNAT bug.  Seems like SPARK is correct here.

> end VT;
> 
> The SPARK Examiner rejects the declaration of C with the message:
> 
>    9     C : constant Boolean := (A + 2) < B; -- Is "<" visible here?
>                                          ^
> ***        Semantic Error    :309: Operator not visible for these types.
> 
> On the other hand, GNAT 3.13 accepts the code with no errors.  Can anyone
> explain which is right, and (more importantly) why?
>  Cheers,
>   Rod Chapman
>   SPARK Team
>   Praxis Critical Systems