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Date: Tue, 25 Nov 2008 23:09:34 +0100
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Subject: Re: Extending discriminant types
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Date: 2008-11-25T23:09:37+01:00
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Adam Beneschan wrote:
> On Nov 25, 12:49 pm, "Jeffrey R. Carter"
> <spam.jrcarter....@spam.acm.org> wrote:
>> Georg Bauhaus wrote:
>>
>>> generic
>>>    type N is (<>);
>>> package U is
>>>    type T (Len: Integer) is private ;
>>>    type Count is new N range 0 .. N'Base'Last;  -- 'BASE
>> It's interesting that this compiles, since N is not an integer type.
>> I suspect
>> you've uncovered a compiler error.
> 
> It also compiles if you remove the 'Base attribute.

In fact, GNAT has let me instantiate this unit in Ada 2005 mode
after I had commented the object declaration mentioning T'Base:

WITH u;
PACKAGE uu IS NEW u(N => INTEGER);

But I'm lost here, is this really a bug?  (I vaguely remember an
issue with using integer types for enum I/O, see below.)
Another compiler complains about N's range using inappropriate
operands. The Ada 83 RM explains generic formal (<>) type in terms
of their operations. And then, in 12.3 on Generic Instantiation,

"For each occurrence, within the generic unit, of a name that denotes a
 given entity, the following list defines which entity is denoted by the
 corresponding occurrence within the instance.
...
"4. For a name that denotes a generic formal type: The corresponding
    name denotes the subtype named by the associated generic actual
    parameter (the actual subtype). "

(Which, I believe, is INTEGER in the instantiation above.) The
following unit is accepted by two different compilers:

with Ada.Text_IO;
package I is new Ada.Text_IO.Enumeration_IO(Integer);