Re: Enumeration representation clause surprise.

["Randy Brukardt" <randy@rrsoftware.com>]

"Markus Schoepflin" <nospam@no.spam> wrote in message 
news:g2oqld$qli$1@nntp.ilk.net...
> Adam Beneschan wrote:
>> On Jun 11, 6:28 am, Samuel Tardieu <s...@rfc1149.net> wrote:
>>> Markus> This is exactly my point. I would have expected a compile time 
>>> error
>>> Markus> for this impossible request.
>>>
>>> Markus> Representation clauses are (amongst other) meant to specify 
>>> specific
>>> Markus> binary layouts for interfacing with the external world. Or am I
>>> Markus> mistaken here?
>>>
>>> I can't find anything in RM chapter 13 which either forbids or allows
>>> such a behaviour. I will submit a patch for GNAT which gives a new
>>> warning for this case:
>>>
>>>     11.       A1 at 0 range 0 .. 0; -- (*)
>>>                  |
>>>         >>> warning: component representation will be biased
>>
>> This seems odd to me.  Having GNAT select a biased component in
>> Christoph's example makes some sense, since a compiler can choose any
>> representation it likes.  But in Markus' case, he specifically asked
>> for a certain representation for the enumeration types---should GNAT
>> take it upon itself to change that, and display a warning that doesn't
>> make it clear that it's disrespecting his request to have the
>> enumeration represented a certain way?
>
> Do you suggest to reword the warning? Or give an error instead?

I agree with Adam (whose message I mysteriously didn't receive). Even if the 
letter of the RM allows this sort of behavior, it seems wrong from an 
expectation basis: an explicitly given representation clause is being 
ignored where there is a conflict between two of them. That's never a good 
thing. So I think this should be an error - a compiler shouldn't be changing 
explicitly given representation items. (I realize that this may not be a 
generally held opinion.)

For the record, Janus/Ada gives:

In File D:\Testing\Win\console\rep.ada at line 11
--------------
   10:      for RECORD_T use record
   11:         A1 at 0 range 0 .. 0; -- (*)
----------------------------------^
*ERROR* More bits are needed to store all values of the type (6.4.17) [RM 
13.5.1(19)]
Continue <Sp> or Abort <^C>?

for this example. But that's not all that interesting, because Janus/Ada 
doesn't support biased representations.

                                 Randy.