Re: GNAT doing macro‑expansion? (Pre/Post conditions)

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Wed, 24 Oct 2012 18:45:27 +0200, Yannick Duchêne (Hibou57)  
<yannick_duchene@yahoo.fr> a écrit:

> Le Wed, 24 Oct 2012 18:40:58 +0200, Yannick Duchêne (Hibou57)  
> <yannick_duchene@yahoo.fr> a écrit:
>
>> same snippet. Obviously, this does not stand for the reliable
> Read “does not stand for the real thing”.
>
>>            type A_Type  is mod 256;
>
> Sorry, forget to say about a second variation: replace the above with  
> `type A_Type is new Positive;` and there will be no more error at  
> runtime (the first error, that is, with the useless package body still  
> there). That one is a lot weird.
>

More on that particular point. I you change `mod 256` into anything the  
modulo is above 256, not necessarily a power of two, as this works with  
`mod 257` two, then the bug disappears. The bug is there for any value  
below and included, 256.

I suspected the element size, and indeed, that's not really the modulo,  
but the size. Replace `type A_Type is mod 256;` with this, with an added  
representation clause:

     type A_Type  is mod 256;
     for A_Type'Size use 16;

The module is still 256, but now the size changed, and the bug is not  
exposed.


-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University