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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Did I find a bug here?
Date: Sat, 3 Apr 2021 11:18:47 +0300
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On 2021-04-03 9:41, J-P. Rosen wrote:
> Le 03/04/2021 à 07:46, reinert a écrit :
>> Could AB_Type(X) in "case AB_Type(X) is" function as such a valid check?
> Yes, 


I believe not. The use of X as an argument to a type conversion is an 
"evaluation" of X, by RM 4.6(28), which can be a bounded error by RM 
13.9.1(9) if X'Valid is False.

That bounded error can lead to an exception or simply to continued 
execution with the invalid value.


> but I recommend "case AB_Type'(X) is", i.e. a qualification rather 
> than a conversion.


That also requires an evaluation of X, by RM 4.7(4), and again can be a 
bounded error if X'Valid is False.

The use of X in X'Valid is explicitly defined to mean that X is "read" 
but is not "evaluated", RM 13.9.2(12)(13). So it seems to be the only 
safe way to check for validity.