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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT bug with assertions
Date: Mon, 13 Aug 2018 16:54:26 +0100
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Date: 2018-08-13T16:54:26+01:00
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Simon Wright <simon@pushface.org> writes:

> AdaMagica <christ-usch.grein@t-online.de> writes:
>
>> Am Sonntag, 12. August 2018 20:51:51 UTC+2 schrieb Simon Wright:
>>> AdaMagica writes:
>>>
>>> > Thank you all for your time to think about this.
>>> >
>>> > I'll send it in to AdaCore. Hopefully it will be correted next year
>>> > in the public version.
>>>
>>> This is a somewhat artificial error, isn't it?
>>>
>>>    function Maybe (X : Priv) return Subt;
>>>    ...
>>>    function Maybe (X : Priv) return Subt is (X);
>>>    ...
>>>    X := Maybe (C);   -- C is -1
>>>
>>> works as we would have expected.
>>
>> What is it that you expect here? What type is X? I didn't try with
>> GNAT, but I would expect Maybe (C) to raise Assertion_Error
>> independently of X.
>
> Oh come on! these are additions to the example that *you* posted upthread.

Oops.

X is of type Subt.

It can get quite misleading to use expression functions when trying to
use the debugger to work out where an exception is raised.

Given

   function Maybe (X : Priv) return Subt is
   begin
      return X;
   end Maybe;

called as

   S : Subt := Maybe (-);
   
Maybe happily returns -1 and leaves it up to the assignment to raise the
necessary exception.

And if the assignment is

   P : Priv := Maybe (-1);

no exception is raised.