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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: To collection (GNAT bug)
Newsgroups: comp.lang.ada
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Date: Mon, 2 Oct 2006 10:23:39 +0200
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On Sun, 01 Oct 2006 19:23:26 GMT, Jeffrey R. Carter wrote:

> Dmitry A. Kazakov wrote:
>> 
>> See ARM 13.9 (17), which requires reversibility of Unchecked_Conversion.
>> That is clearly violated in the example given. Though it is legal not to
>> provide address to access type conversion through Unchecked_Conversion, it
>> is still illegal to provide it wrong.
> 
> It doesn't require it; it merely advises it, and then only "where this 
> clause defines the result". "This clause defines the result" only when 
> "The representation of S is a representation of an object of the target 
> subtype". That is not necessarily true in your case. So lack of 
> reversibility is not an error, even for a compiler that adheres to this 
> advice.

OK, it can be classified as implementation-defined, because the compiler
designer can always claim that Address representation is not the pointer's
one. Who could rebut him?

However, I don't think that it was really the intended behavior in this
case. [ If access String is a fat pointer, then Unchecked_Conversion should
not be allowed at all. Otherwise it must honor the String's dope. ]

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de