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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Valid Attribute and Unchecked Conversion
Date: 1996/10/14
Message-ID: <Dz8oLE.KuB@world.std.com>#1/1
X-Deja-AN: 189196597
references: <3252ED6B.1B74@lmco.com> <53151i$ddd@newsbf02.news.aol.com>
 <dewar.844516953@schonberg> <Dyvo0w.7Dr@thomsoft.com>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-10-14T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <Dyvo0w.7Dr@thomsoft.com>, Keith Thompson <kst@thomsoft.com> wrote:
>But RM95-13.9.1(12) says:
>	A call to an imported function or an instance of
>	Unchecked_Conversion is erroneous if the result
>	is scalar, and the result object has an invalid
>	representation.
>
>Probably that paragraph should have been in 13.9 rather than 13.9.1.

True.

>Note that there are several paragraphs after this in the AARM encouraging
>implementations to behave sensibly:
>
>	[...] We considered requiring such sensible behavior,
>	but it resulted in too much arcane verbiage, and
>	since implementations have little incentive to behave
>	irrationally, such verbiage is not important to have.
>
>So implementations are encouraged to implement sensibly something that
>users are encouraged to avoid.

No, no.  The "sensible" comment in the AARM is *not* referring to the
paragraph about erroneousness of unchecked conversions.  It is referring
to the run-time behavior of "invalid" values, which can arise from many
things, but mainly uninitialized variables.  The intent is that (for
example):

    if X = Red and X /= Red then ...

should be True, despite the fact that X is not initialized (assuming the
compiler fails to catch the bounded error, and raise an exception).

This has nothing to do with the erroneousness defined in paragarph 12,
which would have been clearer if the AARM comment were after paragraph
11, rather than after paragraph 12.

- Bob