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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Strange warning message
Date: Mon, 27 Nov 2017 11:42:09 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:49182
Date: 2017-11-27T11:42:09+01:00
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On 11/27/2017 09:40 AM, reinert wrote:
> 
> when I compile the enclosed program with option "-gnatwa" (i.e. "gnatmake -gnatwa test1k.adb", I get the following annoying warning:
> 
> "warning: useless assignment to "test1", value never referenced"

In order to produce warnings like this, the compiler has to do some data-flow 
analysis. The amount of data-flow analysis required by the ARM is zero. So GNAT 
is making an extra effort to be helpful. That the warning is annoying in this 
case is the price you pay for it being correct in most others.

Clearly GNAT's data-flow analysis is incomplete, or it would also flag the 
assignment in the loop. Complete data-flow analysis is possible (SPARK does it). 
but apparently it's hard, and possibly not really compatible with a compiler's 
main purpose, but even if GNAT did have complete data-flow analysis, you'd still 
get the warning.

I found that even if I change the loop to

for I in Test1'range loop
    Test1 (I) := new Test1_T;
end loop;

GNAT (7.2) still doesn't flag the assignment in the loop, so it's not just that 
GNAT's analysis hasn't caught up with the new "for E of" construct.

-- 
Jeff Carter
"We burst our pimples at you."
Monty Python & the Holy Grail
16