From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Thread: 103376,8e97b70a5f7d5495 X-Google-Attributes: gid103376,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.google.com!news.germany.com!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: "(see below)" Newsgroups: comp.lang.ada Subject: Re: Brain bug or GNAT bug? Date: Thu, 01 Mar 2007 00:39:39 +0000 Message-ID: References: <87irdlx1ol.fsf@ludovic-brenta.org> Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Trace: individual.net iEhMa9I9i5VqjU4L6y0J/gsMiuggcfKbNXA+GvI9o6JPoZlMyk User-Agent: Microsoft-Entourage/11.3.3.061214 Thread-Topic: Brain bug or GNAT bug? Thread-Index: AcdbmhgNVm9oiMeNEdugQwARJIjQTg== Xref: g2news2.google.com comp.lang.ada:9611 Date: 2007-03-01T00:39:39+00:00 List-Id: On 1/3/07 00:11, in article 87irdlx1ol.fsf@ludovic-brenta.org, "Ludovic Brenta" wrote: >> generic >> type modular is mod <>; >> package try is >> >> type DT1 (the_size : modular) is limited private; >> >> -- subtype bounded is modular; >> subtype bounded is modular range 1..9; >> type DT2 (the_size : bounded) is limited private; >> -- | >> -- >>> subtype must be compatible with parent discriminant >> >> function is_empty (the_data : DT2) return Boolean; >> >> private >> type a_thing is null record; >> type a_ptr is access a_thing; >> type a_ptr_array is array (modular range <>) of a_ptr; >> >> type DT1 (the_size : modular) is >> record >> things : a_ptr_array (1 .. the_size); >> end record; >> >> type DT2 (the_size : bounded) is new DT1(the_size); >> end try; >> >> and I get the error message commented-out at line 11. >> >> If I substitute the declaration at line 9 for that at line 10, >> the test program compiles and runs correctly. >> >> Am I blundering here, or is this a GNAT bug? > > Per ARM 3.7(15), the types of DT1.the_size and DT2.the_size must be > statically compatible. In other words, you must guarantee at compile > time that "the_size" is in the range of type "modular". Because > modular's range is not known at compile time but bounded's is, no such > guarantee exists. > > If you remove the "range" constraint on subtype "bounded", then the > range for "bounded" and the range of "modular" are known at compile > time to be identical, so all is well. But if the declaration of bounded compiles, the range of bounded must be a subset of the range of modular, so DT2.the_size must be in the range of modular. The rule at ARM 3.7(15) seems overly restrictive. > What are ou trying to achieve? Implement DT2 as a derivation of DT1 with guaranteed bounds on its size (in the actual s/w, the bounds are also generic parameters, and DT1 is actually declared in a distinct, with-ed package). At the moment bounds checking for DT2 has to be done by a runtime check on use of DT2 objects. I can't help feeling the type system should be able to catch a misuse, at the point of declaration of a DT2 object, but there does not seem any way to do that, thanks to ARM 3.7(15). -- Bill Findlay chez blueyonder.co.uk