[Bug c++/16711] [3.4/3.5 regression] Bug in reporting integral constant expressions problems

["romixlev at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From romixlev at yahoo dot com  2004-07-27 07:07 -------
Subject: Re:  [3.4/3.5 regression] Bug in reporting integral constant expressions problems

Actually, my bug report comes from the following area.
I'm using PROP, a sort of C++ preprocessor, that was
developed may be 10 years ago, which translates a
superset of C++ into "normal" C++. Among other things
it supports algebraic data types and garbage
collection for C++. For a given algebraic data type
there can be several diffeent constructors (in the
sense of algebraic datatypes, not in C++ sense) with
and without arguments. The constructors with arguments
are dynamically creating the objects on the heap. The
unit constructors (i.e. those without arguments) do
not create objects. Instead they are represented by
some special values like (AlgebraicDataType
*)small_integer.
In principle, they are like enumerations. But these
values should be assignable to a pointer to the
AlgebraicDataType, they are always defined like:
#define UniConstructorName (AlgebraicDataType * )
small_integer

Later this macro is used everywhere in the code
generated by PROP.
  
The funny thing about this is that I'm using it since
6-7 years and have never had problems with it undar
any compiler. So, when I switched to GCC 3.4.1 I was
very surprised to see, that suddenly the automatically
generated code was illigal... 

But now, after some clarifications, I see that the
standard really does not allow such conversions into
non-integral types insdie an integral  constant
expression. So, I simply changed PROP to generate a a
code that it standard compliant. Now it works fine.
BTW, there was a small problem while changing PROP.
Since PROP uses bootstraping (i.e. written in itself),
its code was generated with the old version of it and
of course was using these illegal constructs. I had to
change them by hand (!!!) in several hundred places,
before I could compile a new version of PROP. After
that I regenerated it with the new version and now it
does not use illegal constructs. An G++ option that
would explicitly allow old behavior (at least for some
time, before making this constructs totally obsolette)
would help in such situations. But I guess, I ask too
much, or? :)

--- jsm at polyomino dot org dot uk
<gcc-bugzilla@gcc.gnu.org> wrote:

> 
> ------- Additional Comments From jsm at polyomino
> dot org dot uk  2004-07-26 20:30 -------
> Subject: Re:  New: Bug in reporting integral
> constant expressions
>  problems
> 
> On Mon, 26 Jul 2004, romixlev at yahoo dot com
> wrote:
> 
> > G++ 3.4.1 rejects some integral constant
> expressions that were allowed in the
> > earlier versions. The same file compiled with GCC
> as a C file does not produce
> > any errors. This report seems to be related to
> Bugzilla Bug 16489.
> 
> Both the C and C++ standards are clear that casts to
> non-integer types
> aren't allowed in integral constant expressions
> (C90, C++) / integer
> constant expressions (C99).  Thus the bug is the
> nondiagnosis of:
> 
> > // Size of array should be an integral constant
> expression, or?
> > // But G++ does not complain about this expression
> > */
> > int array [((int)(void*)1)];
> 
> The constant expressions bugs for C are bugs 456 and
> 5675.  Addressing
> them is on my C standards conformance roadmap
> <http://www.srcf.ucam.org/~jsm28/gcc/#stdc>, but
> after dealing with other
> miscellaneous obscure bugs such as 13801, and it is
> a fair amount of work
> to implement properly an obscure area of the
> standard with little
> relevance to real code.  Given the obscurity of the
> area (and that there
> is hardly a great deal of user interest in the finer
> points of
> conformance), I'm surprised that this came up as a
> bug report, and still
> more so that real code is actually using such things
> that are/were
> undocumented extensions to constant expressions. 
> (The intention will
> still be that such undocumented extensions are
> liable to be removed
> without notice, as those in C++ have been with the
> implementation of the
> new parser, but it's interesting that such removal
> has had *any* impact on
> user code.)
> 
> 
> 
> -- 
> 
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16711
> 
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
> 



		
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16711