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List:       cfe-dev
Subject:    Re: [cfe-dev] [libcxx] Policy with respect to language extensions
From:       Hal Finkel via cfe-dev <cfe-dev () lists ! llvm ! org>
Date:       2015-08-13 0:23:30
Message-ID: 18952743.745.1439425407679.JavaMail.javamailuser () localhost
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----- Original Message -----
> From: "Richard Smith" <richard@metafoo.co.uk>
> To: "Hal Finkel" <hfinkel@anl.gov>
> Cc: "Marshall Clow" <mclow.lists@gmail.com>, "clang developer list" \
> <cfe-dev@lists.llvm.org>, "Gonzalo BG" <gonzalobg88@gmail.com>, "David Majnemer" \
>                 <david.majnemer@gmail.com>
> Sent: Wednesday, August 12, 2015 7:09:22 PM
> Subject: Re: [cfe-dev] [libcxx] Policy with respect to language extensions
> 
> On Wed, Aug 12, 2015 at 4:01 PM, Hal Finkel < hfinkel@anl.gov >
> wrote:
> 
> ----- Original Message -----
> > From: "Marshall Clow" < mclow.lists@gmail.com >
> > To: "David Majnemer" < david.majnemer@gmail.com >
> > Cc: "Richard Smith" < richard@metafoo.co.uk >, "clang developer
> > list" < cfe-dev@lists.llvm.org >, "Gonzalo BG"
> > < gonzalobg88@gmail.com >
> > Sent: Wednesday, August 12, 2015 5:05:05 PM
> > Subject: Re: [cfe-dev] [libcxx] Policy with respect to language
> > extensions
> > 
> > On Wed, Aug 12, 2015 at 11:32 AM, David Majnemer <
> > david.majnemer@gmail.com > wrote:
> > 
> > 
> > On Wed, Aug 12, 2015 at 2:05 PM, Richard Smith <
> > richard@metafoo.co.uk > wrote:
> > 
> > 
> > 
> > I think it would also be useful to have a "strictly conforming" (or
> > as close as we can reasonably get) mode, controlled by a macro.
> > 
> > 
> > Wouldn't we have to be extra careful or we could get ODR violations
> > across object/shared object boundaries if one wanted extra
> > conformance but another did not?
> > 
> > Yes.
> > 
> > 
> > -- Marshall
> 
> Having a macro to control this seems like a recipe for bad things
> (ODR violation being only one problem).
> 
> 
> 
> What sort of actual bad things do you envisage? People will, of
> course, be able to shoot themselves in the foot by having an
> inconsistent definition in different translation units, but we don't
> necessarily need to support that. And it seems like we should be
> able to avoid this having an ABI impact on sane code.

For these particular extensions, I can't say. However, I tend to find the general \
technique problematic. The issue generally arises for header-only libraries that \
define (or undefined) these kinds of macros, causing very odd interactions (because \
such things don't compose). Tracking down why including one library header breaks \
some other library header because of conflicting desires to define _XOPEN_SOURCE, \
_GNU_SOURCE, etc. is annoying. Doing the same for features of the C++ standard \
library that could affect uses of SFINAE, etc., I suspect, would be especially \
annoying.

 -Hal

> 
> I think the best we can do at this point is have the compiler issue a
> warning in pedantic mode. Maybe we could add some kind of extension
> attribute for this purpose (although we'd need to think carefully
> about when to actually generate the warning - every time some
> extension participates in some overload set might be problematic).
> 
> -Hal
> 
> 
> 
> > 
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev@lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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