Yes, I think this would be an improvement, but it would also be a normative
change. [expr.mul]/4 is very explicit and clear that division by zero is
undefined behavior, even for floating-point types.

Is there a reason we haven't re-synced our floating-point
specification with C's much more explicit and nailed-down formulation,
other than that no-one has done the work? (I seem to recall Hubert
mentioning that the C specification may not properly describe the PPC
double-double representation in some ways. It would be nice to fix that if
my recollection is right.)

On Tue, Sep 28, 2021 at 10:43 AM Gabriel Dos Reis via SG12 <
sg12_at_[hidden]> wrote:

> That would be an improvement in clarity of the current (C++) wording.
> Though I don’t know if ‘should’ would have the normative effect you’re
> seeking…
>
>
>
> -- Gaby
>
>
>
> *From:* Core <core-bounces_at_[hidden]> *On Behalf Of *Jason Merrill
> via Core
> *Sent:* Tuesday, September 28, 2021 6:38 AM
> *To:* Hubert Tong <hubert.reinterpretcast_at_[hidden]>
> *Cc:* Jason Merrill <jason_at_[hidden]>; SG6 numerics <sci_at_[hidden]>;
> C++ Core Language Working Group <core_at_[hidden]>;
> sg12_at_[hidden]
> *Subject:* [isocpp-core] constexpr and FP exceptions (was: Implementation
> defining undefined behavior)
>
>
>
> On Tue, Sep 21, 2021 at 11:48 AM Hubert Tong <
> hubert.reinterpretcast_at_[hidden]> wrote:
>
> On Tue, Sep 21, 2021 at 11:44 AM Herring, Davis via Core <
> core_at_[hidden]> wrote:
>
> > I'm thinking specifically about floating point division by zero, which
> > sometimes hits a hardware exception and sometimes produces
> > floating-point infinity (when is_iec559 is true).
> >
> > As with other cases of undefined behavior, this is made testable by
> > constexpr. Can an implementation say that a construct that is
> > undefined in the C++ standard is defined in that implementation, and
> > therefore accept it in constexpr?
>
> For this case in particular, SG6 (or at least Lawrence) has in the past
> recommended against supporting it <
> https://wiki.edg.com/bin/view/Wg21issaquah2016/CoreWorkingGroup#Core_issue_2168_Narrowing_conver
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.edg.com%2Fbin%2Fview%2FWg21issaquah2016%2FCoreWorkingGroup%23Core_issue_2168_Narrowing_conver&data=04%7C01%7Cgdr%40microsoft.com%7Cb9cde696a68a43f4c08e08d982853f77%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684331085190522%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=kH0jh4F4rIkPc%2F8nIWbzP7WyWXIgTmRoms3BpmndaT8%3D&reserved=0>>
> <https://lists.isocpp.org/sci/2016/03/0000.php
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.isocpp.org%2Fsci%2F2016%2F03%2F0000.php&data=04%7C01%7Cgdr%40microsoft.com%7Cb9cde696a68a43f4c08e08d982853f77%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637684331085200516%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=bbREgnSY4AXbntZGfewg9HG1%2FpZYNzggVywr%2FHU7zwY%3D&reserved=0>
> >.
>
>
>
> It looks to me like those links are talking about narrowing, not division
> by zero. And narrowing isn't undefined, it's ill-formed.
>
>
>
> For division by zero specifically, why doesn't the definition in IEC559
> count as defined behavior, when we advertise that a type conforms to that
> standard? In C's Annex F, it seems to:
>
>
>
> F.8.2 Translation
>
> During translation the IEC 60559 default modes are in effect:
>
> — The rounding direction mode is rounding to nearest.
>
> — The rounding precision mode (if supported) is set so that results are
> not shortened.
>
> — Trapping or stopping (if supported) is disabled on all floating-point
> exceptions.
>
> Recommended practice:
>
> The implementation should produce a diagnostic message for each
> translation-time floating-point exception, other than “inexact”; the
> implementation should then proceed with the translation of the program.
>
>
>
> Jason
> _______________________________________________
> SG12 mailing list
> SG12_at_[hidden]
> Subscription: https://lists.isocpp.org/mailman/listinfo.cgi/sg12
> Link to this post: http://lists.isocpp.org/sg12/2021/09/0973.php
>