[Cfp-interest 3196] Fwd: [SC22WG14.26243] constexpr initialization seemingly can contain UB?

[Fred J. Tydeman]

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From: Aaron Ballman <aaron at aaronballman.com>
To: Joseph Myers <josmyers at redhat.com>
Cc: wg14 <sc22wg14 at open-std.org>
Date: Thu, 1 Aug 2024 13:33:18 -0400
Subject: [SC22WG14.26243] constexpr initialization seemingly can contain UB?

On Thu, Aug 1, 2024 at 12:59PM Joseph Myers <josmyers at redhat.com> wrote:
>
> On Thu, 1 Aug 2024, Aaron Ballman wrote:
>
> > Consider the following example:
> >
> > constexpr float f = 1.0f / 0.0f
> >
> > According to my reading of the C23 standard, this is conforming and
> > should result in `f` holding infinity, presuming that the target
> > supports infinities in their floating-point representation (see
> > 5.2.5.3.3 for what is "in the range of representable values")
> >
> > However, 6.5.6p6 makes this undefined behavior explicitly:
> > The result of the / operator is the quotient from the division of the
> > first operand by the second; the result of the % operator is the
> > remainder. In both operations, if the value of the second operand is
> > zero, the behavior is undefined.
>
> It was previously stated in response to DR#442 that "WG14 treats normative
> annexes such as Annex F as if they were linear extensions of the standard
> itself. When Annex F is requested via definition of __STDC_IEC_559__ then
> 6.5#5 is superseded and floating point exceptions become well defined.".
> I think that applies to the specification of the division operator just as
> to the general statement about exceptional conditions - the rules for
> translation-time evaluation thus apply, as long as Annex F is implemented,
> to give the result of evaluating that expression in the default rounding
> mode with default exception handling and exceptions discarded.  See the
> example in F.8.5.

Agreed, but Annex F doesn't save us here -- there are targets for
which my implementation cannot conform to Annex F for <reasons>, and
we don't claim conformance on any target currently by defining the
__STDC_IEC_559__ macro.

~Aaron

>
> --
> Joseph S. Myers
> josmyers at redhat.com
>

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---
Fred J. Tydeman        Tydeman Consulting
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