[cfe-commits] libc++ and const expr.

[Matthieu Monrocq]

Le 4 février 2012 18:04, Eli Friedman <eli.friedman at gmail.com> a écrit :

> On Sat, Feb 4, 2012 at 8:53 AM, Howard Hinnant <hhinnant at apple.com> wrote:
> > On Feb 4, 2012, at 5:25 AM, Jean-Daniel Dupas wrote:
> >
> >> Hi,
> >>
> >> Am I the only one having trouble compiling libc++ with clang TOT.
> >> For sometime now, clang refuse to compile (and use it) because of the
> following issue:
> >>
> >> ../include/ratio:193:19: error: static_assert expression is not an
> integral constant expression
> >>     static_assert(_Xp != nan && _Yp != nan && __a_x <= max / __a_y,
> "overflow in __ll_mul");
> >>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> ../include/ratio:308:13: note: in instantiation of template class
> 'std::__1::__ll_mul<1, 1>' requested here
> >>             __ll_mul<_R1::num / __gcd_n1_n2, _R2::den /
> __gcd_d1_d2>::value,
> >>             ^
> >> ../include/ratio:315:33: note: in instantiation of template class
> 'std::__1::__ratio_divide<std::__1::ratio<1, 1000000000>,
> std::__1::ratio<1, 1000000000> >' requested
> >>       here
> >> template <class _R1, class _R2> using ratio_divide
> >>                                 ^
> >> ../include/chrono:410:18: note: in instantiation of template type alias
> 'ratio_divide' requested here
> >>                 (ratio_divide<_Period2, period>::type::den == 1 &&
> >>                  ^
> >> ../include/chrono:406:9: note: while substituting deduced template
> arguments into function template 'duration' [with _Rep2 = long long,
> _Period2 = <no value>]
> >>         duration(const duration<_Rep2, _Period2>& __d,
> >>         ^
> >> ../include/ratio:193:26: note: initializer of 'nan' is not a constant
> expression
> >>     static_assert(_Xp != nan && _Yp != nan && __a_x <= max / __a_y,
> "overflow in __ll_mul");
> >>                          ^
> >> ../include/ratio:187:27: note: declared here
> >>     static const intmax_t nan = (1LL << (sizeof(intmax_t) * CHAR_BIT -
> 1));
> >>                           ^
> >> ../include/ratio:187:27: note: declared here
> >> ../include/ratio:189:27: note: declared here
> >>     static const intmax_t max = -min;
> >
> > I haven't noticed because I'm not using TOT clang, and I've been
> distracted by libc++abi for the past couple of months.
> >
> > But I note the crux of this issue appears to be that this:
> >
> >   static const intmax_t nan = (1LL << (sizeof(intmax_t) * CHAR_BIT - 1));
> >
> > is no longer consider a compile time constant expression.
> >
> > This is one of two things:
> >
> > 1.  A clang bug.
> > 2.  A standards defect.  I.e. this will break a lot of C++03 code if the
> standard really says this.
> >
> > If it is 2) it would be good for me to know immediately.  The next C++
> standards meeting is next week (Feb. 6-10) and it would be good to give
> such a defect a high profile/priority at the meeting.
> >
> > Does anyone know if this is 1) or 2)?
>
> [expr.shift]p2: [...] if E1 has a signed type and non-negative value,
> and E1×2E2 is representable in the result type, then that is the
> resulting value; otherwise, the behavior is undefined.
>
> -Eli
>

This should read that for signed types, E1 << E2 is equivalent to E1 *
2^(E2), footnotes and superscript just don't work as much as we would want
them to :x

-- Matthieu
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