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

[Eli Friedman]

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