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947.duration arithmetic: contradictory requirements

Section: 30.5.6[time.duration.nonmember]Status:ResolvedSubmitter: Pete BeckerOpened: 2008-12-20Last modified: 2016-01-28

Priority:Not Prioritized

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Discussion:

In 30.5.6[time.duration.nonmember], paragraph 8 says that callingdur / rep whenrep is an instantiation ofduration requires a diagnostic. That's followed by anoperator/ that takes two durations. Sodur1 / dur2 is legal under the second version,but requires a diagnostic under the first.

[Howard adds:]

Please see the thread starting with c++std-lib-22980 for more information.

[Batavia (2009-05):]

Move to Open, pending proposed wording (and preferably an implementation).

[2009-07-27 Howard adds:]

I've addressed this issue under the proposed wording for1177⁽ⁱ⁾ whichcleans up several places under 30.5[time.duration] which used thephrase "diagnostic required".

For clarity's sake, here is an example implementation of the constrainedoperator/:

template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value>struct __duration_divide_result{};template <class _Duration, class _Rep2,    bool = is_convertible<_Rep2,                          typename common_type<typename _Duration::rep, _Rep2>::type>::value>struct __duration_divide_imp{};template <class _Rep1, class _Period, class _Rep2>struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true>{    typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type;};template <class _Rep1, class _Period, class _Rep2>struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false>    : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2>{};template <class _Rep1, class _Period, class _Rep2>inlinetypename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::typeoperator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s){    typedef typename common_type<_Rep1, _Rep2>::type _Cr;    duration<_Cr, _Period> __r = __d;    __r /= static_cast<_Cr>(__s);    return __r;}

__duration_divide_result is basically a custom-builtenable_ifthat will containtype only ifRep2 is not adurationand ifRep2 is implicitly convertible tocommon_type<typename Duration::rep, Rep2>::type.__is_durationis simply a private trait that answersfalse, but is specialized forduration to answertrue.

The constrainedoperator% works identically.

[2009-10 Santa Cruz:]

MarkNAD EditorialResolved, fixed by1177⁽ⁱ⁾.

Proposed resolution: