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### 947. `duration` arithmetic: contradictory requirements

Section: 27.5.6 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 27.5.6 [time.duration.nonmember], paragraph 8 says that calling `dur / rep` when `rep` is an instantiation of `duration` requires a diagnostic. That's followed by an `operator/` that takes two durations. So `dur1 / 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 for 1177 which cleans up several places under 27.5 [time.duration] which used the phrase "diagnostic required".

> For clarity's sake, here is an example implementation of the constrained `operator/`:

> >     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>
> >     inline
> >     typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type
> >     operator/(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-built `enable_if` that will contain `type` only if `Rep2` is not a `duration` and if `Rep2` is implicitly convertible to `common_type<typename Duration::rep, Rep2>::type`. `__is_duration` is simply a private trait that answers `false`, but is specialized for `duration` to answer `true`.

> The constrained `operator%` works identically.

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial Resolved, fixed by 1177.

Proposed resolution: