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### 3104. Fixing `duration` division

Section: 27.5.6 [time.duration.nonmember] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-04-17 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.duration.nonmember].

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Discussion:

[time.duration.nonmember]/1 states

> In the function descriptions that follow, `CD` represents the return type of the function.

From what I could find, many definitions of `CD` in the paragraphs of [time.duration.nonmember] were lifted to [time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10, which the change rendered ill-formed:

>     template<class Rep1, class Period1, class Rep2, class Period2>
>       constexpr common_type_t<Rep1, Rep2>
>         operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

> > Returns: `CD(lhs).count() / CD(rhs).count()`.

In this case, we want `CD` to mean `common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>`. That way, the division has the expected semantics of dividing two quantities of the same dimension.

[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 27.5.6 [time.duration.nonmember] as indicated:

>    > -1- In the function descriptions that follow, unless stated otherwise, let `CD` represents the return type of the function.

>    > […]

>    >     template<class Rep1, class Period1, class Rep2, class Period2>
>    >       constexpr common_type_t<Rep1, Rep2>
>    >         operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>    > > Let `CD` be `common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>`.

>    > > -10- Returns: `CD(lhs).count() / CD(rhs).count()`.