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### 2094. `duration` conversion overflow shouldn't participate in overload resolution

Section: 27.5.2 [time.duration.cons] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2011-10-31 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

27.5.2 [time.duration.cons] says:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`.

The evaluation of `ratio_divide<Period2, period>::den` could make `ratio_divide<Period2, period>::num` overflow.

This occur for example when we try to create a millisecond (`period`=`ratio<1,1000>`) from an exa-second (`Period2`=`ratio<1018>`).

`ratio_divide<ratio<1018>, ratio<1,1000>>::num` is `1021` which overflows which makes the compiler error.

If the function `f` is overloaded with milliseconds and seconds

>     void f(milliseconds);
>     void f(seconds);

The following fails to compile.

>     duration<int,exa> r(1);
>     f(r);

While the conversion to seconds work, the conversion to milliseconds make the program fail at compile time. In my opinion, this program should be well formed and the constructor from `duration<int,exa>` to milliseconds shouldn't participate in overload resolution as the result can not be represented.

I think the wording of the standard can be improved so no misinterpretations are possible by adding that "no overflow is induced by the conversion".

[2012, Kona]

Move to Review.

Pete: The wording is not right.

Howard: Will implement this to be sure it works.

Jeffrey: If ratio needs a new hook, should it be exposed to users for their own uses?

Pete: No.

Move to Review, Howard to implement in a way that mere mortals can understand.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to the FDIS.

Change the following paragraphs of 27.5.2 [time.duration.cons] p4 indicated:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`. [ Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note ]