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Showing content from https://cplusplus.github.io/LWG/issue3090 below:

What is §[time.duration.cons]p4's "no overflow is induced in the conversion" intended to mean?

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Section: 30.5.2 [time.duration.cons] Status: New Submitter: Richard Smith Opened: 2018-03-22 Last modified: 2020-09-12

Priority: 3

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

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Discussion:

30.5.2 [time.duration.cons] p4 says:

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_v<rep> is true or both ratio_divide<Period2, period>::den is 1 and treat_as_floating_point_v<Rep2> is false.

with this example:

duration<int, milli> ms(3);
duration<int, micro> us = ms;  // OK
duration<int, milli> ms2 = us; // error

It's unclear to me what "no overflow is induced in the conversion" means in the above. What happens here:

duration<int, milli> ms(INT_MAX);
duration<int, micro> us = ms;  // ???

An overflow is clearly induced in the conversion here: internally, we'll multiply INT_MAX by 1000. But that cannot be determined statically (in general), and so can't affect the result of overload resolution.

This refers to the compile-time conversion factor to convert Period2 to Period. If that conversion factor is not representable as a (reduced) ratio<N, D>, then the constructor is SFINAE'd out. This might happen (for example) converting years to picoseconds.

I would not have guessed that from the wording. Maybe replacing "no overflow is induced in the conversion" with "the result of ratio_divide<Period2, Period> is representable as a ratio" or similar would help?

[2018-06-18 after reflector discussion]

Priority set to 3

[2020-09-12 Jonathan adds a proposed resolution]

Since the result of the ratio_divide has to be a ratio, if it's not representable then the result simply isn't a valid type. Implementations are not required to make ratio_divide SFINAE-friendly to implement this constraint. They can perform the equivalent calculations to check if they would overflow, without actually using ratio_divide.

Proposed resolution:

This wording is relative to N4861.

1. Modify 30.5.2 [time.duration.cons] as indicated:

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

   -3- Constraints: is_convertible_v<const Rep2&, rep> is true. ratio_divide<typename Period2::type, period> is a valid ratio specialization. Either:

   • treat_as_floating_point_v<rep> is true; or
   • ratio_divide<Period2, period>::den is 1 and treat_as_floating_point_v<Rep2> is false.

   No overflow is induced in the conversion and treat_as_floating_point_v<rep> is true or both ratio_divide<Period2, period>::den is 1 and treat_as_floating_point_v<Rep2> is false. [Note: This requirement prevents implicit truncation error s when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note]

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