Proposed resolution:

This wording is relative to N4849.

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

   template<class Rep2>
     constexpr explicit duration(const Rep2& r);
   

   -1- Constraints: is_convertible_v<const Rep2&, rep> is true and

   1. (1.1) — treat_as_floating_point_v<rep> is true or

   2. (1.2) — treat_as_floating_point_v<Rep2> is false.

   […]

   -2- Postconditions: count() == static_cast<rep>(r).

2. Modify [time.duration.nonmember] as indicated:

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const duration<Rep1, Period>& d, const Rep2& s);
   

   -4- Constraints: is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is true.

   […]

   template<class Rep1, class Rep2, class Period>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const Rep1& s, const duration<Rep2, Period>& d);
   

   -6- Constraints: is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>> is true.

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator/(const duration<Rep1, Period>& d, const Rep2& s);
   

   -8- Constraints: is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is true and Rep2 is not a specialization of duration.

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator%(const duration<Rep1, Period>& d, const Rep2& s);
   

   -12- Constraints: is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is true and Rep2 is not a specialization of duration.

   […]

[ 2020-02 Status to Immediate on Thursday night in Prague. ]

[ 2020-02-13, Prague ]

Rebase to most recent working draft

[ 2018-12-05 Jonathan provides new wording ]

In San Diego Geoff noticed that the current WP does not use CR. Jonathan provides new wording consistent with the editorial changes that removed CR.

This wording is relative to N4713.

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

   template<class Rep2>
     constexpr explicit duration(const Rep2& r);
   

   -1- Remarks: This constructor shall not participate in overload resolution unless is_convertible_v<const Rep2&, rep> is trueRep2 is implicitly convertible to rep and

   1. (1.1) — treat_as_floating_point_v<rep> is true or

   2. (1.2) — treat_as_floating_point_v<Rep2> is false.

   […]

   -2- Effects: Constructs an object of type duration.

   -3- Postconditions: count() == static_cast<rep>(r).

2. Modify [time.duration.nonmember] as indicated:

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const duration<Rep1, Period>& d, const Rep2& s);
   

   -4- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, CR(Rep1, Rep2)> is trueRep2 is implicitly convertible to CR(Rep1, Rep2).

   […]

   template<class Rep1, class Rep2, class Period>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const Rep1& s, const duration<Rep2, Period>& d);
   

   -6- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep1&, CR(Rep1, Rep2)> is trueRep1 is implicitly convertible to CR(Rep1, Rep2).

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator/(const duration<Rep1, Period>& d, const Rep2& s);
   

   -8- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, CR(Rep1, Rep2)> is trueRep2 is implicitly convertible to CR(Rep1, Rep2) and Rep2 is not a specialization of duration.

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator%(const duration<Rep1, Period>& d, const Rep2& s);
   

   -11- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, CR(Rep1, Rep2)> is trueRep2 is implicitly convertible to CR(Rep1, Rep2) and Rep2 is not a specialization of duration.

   […]

This wording is relative to N4791.

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

   template<class Rep2>
     constexpr explicit duration(const Rep2& r);
   

   -1- Remarks: This constructor shall not participate in overload resolution unless is_convertible_v<const Rep2&, rep> is trueRep2 is implicitly convertible to rep and

   1. (1.1) — treat_as_floating_point_v<rep> is true or

   2. (1.2) — treat_as_floating_point_v<Rep2> is false.

   […]

   -2- Effects: Constructs an object of type duration.

   -3- Postconditions: count() == static_cast<rep>(r).

2. Modify [time.duration.nonmember] as indicated:

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const duration<Rep1, Period>& d, const Rep2& s);
   

   -4- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is trueRep2 is implicitly convertible to common_type_t<Rep1, Rep2>.

   […]

   template<class Rep1, class Rep2, class Period>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator*(const Rep1& s, const duration<Rep2, Period>& d);
   

   -6- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>> is trueRep1 is implicitly convertible to common_type_t<Rep1, Rep2>.

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator/(const duration<Rep1, Period>& d, const Rep2& s);
   

   -8- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is trueRep2 is implicitly convertible to common_type_t<Rep1, Rep2> and Rep2 is not a specialization of duration.

   […]

   template<class Rep1, class Period, class Rep2>
     constexpr duration<common_type_t<Rep1, Rep2>, Period>
       operator%(const duration<Rep1, Period>& d, const Rep2& s);
   

   -11- Remarks: This operator shall not participate in overload resolution unless is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>> is trueRep2 is implicitly convertible to common_type_t<Rep1, Rep2> and Rep2 is not a specialization of duration.

   […]

[ 2018-11 San Diego Thursday night issue processing ]

Jonathan to provide updated wording; the underlying text has changed.

[ 2018-06 Rapperswil Thursday issues processing ]

P3; Status to Open

The converting constructor in std::chrono::duration is currently specified as ([time.duration.cons] p1):

template<class Rep2>
  constexpr explicit duration(const Rep2& r);

Remarks: This constructor shall not participate in overload resolution unless Rep2 is implicitly convertible to rep and […]

But the parameter is of type Rep2 const, so we should check that Rep2 const is implicitly convertible to rep, not just Rep2. This means that for a type like:

struct X { operator int64_t() /* not const */; };

std::is_constructible_v<std::chrono::seconds, X> is true, but actual construction will fail to compile.

Further analysis of sub-clause [time.duration] revealed similar specification problems in some other functions.