[ 2019-09-14; Tomasz and Howard provide concrete wording ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

[Drafting note: Suggested wording below assumes that we can add a Constraints: to a signature where the constraint does not apply to a deduced template. We have examples of such constraints in other parts of the WD (e.g. [unique.ptr.single.ctor]/p15, [unique.ptr.single.asgn]/p1). And we have the old form "does not participate …" being used for non-deduced templates in several places as well (e.g. [pairs.pair]/p5).

There are several ways of implementing such a constraint, such as adding a gratuitous template parameter.]

1. Modify [time.cal.ym.members] as indicated:

   constexpr year_month& operator+=(const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -4- Effects: *this = *this + dm.

   […]

   constexpr year_month& operator-=(const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -6- Effects: *this = *this - dm.

   […]

2. Modify [time.cal.ym.nonmembers] as indicated:

   constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -3- Returns: A year_month value z such that z - ym == dm.

   […]

   constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -5- Returns: ym + dm.

   constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -6- Returns: ym + -dm.

3. Modify [time.cal.ymd.members] as indicated:

   constexpr year_month_day& operator+=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -7- Effects: *this = *this + m.

   […]

   constexpr year_month_day& operator-=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -9- Effects: *this = *this - m.

   […]

4. Modify [time.cal.ymd.nonmembers] as indicated:

   constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -3- Returns: (ymd.year() / ymd.month() + dm) / ymd.day().

   […]

   constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -5- Returns: ymd + dm.

   constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -6- Returns: ymd + (-dm).

5. Modify [time.cal.ymdlast.members] as indicated:

   constexpr year_month_day_last& operator+=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -2- Effects: *this = *this + m.

   […]

   constexpr year_month_day_last& operator-=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -4- Effects: *this = *this - m.

   […]

6. Modify [time.cal.ymdlast.nonmembers] as indicated:

   constexpr year_month_day_last
     operator+(const year_month_day_last& ymdl, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -3- Returns: (ymdl.year() / ymdl.month() + dm) / last.

   constexpr year_month_day_last
     operator+(const months& dm, const year_month_day_last& ymdl) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -4- Returns: ymdl + dm.

   constexpr year_month_day_last
     operator-(const year_month_day_last& ymdl, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -5- Returns: ymdl + (-dm).

7. Modify [time.cal.ymwd.members] as indicated:

   constexpr year_month_weekday& operator+=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -6- Effects: *this = *this + m.

   […]

   constexpr year_month_weekday& operator-=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -8- Effects: *this = *this - m.

   […]

8. Modify [time.cal.ymwd.nonmembers] as indicated:

   constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -2- Returns: (ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed().

   constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -3- Returns: ymwd + dm.

   constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -4- Returns: ymwd + (-dm).

9. Modify [time.cal.ymwdlast.members] as indicated:

   constexpr year_month_weekday_last& operator+=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -2- Effects: *this = *this + m.

   […]

   constexpr year_month_weekday_last& operator-=(const months& m) noexcept;
   

   -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

   -4- Effects: *this = *this - m.

   […]

10. Modify [time.cal.ymwdlast.nonmembers] as indicated:

    constexpr year_month_weekday_last
      operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
    

    -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

    -2- Returns: (ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last().

    constexpr year_month_weekday_last
      operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;
    

    -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

    -3- Returns: ymwdl + dm.

    constexpr year_month_weekday_last
      operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;
    

    -?- Constraints: The argument supplied by the caller for the months parameter is not implicitly convertible to years.

    -4- Returns: ymwdl + (-dm).