Proposed resolution:

This wording is relative to N4810.

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

   constexpr operator sys_days() const noexcept;
   

   -18- Returns: If ok(), returns a sys_days holding a count of days from the sys_days epoch to *this (a negative value if *this represents a date prior to the sys_days epoch). Otherwise, if y_.ok() && m_.ok() is true, returns sys_days{y_/m_/1d} + (d_ - 1d)a sys_days which is offset from sys_days{y_/m_/last} by the number of days d_ is offset from sys_days{y_/m_/last}.day(). Otherwise the value returned is unspecified.

[ 2019-06-08; Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector. ]

The current specification of the year_month_day conversion function to sys_days, uses the day member function on the sys_days (a.k.a. time_point<system_clock, days>), that does not exist.

In [time.cal.ymd.members] p18, the expression sys_days{y_/m_/last}.day() is ill-formed:

[…] Otherwise, if y_.ok() && m_.ok() is true, returns a sys_days which is offset from sys_days{y_/m_/last} by the number of days d_ is offset from sys_days{y_/m_/last}.day(). Otherwise the value returned is unspecified.