Proposed resolution:

This wording is relative to N4810.

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

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

   -3- Returns: A year_month value z such that z.ok() && z - ym == dm is true.

   Complexity: 𝒪(1) with respect to the value of dm.

[ 2019-07 Issue Prioritization ]

Status to Tentatively Ready after five positive votes on the reflector.

[ 2019-06-24; LWG discussion ]

During discussions on the LWG reflector there was a preference to add "is true" at the end of the modified Returns: element. This additional edit has been applied to Tomasz' original wording below.

The current specification of the addition of year_month and months does not define a unique result value.

To illustrate, both year(2019)/month(1) and year(2018)/month(13) are valid results of year(2018)/month(12) + months(1) addition, according to the spec in [time.cal.ym.nonmembers].