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### 3221. Result of `year_month` arithmetic with `months` is ambiguous

Section: 27.8.13.3 [time.cal.ym.nonmembers] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-16 Last modified: 2021-02-25

Priority: 0

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Discussion:

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 27.8.13.3 [time.cal.ym.nonmembers].

[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.

[2019-07 Issue Prioritization]

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

Proposed resolution:

This wording is relative to N4810.

1. Modify 27.8.13.3 [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`.