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### 3613. Specify that `nullopt_t` is copyable

Section: 20.6.4 [optional.nullopt] Status: New Submitter: Frank Birbacher Opened: 2021-10-01 Last modified: 2021-10-14

Priority: 3

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Discussion:

The Standard defines a number of types that are used to create overload disambiguators for constructors, like `nullopt_t` and `allocator_tag_t`. Values of such types are passed by value to such constructors to give it particular meaning. For pass-by-value these types need to be copy-constructible and for consistency should also be copy-assignable. Of those types the specification of `nullopt_t` doesn't clearly state that the type is copyable, 20.6.4 [optional.nullopt].

The reason that `nullopt_t` is defined differently from other such types is to avoid ambiguity in expressions that assign an empty brace initializer to an `optional`. The meaning of such assignment should be to engage the `optional` instead of taking the braces to create a temporary `nullopt_t` for assignment and thus reset the `optional`. The RHS of such assignment should be a temporary empty `optional` instead of a temporary `nullopt_t`.

Types that aren't affected: `nullptr_t` (fundamental type), `allocator_tag_t`, `piecewise_construct_t`, `in_place_t`, `in_place_type_t`, `in_place_index_t` (all basically defined as a `class T { explicit T() = default; }` which works fine for pass-by-value)

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

[Daniel commented:]

I would prefer to see the wording use `is_trivially_copy_constructible_v` and `trivially_copy_assignable_v`, which is consistent with similar usage of trivial-requirements in `std::optional`.

[Tim commented:]

We need to say that it models copyable and is trivially copyable (not sure if we need the latter but might as well - does anyone do it differently?). "has a copy constructor" isn't enough - `T(T&)` is a copy constructor.

Proposed resolution:

This wording is relative to N4892.

1. Modify 20.6.4 [optional.nullopt] as indicated:

   >     struct nullopt_t{see below};
   >     inline constexpr nullopt_t nullopt(unspecified);

   > > -1- The struct `nullopt_t` is an empty class type used as a unique type to indicate the state of not containing a value for `optional` objects. In particular, `optional<T>` has a constructor with `nullopt_t` as a single argument; this indicates that an optional object not containing a value shall be constructed.

   > > -2- Type `nullopt_t` shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate, and shall have a copy constructor and a copy assignment operator, both shall be public and trivial.