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### 2811. "Selected constructor" wording is incorrect for `optional`/`variant`/`any`

Section: 20.6.3.2 [optional.ctor], 20.6.3.4 [optional.assign], 20.7.3.2 [variant.ctor], 20.8.4.2 [any.cons], 20.8.4.4 [any.modifiers] Status: New Submitter: Tim Song Opened: 2016-10-29 Last modified: 2020-06-13

Priority: 3

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Discussion:

Throughout `optional`/`variant`/`any`'s specification references are made to "the selected constructor of `T`". For example, 20.6.3.2 [optional.ctor]/16 says of the constructor from `const T&`:

> -16- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

Similarly, the in-place constructor has this wording (20.6.3.2 [optional.ctor]/25-26):

> -25- Throws: Any exception thrown by the selected constructor of `T`.

> -26- Remarks: If `T`'s constructor selected for the initialization is a `constexpr` constructor, this constructor shall be a `constexpr` constructor.

If `T` is a scalar type, it has no constructor at all. Moreover, even for class types, the in-place constructor wording ignores any implicit conversion done on the argument before the selected constructor is called, which 1) may not be valid in constant expressions and 2) may throw an exception; such exceptions aren't thrown "by the selected constructor of `T`" but outside it.

The wording should probably be recast to refer to the entire initialization.

[Issues Telecon 16-Dec-2016]

Priority 3; Jonathan to provide wording.

[2020-06-11; Nina Dinka Ranns comments and provides initial wording]

This wording depends on the current resolution for LWG 2833, which covers the `constexpr` portion of this issue.

Proposed resolution:

This wording is relative to N4861.

1. Modify 20.6.3.2 [optional.ctor] as indicated:

   >     constexpr optional(const optional& rhs);

   > > -3- […]

   > > -4- […]

   > > -5- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     constexpr optional(optional&& rhs) noexcept(see below);

   > > -7- […]

   > > […]

   > > -10- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);

   > > -12- […]

   > > […]

   > > -15- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     template<class U, class... Args>
   >       constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

   > > -17- […]

   > > […]

   > > -20- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     template<class U = T> constexpr explicit(see below) optional(U&& v);

   > > -22- […]

   > > […]

   > > -25- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     template<class U> explicit(see below) optional(const optional<U>& rhs);

   > > -27- […]

   > > […]

   > > -30- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

   >     template<class U> explicit(see below) optional(optional<U>&& rhs);

   > > -32- […]

   > > […]

   > > -35- Throws: Any exception thrown by the selected constructor of `T` initialization of the contained value.

2. Modify 20.7.3.2 [variant.ctor] as indicated:

   >     template<class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&... args);

   > > -20- […]

   > > […]

   > > -23- Throws: Any exception thrown by calling the selected constructor of `T` the initialization of the contained value.

   >     template<class T, class U, class... Args>
   >       constexpr explicit variant(in_place_type_t<T>, initializer_list<U> il, Args&&... args);

   > > -25- […]

   > > […]

   > > -28- Throws: Any exception thrown by calling the selected constructor of `T` the initialization of the contained value.

   >     template<size_t I, class... Args> constexpr explicit variant(in_place_index_t<I>, Args&&... args);

   > > -30 […]

   > > […]

   > > -33- Throws: Any exception thrown by calling the selected constructor of `TI` the initialization of the contained value.

3. Modify 20.8.4.2 [any.cons] as indicated:

   >     any(const any& other);

   > > -2- Effects: […]

   > > -3- Throws: Any exceptions arising from calling the selected constructor for thrown by the initialization of the contained value.

   > […]

   >     template<class T>
   >       any(T&& value);

   > > -5- […]

   > > […]

   > > -9- Throws: Any exception thrown by the selected constructor of `VT` initialization of the contained value.

   >     template<class T, class... Args>
   >       explicit any(in_place_type_t<T>, Args&&... args);

   > > -10- […]

   > > […]

   > > -15- Throws: Any exception thrown by the selected constructor of `VT` initialization of the contained value.

   >     template<class T, class U, class... Args>
   >       explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);

   > > -16- […]

   > > […]

   > > -21- Throws: Any exception thrown by the selected constructor of `VT` initialization of the contained value.

4. Modify 20.8.4.4 [any.modifiers] as indicated:

   >     template<class T, class... Args>
   >       decay_t<T>& emplace(Args&&... args);

   > > -1- […]

   > > […]

   > > -7- Throws: Any exception thrown by the selected constructor of `VT` initialization of the contained value.

   > > -8- […]

   >     template<class T, class U, class... Args>
   >       decay_t<T>& emplace(initializer_list<U> il, Args&&... args);

   > > -9- […]

   > > […]

   > > -15- Throws: Any exception thrown by the selected constructor of `VT` initialization of the contained value.