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4460. Missing Throws: for last variant constructor

Section: 22.6.3.2 [variant.ctor] Status: New Submitter: Jonathan Wakely Opened: 2025-11-07 Last modified: 2025-11-07

Priority: Not Prioritized

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Discussion:

All variant constructors except the last one have a Throws: element saying what they're allowed to throw.

This originates from an editorial pull request, where the submitter said:

"It looks like this defect is an artifact of a change between P0088R0 and P0088R1. Note how in R0 neither one of the emplaced_type_t/emplaced_index_t (as they were then called) + initializer_list constructors have a throws clause. In R1 only one of them gained it."

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.6.3.2 [variant.ctor], as indicated:

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

   -35- Constraints:

   1. (35.1) — I is less than sizeof...(Types) and
   2. (35.2) — is_constructible_v<TI, initializer_list<U>&, Args...> is true.

   -36- Effects: Direct-non-list-initializes the contained value of type TI with il, std::forward<Args>(args)....

   -37- Postconditions: index() is I.

   -?- Throws: Any exception thrown by calling the selected constructor of TI.

   -38- Remarks: If TI’s selected constructor is a constexpr constructor, this constructor is a constexpr constructor.