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### 2991. `variant` copy constructor missing `noexcept(see below)`

Section: 22.6.3.2 [variant.ctor] Status: LEWG Submitter: Peter Dimov Opened: 2017-06-27 Last modified: 2017-07-12

Priority: Not Prioritized

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Discussion:

The copy constructor of `std::variant` is not conditionally `noexcept` (I think it was in the original proposal.)

It should be, for two reasons: first, this would be consistent with the other three constructors

>     constexpr variant() noexcept(see below);

>     variant(variant&&) noexcept(see below);

>     template <class T>
>     constexpr variant(T&&) noexcept(see below);

and second, `variant` itself makes use of `is_nothrow_copy_constructible`, so it's inconsistent for it to take a stance against it.

[2017-07 Toronto Tuesday PM issue prioritization]

Status to LEWG

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.6.3 [variant.variant], class template `variant` synopsis, as indicated:

   >     template <class... Types>
   >       class variant {
   >       public:
   >         // 23.7.3.1, constructors
   >         constexpr variant() noexcept(see below);
   >         variant(const variant&) noexcept(see below);
   >         variant(variant&&) noexcept(see below);
   >         […]
   >       };

2. Edit 22.6.3.2 [variant.ctor] as indicated:

   >     variant(const variant& w) noexcept(see below);

   > > […]

   > > -8- Remarks: This function shall not participate in overload resolution unless `is_copy_constructible_v<Ti>` is `true` for all `i`. The expression inside `noexcept` is equivalent to the logical AND of `is_nothrow_copy_constructible_v<Ti>` for all `i`.