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.