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2991. variant copy constructor missing noexcept(see below)

Section: 22.6.3.2 [variant.ctor] Status: Open Submitter: Peter Dimov Opened: 2017-06-27 Last modified: 2024-11-19

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

[Wrocław 2024-11-18; LEWG approves the direction]

In P0088R1 the copy constructor was conditionally noexcept in the synopsis, but not the detailed description. This was pointed out during LWG review in Jacksonville. The approved paper, P008R3, doesn't have it in either place.

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.