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2432. initializer_list assignability

Section: 17.10 [support.initlist] Status: NAD Submitter: David Krauss Opened: 2014-09-30 Last modified: 2023-11-13

Priority: 2

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Discussion:

std::initializer_list::operator= 17.10 [support.initlist] is horribly broken and it needs deprecation:

std::initializer_list<foo> a = {{1}, {2}, {3}};
a = {{4}, {5}, {6}};
// New sequence is already destroyed.

Assignability of initializer_list isn't explicitly specified, but most implementations supply a default assignment operator. I'm not sure what 16.3 [description] says, but it probably doesn't matter.

[Lenexa 2015-05-05: Send to EWG as discussed in Telecon]

[2022-08-24; Reflector poll]

Set status to Tentatively NAD after reflector poll in October 2021.

"If somebody wants to revisit it, they'll need to write a paper to demonstrate what the change would break, whether that would be a problem in practice, and convince the evolution groups to make a change. But it's not an LWG issue."

[2022-11-25; see EWG 1369]

[2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD.]

[Varna 2023-06-12; EWG discussed the issue again]

EWG consensus: "Send LWG2432 back to LWG to encourage them to either add a note for their implementer to issue a warning as QoI, or to use the tools (explicit convertible_to operator=) at their disposal to accomplish their goal"

The operator= suggestion refers to:

initializer_list& operator=(convertible_to<initializer_list> auto&& rhs) { /* ... */ }
initializer_list& operator=(initializer_list const&) requires false = default;

Proposed resolution:

1. Edit 17.10 [support.initlist] p1, class template initializer_list synopsis, as indicated:

   namespace std {
     template<class E> class initializer_list {
     public:
       […]
       constexpr initializer_list() noexcept;
     
       initializer_list(const initializer_list&) = default;
       initializer_list(initializer_list&&) = default;
       initializer_list& operator=(const initializer_list&) = delete;
       initializer_list& operator=(initializer_list&&) = delete;
       
       constexpr size_t size() const noexcept;
       […]
     };
     […]
   }