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3428. single_view's in place constructor should be explicit

Section: 26.6.3.2 [range.single.view] Status: WP Submitter: Tim Song Opened: 2020-04-07 Last modified: 2020-11-09

Priority: 0

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Discussion:

The in_place_t constructor template of single_view is not explicit:

template<class... Args>
  requires constructible_from<T, Args...>
constexpr single_view(in_place_t, Args&&... args);

so it defines an implicit conversion from std::in_place_t to single_view<T> whenever constructible_from<T> is modeled, which seems unlikely to be the intent.

[2020-04-18 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 26.6.3.2 [range.single.view] as indicated:

   namespace std::ranges {
     template<copy_constructible T>
       requires is_object_v<T>
     class single_view : public view_interface<single_view<T>> {
       […]
     public:
       […]
       template<class... Args>
         requires constructible_from<T, Args...>
       constexpr explicit single_view(in_place_t, Args&&... args);
   
       […]
     };
   }
   

   […]

   template<class... Args>
   constexpr explicit single_view(in_place_t, Args&&... args);
   

   -3- Effects: Initializes value_ as if by value_{in_place, std::forward<Args>(args)...}.