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3428. single_view's in place constructor should be explicit

Section: 24.6.3.2 [range.single.view] Status: WP Submitter: Tim Song Opened: 2020-04-07 Last modified: 2020-11-15

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 24.6.3.2 [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)...}.