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3428.single_view's in place constructor should be explicit

Section: 25.6.3.2[range.single.view]Status:C++23Submitter: Tim SongOpened: 2020-04-07Last modified: 2023-11-22

Priority:0

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Discussion:

Thein_place_t constructor template ofsingle_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 fromstd::in_place_t tosingle_view<T> wheneverconstructible_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 toN4861.

1. Modify 25.6.3.2[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...>    constexprexplicit single_view(in_place_t, Args&&... args);    […]  };}

   […]

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

   -3-Effects: Initializesvalue_ as if byvalue_{in_place, std::forward<Args>(args)...}.