3847. ranges::to can still return views

Section: 25.5.7.2 [range.utility.conv.to], 25.5.7.3 [range.utility.conv.adaptors] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2024-01-29

Priority: 2

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Discussion:

The intention of ranges::to is to construct a non-view object, which is reflected in its constraint that object type C should not model a view.

The specification allows C to be a const-qualified type which does not satisfy view such as const string_view, making ranges::to return a dangling view. We should ban such cases.

[2023-02-01; Reflector poll]

Set priority to 2 after reflector poll. Just require C to be a cv-unqualified object type.

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

1. Modify 25.2 [ranges.syn] as indicated:

   #include <compare>              // see 17.11.1 [compare.syn]
   #include <initializer_list>     // see 17.10.2 [initializer.list.syn]
   #include <iterator>             // see 24.2 [iterator.synopsis]
   
   namespace std::ranges {
     […]
     // 25.5.7 [range.utility.conv], range conversions
     template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>)
       constexpr C to(R&& r, Args&&... args);                                          // freestanding
     template<template<class...> class C, input_range R, class... Args>
       constexpr auto to(R&& r, Args&&... args);                                       // freestanding
     template<class C, class... Args> requires (!view<remove_cv_t<C>>)
       constexpr auto to(Args&&... args);                                              // freestanding
     template<template<class...> class C, class... Args>
       constexpr auto to(Args&&... args);                                              // freestanding
     […]
   }
   

2. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>)
   constexpr C to(R&& r, Args&&... args);
   

   -1- Returns: An object of type C constructed from the elements of r in the following manner:

   […]

3. Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:

   template<class C, class... Args> requires (!view<remove_cv_t<C>>)
     constexpr auto to(Args&&... args);
   template<template<class...> class C, class... Args>
     constexpr auto to(Args&&... args);
   

   -1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object]) f that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

   […]

[2023-02-02; Jonathan provides improved wording]

[Issaquah 2023-02-08; LWG]

Unanimous consent to move to Immediate. This also resolves LWG 3787.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   template<class C, input_range R, class... Args> requires (!view<C>)
   constexpr C to(R&& r, Args&&... args);
   

   -?- Mandates: C is a cv-unqualified class type.

   -1- Returns: An object of type C constructed from the elements of r in the following manner:

   […]

2. Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:

   template<class C, class... Args> requires (!view<C>)
     constexpr auto to(Args&&... args);
   template<template<class...> class C, class... Args>
     constexpr auto to(Args&&... args);
   

   -?- Mandates: For the first overload, C is a cv-unqualified class type.

   -1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object]) f that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

   […]