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3787. ranges::to's template parameter C should not be a reference type

Section: 26.5.7.2 [range.utility.conv.to], 26.5.7.3 [range.utility.conv.adaptors] Status: Resolved Submitter: Hewill Kang Opened: 2022-09-25 Last modified: 2023-03-22

Priority: 3

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Discussion:

It doesn't make sense for the template parameter C of ranges::to be a reference, which allows us to write something like ranges::to<vector<int>&&>(std::move(v)) or ranges::to<const vector<int>&&>(v), we should forbid this.

The proposed resolution constrains the template parameter C to be object type to conform to its description: "The range conversion functions construct an object (usually a container) from a range".

[2022-09-28; Reflector poll]

Set priority to 3 after reflector poll. Some suggestions that it should be Mandates: not Constraints:, but no consensus.

[Issaquah 2023-02-08; LWG]

This would be resolved by LWG 3847.

[2023-03-22 LWG 3847 was approved in Issaquah. Status changed: New → Resolved.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 26.2 [ranges.syn], header <ranges> synopsis, as indicated:

   #include <compare>              // see 17.11.1 [compare.syn]
   #include <initializer_list>     // see 17.10.2 [initializer.list.syn]
   #include <iterator>             // see 25.2 [iterator.synopsis]
   
   namespace std::ranges {
     […]
     // 26.5.7 [range.utility.conv], range conversions
     template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<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 is_object_v<C> && (!view<C>)
       constexpr auto to(Args&&... args);                                              // freestanding
     template<template<class...> class C, class... Args>
       constexpr auto to(Args&&... args);                                              // freestanding
     […]
   }
   

2. Modify 26.5.7.2 [range.utility.conv.to] as indicated:

   template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<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 26.5.7.3 [range.utility.conv.adaptors] as indicated:

   template<class C, class... Args> requires is_object_v<C> && (!view<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 (26.7.2 [range.adaptor.object]) f that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

   […]