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3845. ranges::to's from_range_t tag branch has the wrong constraint

Section: 25.5.7.2 [range.utility.conv.to] Status: New Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2024-01-29

Priority: 4

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Discussion:

In bullet (1.1.2), ranges::to checks whether the container type C models constructible_from<from_range_t, ...> and constructs it via C(from_range, ...).

Since from_range is a constexpr variable here, it would be more accurate to constrain C to be constructible from a const lvalue tag rather than an rvalue tag.

[2023-02-02; Reflector poll]

Set priority to 4 after reflector poll. Several votes for "Tentatively Ready", but also two objections, preferring NAD. The proposed change would appear to bless unconventional uses of from_range, and we don't want to support or encourage that. The current wording is simpler, and works for the intended cases.

Proposed resolution:

This wording is relative to N4917.

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);
   

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

   1. (1.1) — If convertible_to<range_reference_t<R>, range_value_t<C>> is true:

      1. (1.1.1) — If constructible_from<C, R, Args...> is true:

         C(std::forward<R>(r), std::forward<Args>(args)...)

      2. (1.1.2) — Otherwise, if constructible_from<C, const from_range_t&, R, Args...> is true:

         C(from_range, std::forward<R>(r), std::forward<Args>(args)...)

      3. […]