1 Abstract

This paper provides wording for [LWG3524] and resolves related issues caused by the erroneous use of list-initialization in ranges wording.

2 Discussion

As discussed in [LWG3524], the use of list-initialization in the ranges specification implies ordering guarantees that are unintended and unimplementable in ordinary C++, as well as narrowing checks that are unnecessary and sometimes unimplementable.

Other issues have also been reported against the misuse of list-initialization in ranges wording. See [subrange] and [take.view].

The approach taken in this paper is to remove uses of list-initialization that are actively harmful, as well as any neighboring uses to maintain some level of local consistency. I have not attempted to remove all uses of list-initialization in the normative text of Clause 24.

During the drafting of this paper, two issues were discovered with views::single:

The implicit deduction guides fail to remove cv-qualification for const rvalues or decay array/function types. This is inconsistent with range-v3 and appears to be an oversight.
views::single(a_single_view) unexpectedly copies instead of wraps, similar to [LWG3474].

The proposed wording below fixes these issues as well.

3 Wording

This wording is relative to [N4885].

Edit 24.5.4.2 [range.subrange.ctor] p6 as indicated:

template<not-same-as<subrange> R>
  requires borrowed_range<R> &&
           convertible-to-non-slicing<iterator_t<R>, I> &&
           convertible_to<sentinel_t<R>, S>
constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);

6 Effects: Equivalent to:

(6.1) If StoreSize is true, subrange{(r, ranges::size(r)}).

(6.2) Otherwise, subrange{(ranges::begin(r), ranges::end(r)}).

Edit 24.6.3.1 [range.single.overview] p2 as indicated:

2 The name views::single denotes a customization point object (16.3.3.3.6 [customization.point.object]). Given a subexpression E, the expression views::single(E) is expression-equivalent to single_view<decay_t<decltype((E))>>{(E}).

Add a deduction guide to 24.6.3.2 [range.single.view]:

 namespace std::ranges {
   template<copy_constructible T>
     requires is_object_v<T>
   class single_view : public view_interface<single_view<T>> {
     // [...]
   };

+  template<class T>
+    single_view(T) -> single_view<T>;
 }

Edit 24.6.4.1 [range.iota.overview] p2 as indicated:

2 The name views::iota denotes a customization point object (16.3.3.3.6 [customization.point.object]). Given subexpressions E and F, the expressions views::iota(E) and views::iota(E, F) are expression-equivalent to iota_view{(E}) and iota_view{(E, F}), respectively.

Edit 24.7.5.1 [range.filter.overview] p2 as indicated:

2 The name views::filter denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and P, the expression views::filter(E, P) is expression-equivalent to filter_view{(E, P}).

Edit 24.7.6.1 [range.transform.overview] p2 as indicated:

2 The name views::transform denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and F, the expression views::transform(E, F) is expression-equivalent to transform_view{(E, F}).

Edit 24.7.7.1 [range.take.overview] p2 as indicated:

2 The name views::take denotes a range adaptor object (24.7.2 [range.adaptor.object]). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::take(E, F) is ill-formed. Otherwise, the expression views::take(E, F) is expression-equivalent to:

(2.1) If T is a specialization of ranges::empty_view (24.6.2.2 [range.empty.view]), then ((void) F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced.

(2.2) Otherwise, if T models random_access_range and sized_range and is

(2.2.1) […]

then T{(ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}), except that E is evaluated only once.

(2.3) Otherwise, ranges::take_view{(E, F}).

Edit 24.7.7.2 [range.take.view] as indicated:

namespace std::ranges {
  template<view V>
  class take_view : public view_interface<take_view<V>> {

    // [...]

    constexpr auto begin() requires (!simple-view<V>) {
      if constexpr (sized_range<V>) {
        if constexpr (random_access_range<V>)
          return ranges::begin(base_);
        else {
          auto sz = size();
          return counted_iterator{(ranges::begin(base_), sz});
        }
      } else
        return counted_iterator{(ranges::begin(base_), count_});
    }

    constexpr auto begin() const requires range<const V> {
      if constexpr (sized_range<const V>) {
        if constexpr (random_access_range<const V>)
          return ranges::begin(base_);
        else {
          auto sz = size();
          return counted_iterator{(ranges::begin(base_), sz});
        }
      } else
        return counted_iterator{(ranges::begin(base_), count_});
    }

    // [...]
  };

  template<class R>
    take_view(R&&, range_difference_t<R>)
      -> take_view<views::all_t<R>>;
}

Edit 24.7.8.1 [range.take.while.overview] p2 as indicated:

2 The name views::take_while denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and F, the expression views::take_while(E, F) is expression-equivalent to take_while_view{(E, F}).

Edit 24.7.9.1 [range.drop.overview] p2 as indicated:

2 The name views::drop denotes a range adaptor object (24.7.2 [range.adaptor.object]). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::drop(E, F) is ill-formed. Otherwise, the expression views::drop(E, F) is expression-equivalent to:

(2.1) If T is a specialization of ranges::empty_view (24.6.2.2 [range.empty.view]), then ((void) F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced.

(2.2) Otherwise, if T models random_access_range and sized_range and is

(2.2.1) […]

then T{(ranges::begin(E) + min<D>(ranges::size(E), F), ranges::end(E)}), except that E is evaluated only once.

(2.3) Otherwise, ranges::drop_view{(E, F}).

Edit 24.7.10.1 [range.drop.while.overview] p2 as indicated:

2 The name views::drop_while denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and F, the expression views::drop_while(E, F) is expression-equivalent to drop_while_view{(E, F}).

Edit 24.7.12.1 [range.split.overview] p2 as indicated:

2 The name views::split denotes a range adaptor object (24.7.2 [range.adaptor.object]). Given subexpressions E and F, the expression views::split(E, F) is expression-equivalent to split_view{(E, F}).

Edit 24.7.12.2 [range.split.view] p2 as indicated:

template<input_range R>
  requires constructible_from<V, views::all_t<R>> &&
           constructible_from<Pattern, single_view<range_value_t<R>>>
constexpr split_view(R&& r, range_value_t<R> e);
2 Effects: Initializes base_ with views::all(std::forward<R>(r)), and pattern_ with single_view{std::move(e)} views::single(std::move(e)).

Edit 24.7.13 [range.counted] p2 as indicated:

2 The name views::counted denotes a customization point object (16.3.3.3.6 [customization.point.object]). Let E and F be expressions, let T be decay_t<decltype((E))>, and let D be iter_difference_t<T>. If decltype((F)) does not model convertible_to<D>, views::counted(E, F) is ill-formed.

[ Note 1: This case can result in substitution failure when views::counted(E, F) appears in the immediate context of a template instantiation. — end note ]

Otherwise, views::counted(E, F) is expression-equivalent to:

(2.1) If T models contiguous_iterator, then span{(to_address(E), static_cast<D>(F)}).

(2.2) Otherwise, if T models random_access_iterator, then subrange{(E, E + static_cast<D>(F)}), except that E is evaluated only once.

(2.3) Otherwise, subrange{(counted_iterator{(E, F}), default_sentinel}).

4 References

[LWG3474] Barry Revzin. Nesting join_views is broken because of CTAD.
https://wg21.link/lwg3474

[LWG3524] Tim Song. Unimplementable narrowing and evaluation order requirements for range adaptors.
https://wg21.link/lwg3524

[N4885] Thomas Köppe. 2021-03-17. Working Draft, Standard for Programming Language C++.
https://wg21.link/n4885

[subrange] Jonathan Wakely. 2020. Yet Another Braced Init Bug.
https://lists.isocpp.org/lib/2020/10/17806.php

[take.view] Michael Schellenberger Costa. 2021. Fix error in take_view::begin by casting to the appropriate size.
https://github.com/microsoft/STL/pull/1844

Document #:	P2367R0
Date:	2021-04-29
Project:	Programming Language C++
Audience:	LWG
Reply-to:	Tim Song <t.canens.cpp@gmail.com>