Issue 3875: std::ranges::repeat_view<T, IntegerClass>::iterator may be ill-formed

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3875. `std::ranges::repeat_view<T, IntegerClass>::iterator` may be ill-formed

Section: 26.6.5 [range.repeat] Status: C++23 Submitter: Jiang An Opened: 2023-02-05 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

26.6.5.3 [range.repeat.iterator] specifies difference_type as

using difference_type = conditional_t<is-signed-integer-like<index-type>,
  index-type,
  IOTA-DIFF-T(index-type)>;

which always instantiates IOTA-DIFF-T(index-type), and thus possibly makes the program ill-formed when index-type is an integer-class type (index-type is same as Bound in this case), because IOTA-DIFF-T(index-type) is specified to be iter_difference_t<index-type> which may be ill-formed (26.6.4.2 [range.iota.view]/1.1).

I think the intent is using index-type as-is without instantiating IOTA-DIFF-T when is-signed-integer-like<index-type> is true.

However, when Bound is an unsigned integer-class type, it's unclear which type should the difference type be, or whether repeat_view should be well-formed when the possibly intended IOTA-DIFF-T(Bound) is ill-formed.

[2023-02-09 Tim adds wording]

We should reject types for which there is no usable difference type.

[Issaquah 2023-02-09; LWG]

Move to Immediate for C++23

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

Proposed resolution:

This wording is relative to N4928.

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

// […]
namespace std::ranges {
  template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires see below(is_object_v<W> && same_as<W, remove_cv_t<W>> &&
              (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))
  class repeat_view;

Modify 26.6.5.2 [range.repeat.view] as indicated:

namespace std::ranges {
  
  template<class T>
  concept integer-like-with-usable-difference-type =   //exposition only
    is-signed-integer-like<T> || (is-integer-like<T> && weakly_incrementable<T>);

  template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
              (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>))
  class repeat_view {
    […]
  };
}

Modify 26.6.5.3 [range.repeat.iterator] as indicated:

namespace std::ranges {
  template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires (is_object_v<W> && same_as<W, remove_cv_t<W>> &&
              (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>))
  class repeat_view<W, Bound>::iterator {
  private:
    using index-type =                  // exposition only
      conditional_t<same_as<Bound, unreachable_sentinel_t>, ptrdiff_t, Bound>;
    […]
  public:
    […]
    using difference_type = see belowconditional_t<is-signed-integer-like<index-type>,
        index-type,
        IOTA-DIFF-T(index-type)>;
    […]
  };
}

-?- If is-signed-integer-like<index-type> is true, the member typedef-name difference_type denotes index-type. Otherwise, it denotes IOTA-DIFF-T(index-type) (26.6.4.2 [range.iota.view]).

3875. std::ranges::repeat_view<T, IntegerClass>::iterator may be ill-formed

3875. `std::ranges::repeat_view<T, IntegerClass>::iterator` may be ill-formed