Issue 4074: compatible-joinable-ranges is underconstrained (original) (raw)

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4074. _compatible-joinable-ranges_ is underconstrained

Section: 25.7.15.2 [range.join.with.view] Status: WP Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2024-07-08

Priority: Not Prioritized

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Discussion:

join_with_view requires the value type, reference and rvalue reference of the inner range and pattern range to share common (reference) types through _compatible-joinable-ranges_.

However, unlike what concat_view and generator do, this concept only requires that these three types be valid and does not further check the relationship between them to be compatible with the indirectly_readable requirement for input_iterator. This results in a validly-constructed join_with_view that may not model input_range, which seems unintended.

The proposed resolution aliases _compatible-joinable-ranges_ to _concatable_i.e. specialization for two ranges to fully constrain, and I believe this could also be a better fit for LWG 3971(i).

Previous resolution [SUPERSEDED]:

This wording is relative to N4981.

1. Modify 25.7.15.2 [range.join.with.view] as indicated:

   namespace std::ranges {
   template<class R, class P>
   concept compatible-joinable-ranges = concatable<R, P>; // exposition only
   common_with<range_value_t, range_value_t

   > &&

common_reference_with<range_reference_t, range_reference_t

> &&

common_reference_with<range_rvalue_reference_t, range_rvalue_reference_t

>;

[…]
}

[2024-04-24; Hewill Kang provides improved wording]

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

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

   #include // see 17.12.1 [compare.syn]
   #include // see 17.11.2 [initializer.list.syn]
   #include // see 24.2 [iterator.synopsis]

   namespace std::ranges {
   […]
   // 25.7.15 [range.join.with], join with view
   template<class R, class P>
   concept compatible-joinable-ranges = see below; // exposition only

   template<input_range V, forward_range Pattern>
   requires view && input_range<range_reference_t>
   && view
   && compatible-joinable-ranges<range_reference_t, Pattern>
   see below

class join_with_view; // freestanding
[…]
} 2. Modify 25.7.15.2 [range.join.with.view] as indicated:
namespace std::ranges {
template<class R, class P>
concept compatible-joinable-ranges = // exposition only
common_with<range_value_t, range_value_t

> &&

common_reference_with<range_reference_t, range_reference_t

> &&

common_reference_with<range_rvalue_reference_t, range_rvalue_reference_t

>;

[…]

template<input_range V, forward_range Pattern>
requires view && input_range<range_reference_t>
&& view
&& compatible-joinable-rangesconcatable<range_reference_t, Pattern>
class join_with_view : public view_interface<join_with_view<V, Pattern>> {
[…]
constexpr auto begin() const
requires forward_range &&
forward_range &&
is_reference_v<range_reference_t> &&
input_range<range_reference_t> &&
concatable<range_reference_t, const Pattern> {
return iterator{*this, ranges::begin(base)};
}
[…]
constexpr auto end() const
requires forward_range && forward_range &&
is_reference_v<range_reference_t> &&
input_range<range_reference_t> &&
concatable<range_reference_t, const Pattern> {
[…]
}
};
} 3. Modify 25.7.15.3 [range.join.with.iterator] as indicated:
namespace std::ranges {
template<input_range V, forward_range Pattern>
requires view && input_range<range_reference_t>
&& view && compatible-joinable-rangesconcatable<range_reference_t, Pattern>
template
class join_with_view<V, Pattern>::iterator {
[…]
};
} 4. Modify 25.7.15.4 [range.join.with.sentinel] as indicated:
namespace std::ranges {
template<input_range V, forward_range Pattern>
requires view && input_range<range_reference_t>
&& view && compatible-joinable-rangesconcatable<range_reference_t, Pattern>
template
class join_with_view<V, Pattern>::sentinel {
[…]
};
}