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views::adjacent<0>
should reject non-forward rangesSection: 26.7.27.1 [range.adjacent.overview], 26.7.28.1 [range.adjacent.transform.overview] Status: WP Submitter: Hewill Kang Opened: 2024-05-10 Last modified: 2024-07-08
Priority: Not Prioritized
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Discussion:
Following-up LWG 4082 and LWG 4083, the current wording makes
views::adjacent<0>(r)
and views::adjacent_transform<0>(r, [] { return 0; })
well-formed even when r
is just an input range or an output range, which seems to be an oversight.
[2024-06-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 26.7.27.1 [range.adjacent.overview] as indicated:
-2- The name
views::adjacent<N>
denotes a range adaptor object (26.7.2 [range.adaptor.object]). Given a subexpressionE
and a constant expressionN
, the expressionviews::adjacent<N>(E)
is expression-equivalent to
(2.1) —
((void)E, auto(views::empty<tuple<>>))
ifN
is equal to0
anddecltype((E))
modelsforward_range
,(2.2) — otherwise,
adjacent_view<views::all_t<decltype((E))>, N>(E)
.
Modify 26.7.28.1 [range.adjacent.transform.overview] as indicated:
-2- The name
views::adjacent_transform<N>
denotes a range adaptor object (26.7.2 [range.adaptor.object]). Given subexpressionsE
andF
and a constant expressionN
:
(2.1) — If
N
is equal to0
anddecltype((E))
modelsforward_range
,views::adjacent_transform<N>(E, F)
is expression-equivalent to((void)E, views::zip_transform(F))
, except that the evaluations ofE
andF
are indeterminately sequenced.(2.2) — Otherwise, the expression
views::adjacent_transform<N>(E, F)
is expression-equivalent toadjacent_transform_view<views::all_t<decltype((E))>, decay_t<decltype((F))>, N>(E, F)
.