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Section: 184.108.40.206 [range.iota.view] Status: WP Submitter: Jiang An Opened: 2021-09-23 Last modified: 2022-11-17
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Unsigned integer types satisfy advanceable, but don't model it, since
every two values of an unsigned integer type are reachable from each other, and modular arithmetic is performed on unsigned integer types,
which makes the last three bullets of the semantic requirements of advanceable (220.127.116.11 [range.iota.view]/5) can't be satisfied, and some (if not all) uses of iota_views of unsigned integer types ill-formed, no diagnostic required.Some operations that are likely to expect the semantic requirements of advanceable behave incorrectly for unsigned integer types. E.g. according to 18.104.22.168 [range.iota.view]/6 and 22.214.171.124 [range.iota.view]/16, std::ranges::iota_view<std::uint8_t, std::uint8_t>(std::uint8_t(1)).size() is well-defined IMO, because
Bound() is std::uint8_t(0), which is reachable from std::uint8_t(1), and not modeling advanceable shouldn't affect the validity, as both W and Bound are integer types.
However, it returns unsigned(-1) on common implementations (where sizeof(int) > sizeof(std::uint8_t)), which is wrong.Perhaps the semantic requirements of advanceable should be adjusted, and a refined definition of reachability in 26.6.4 [range.iota] is needed to avoid reaching a from b when a > b (the iterator type is also affected).
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
[Tim Song commented:]
The advanceable part of the issue is NAD.
This is no different from NaN and
see 126.96.36.199 [structure.requirements]/8.
The part about
iota_view<uint8_t, uint8_t>(1) is simply this:
when we added "When
Bound model ..." to
we forgot to add its equivalent to the single-argument constructor.
We should do that.
[2022-10-12; Jonathan provides wording]
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after seven votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
This wording is relative to N4917.
Modify 188.8.131.52 [range.iota.view] as indicated:
constexpr explicit iota_view(W value);
-6- Preconditions: Bound denotes unreachable_sentinel_t or Bound() is reachable from value.
-7- Effects: Initializes value_ with value.constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);
-8- Preconditions: Bound denotes unreachable_sentinel_t or bound is reachable from value. When W and Bound model totally_ordered_with, then bool(value <= bound) is true.
-9- Effects: Initializes value_ with value and bound_ with bound.