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4403. simd::basic_vec CTAD misses difference type casting

Section: 29.10.7.2 [simd.ctor] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-10-04 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

Currently, basic_vec can take an object r of range type R whose size is a constant expression and deduced to vec<ranges::range_value_t<R>, ranges::size(r)>.

However, such a deduced type is ill-formed when R has a an integer-class type size which cannot be implicitly converted to simd-size-type, which is a signed integer type.

It is necessary to perform difference type casting here, and the narrowing conversion will still correctly be rejected due to the constructor's constraints.

[2025-10-17; Reflector poll]

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

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   template<class R, class... Ts>
     basic_vec(R&& r, Ts...) -> see below;
   

   -17- Constraints:

   1. (17.1) — R models ranges::contiguous_range and ranges::sized_range, and

   2. (17.2) — ranges::size(r) is a constant expression.

   -18- Remarks: The deduced type is equivalent to vec<ranges::range_value_t<R>, static_cast<simd-size-type>(ranges::size(r))>