Proposed resolution:

This wording is relative to N5014.

1. Modify [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))>

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.