Proposed resolution:

This wording is relative to N5014.

1. Modify [simd.ctor] as indicated:

   template<class U> constexpr explicit(see below) basic_vec(U&& value) noexcept;
   

   -1- Let `From` denote the type remove_cvref_t<U>.

   -2- Constraints: `value_type` satisfies constructible_from<U>.

   -3- Effects: Initializes each element to the value of the argument after conversion to `value_type`.

   -4- Remarks: The expression inside explicit evaluates to `false` if and only if `U` satisfies convertible_to<value_type>, and either

   • (4.1) — `U` satisfies convertible_to<value_type> and `From` is not an arithmetic type and does not satisfy constexpr-wrapper-like,
   • (4.2) — `From` is an arithmetic type and the conversion from `From` to `value_type` is value-preserving ([simd.general]), or
   • (4.3) — `From` satisfies constexpr-wrapper-like, remove_const_t<decltype(From::value)> is an arithmetic type, and `From::value` is representable by `value_type`.

[simd.ctor] p4 requires all implicit conversions via broadcast constructor to satisfy convertible_to<value_type>. However, std::cw<0.f> is not convertible to `float16_t` because `float` does not satisfy convertible_to<float16_t>. Consequently, with the current wording we don't arrive at bullet (4.3) which allows not-value-preserving conversions if the value is known to not change on conversion. The `convertible_to` constraint is implicit in bullet (4.2), not needed/wrong for bullet (4.3) and thus needs to be moved into bullet (4.1).