Proposed resolution:

This wording is relative to N5014.

1. Modify [simd.overview] as indicated:

   namespace std::simd {
     template<class T, class Abi> class basic_vec {
     public:
       […]
       template<class U = T>
         constexpr explicit(see below) basic_vec(U&& value) noexcept;
       […]
     };
   }
   

2. Modify [simd.ctor] as indicated:

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

   -1- […]

simd::basic_vec supports complex after P2663R7, defaulting the template parameter of its broadcast constructor seems to be reasonable, as this allows the intuitive spelling:

simd::vec<complex<double>> sc1   ({-1.0, 0.5}); // current ill-formed
simd::vec<complex<double>> sc2 = {{-1.0, 0.5}}; // current ill-formed