Proposed resolution:

This wording is relative to N5014.

1. Modify [simd.syn] as indicated:

   namespace std::simd {
     […]
     // [simd.alg], algorithms
     […]
     template<copyableclass T, copyableclass U>
       constexpr auto select(bool c, const T& a, const U& b)
       -> remove_cvref_t<decltype(c ? a : b)>;
     […]
   };
   

2. Modify [simd.alg] as indicated:

   template<copyableclass T, copyableclass U>
     constexpr auto select(bool c, const T& a, const U& b)
       -> remove_cvref_t<decltype(c ? a : b)>;
   

   -9- Effects: Equivalent to: return c ? a : b;

This function currently only requires that `c ? a : b` be a well-formed expression, which simply returns `c ? a : b`.

Given that it seems intended to work with `basic_vec`, `basic_mask`, or vectorizable type, requiring `T` and `U` to be copyable seems reasonable since they are trivially copyable. It shouldn't take non-copyable objects and produce hard errors in the function body.