If value is present, the type alignment<T, U> is a
BinaryTypeTrait with a base characteristic of
integral_constant<size_t, N> for some unspecified
N ([simd.ctor], [simd.loadstore]).
Where present, the member typedef type names basic_vec<T, Abi1>
if V is a specialization of basic_vec or
basic_mask<sizeof(T), Abi1> if V is a specialization of
basic_mask.
Where present, the member typedef type names
basic_vec<typename V::value_type, Abi1>
if V is a specialization of basic_vec, or
basic_mask<mask-element-size<V>, Abi1>
if V is a specialization of basic_mask.