message: quench a warning on GCC
diff: @@ -446,7 +446,7 @@ private:
  if constexpr (detail::broadcast<T, Derived>) {
  auto s = static_cast<cast_t<T>>(value);
  bool unused[] = { (coeff(Is) = s, false)..., false };
  - (void) unused;
  + (void) unused; (void) s;
  } else {
  if constexpr (Move) {
  bool unused[] = { (coeff(Is) = std::move(value.derived().coeff(Is)), false)..., false };
  